algorithms call for the best possible parameterisation of the subglacial .... wave velocity profile and its relation to
IGT Projects 2016 Contents Subglacial Aquifers: Characterisation with Geophysical Techniques ......................... 2 Constructing geochemical analogues of the asthenosphere: Earth’s largest geochemical reservoir ................................................................................................ 7 High-rate volcano deformation monitoring from space ............................................. 10 Volcanoes as a source of aerosol: experimental constraints and field observations 13 Excitement of torsional waves in Earth’s core .......................................................... 17 Hydration of the lower oceanic crust and upper mantle: implications for element fluxes at subduction zones ....................................................................................... 20 Boundary Controls on Convection from the Top and Bottom of Earth’s Fluid Core .. 23 Robustness of isotope signatures as proxies of formation conditions in Earth Sciences: experimental study on kinetic isotopic fractionation in carbonates ........... 26 Using seismic and tilt measurements to forecast eruptions of silicic volcanoes ....... 31 Detecting microearthquakes in fault zones and geothermal fields—automatically ... 35 Slab Graveyards and Plume Generation Zones: Mixing and melting in the Earth's lowermost mantle ..................................................................................................... 39 Petrological & geochemical insights into subduction initiation- the case of Izu-BoninMariana volcanic arc ................................................................................................ 43 Quantifying Earth’s deep water cycle by combining seismology and mineral physics ................................................................................................................................. 47 What controls the magmatic plumbing systems of spreading centres in Afar?......... 51
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Subglacial Aquifers: Characterisation with Geophysical Techniques Dr Adam Booth (SEE), Dr Jared West (SEE), Dr Phil Livermore (SEE) Contact email:
[email protected] The hydrology of the subglacial environment exerts a significant control on the flow dynamics of glaciers and ice masses (Bell, 2008; for basic background see Benn and Evans, 2013). The supply of subglacial water influences ice flow by modulating basal friction and the strength of subglacial sediment and its potential to deform. Subglacial water can be measured directly at point-samples in boreholes, or inferred more widely from remotelysensed data including surface-based geophysical observations. Such measurements are particularly important for modelling the dynamic hydrology beneath numerous Antarctic ice streams: groundwater flux between linked subglacial aquifers is an important influence on ice stream stability (Fricker et al., 2014), and the latest generation of predictive modelling algorithms call for the best possible parameterisation of the subglacial environment (Christofferson et al., 2014). However, while geophysical measurements usefully detect subglacial water, they offer little quantitative control on the amount of water stored within a subglacial reservoir. While standard glacio-geophysical methods – including seismic reflection and ground penetrating radar (GPR) surveys – offer powerful constraint on the englacial and basal properties of an ice mass, they are more poorly suited to measuring the properties of material beyond the immediate vicinity (~2 m) of the glacier bed (e.g., Booth et al., 2012). Correspondingly, geophysical images of subglacial structure are not widespread. The seismic reflectivity at the glacier bed tends to be high and, accordingly, the transmission of seismic energy into the subglacial environment is weak and incoherent (Figure 1). This effect is compounded for the GPR case, since attenuation rates are typically increased when propagating through wet sediment therefore energy is absorbed before propagating to significant depth. A new suite of geophysical tools is therefore required to sample the subglacial environment. This project will explore the combined applicability of seismic surface wave analysis (MASW) and time-domain electromagnetic (TEM) methods. The MASW technique is used extensively in engineering surveys to derive mechanical subsurface properties from the Rayleigh wave (groundroll) component of the seismic wavefield (Armstrong et al., 2009; Kim et al., 2010). Rayleigh wave velocity varies with frequency, and measured velocity:frequency trends can be expressed as an S-wave velocity profile (Reynolds, 2011). S-wave velocity has been shown to be sensitive to the water content in subglacial sediment (Peters et al., 2008) hence there is scope for useful MASW applications. TEM methods are extensively applied in groundwater mapping, and have had recent success in mapping subglacial hydrology in Antarctica (Mikucki et al., 2015; Figure 2). The principle 2
of TEM is to measure the decay of magnetic fields induced in subsurface conductors, and interpret these in terms of the electrical resistivity structure of the ground surface. Empirical scaling laws exist (e.g., Archie’s Law) by which water saturation can be inferred from a measure of electrical conductivity, but these likely require refinement for subglacial sediment hence laboratory analysis of sediment samples (Kilner et al., 2005; West et al., 2007) is a potential avenue for study.
Figure 1. Schematic cross-section through a glacier. Water at the glacier bed lubricates ice flow, and can be stored in porous sediment in subglacial aquifers. Glaciers can be extensively surveyed by geophysical methods, but GPR and seismic reflection methods offer little insight into the properties of subglacial sediment beyond the immediate glacier bed. This project will test the applicability of MASW and TEM systems to extend the characterisation of subglacial aquifers, at accessible glacier sites in Arctic and/or Alpine Europe.
Figure 2. TEM profile from Antarctica’s Taylor valley. The electrical properties of ice masses and subglacial sediment can be clearly distinguished, with the resistivity of the latter being indicative of water-saturated material (Mikucki et al., 2015). 3
Field acquisitions using these methods are planned at accessible European glacier sites, with a view to establishing their feasibility ahead of future Antarctic deployments. Potential field sites include glaciers on the Svalbard archipelago, outlets from Norway’s Hardanger ice cap and in the Swiss Alps. On acquiring data, seismic and TEM records will be combined in a joint interpretation/inversion strategy, with observations supplied to a coupled sediment/ice flow model (in collaboration with Dr Poul Christoffersen at the Scott Polar Research Institute). Objectives: In this project, you will work with leading scientists at Leeds and the Scott Polar Research Institute to perform geophysical surveys to offer novel insight into the physical properties of the subglacial environment. Potential objectives for the studentship include, but are not limited to: 1. Forward-modelling of MASW and TDEM responses for a range of subglacial environments and hydrological settings, including development of modelling algorithms where appropriate. 2. In-field deployment of MASW and TDEM surveys at Arctic and/or Alpine glacier sites, alongside more conventional geophysical imaging methods. 3. Derivation of physical properties from MASW and EM surveys to assess the composition and, where appropriate, thickness and water content of a subglacial reservoir. 4. Development of a joint inversion strategy for combining observations from the seismic and electromagnetic data archives. 5. Petrophysical characterisation of subglacial sediment samples, to facilitate the interpretation of field-scale geophysical measurements. 6. Integration of hydrological observations into a coupled sediment/ice flow model. Potential for high impact outcome: The next generation of climate-coupled glacier flow models requires improved parameterisation of the subglacial environment. The geophysical methods proposed here represent innovative developments in the constraint of subglacial hydrology, and contribute to a broader understanding of the coupled ice-ocean-atmosphere climate system. The Institute of Applied Geoscience at the University of Leeds has broad experience of a range of geophysical survey methods, in particular for cryosphere applications. We anticipate that the project will generate at least three high-impact papers, demonstrating new interpretative geophysical approaches, their application in ice-flow modelling, and reporting observations from study sites. The approaches developed will be incorporated in major funding applications to support Antarctic field deployment, a proven pathway to impactful publication and dissemination. Training: The student will work under the supervision of Dr Adam Booth, Dr Jared west and Dr Phil Livermore, within the Institute of Applied Geophysics in the University of Leeds School of Earth and Environment. This project provides a high level of specialist scientific training in:
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appreciation of the significant research themes in the discipline of cryosphere science, design of geophysical surveys and modelling of geophysical responses, acquisition of geophysical field data in glaciated environments, including at sites throughout the European Alps and Arctic, quantitative interpretation of geophysical observations, and development and sensitivity analysis of coupled sediment/ice flow models
Co-supervision will involve regular meetings between all partners. The student can undertake an MSc-level foundation course in Antarctic Glaciology during a one-month study programme at the University Centre on Svalbard, in which participants are trained in theoretical and practical techniques, and/or the Karthaus summer school in northern Italy, which focuses on computational aspects of glaciology. The successful candidate will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, which will be bolstered with the input of Dr Christoffersen. Practical support in undertaking your PhD, from its planning to its examination, is also offered. References
Armstrong M. (2009), Multichannel Analysis of Survey Waves (MASW) determined surfacewave velocity profile and its relation to observations of the near-surface polar firn layers. Project Report, GCAS 11, 2008/2009, University of Canterbury, New Zealand. Bell R. (2008), The role of subglacial water in ice-sheet mass balance. Nature Geoscience,1, 297-304. Benn D.I. and Evans, D.J.A. (2013), Glaciers and Glaciation, 3rd Edition, Routledge Booth A.D., Clark R.A., Kulessa B., Murray T., Carter J., Doyle S. and Hubbard A. (2012), Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland. The Cryosphere, 6, 909-922. Christoffersen P., Bougamont M., Carter S.P., Fricker H.A. and Tulaczyk S. (2014), Significant groundwater contribution to Antarctic ice streams hydrologic budget. Geophysical Research Letters, 41(6), 2003-2010. Fricker H.A., Carter S.P., Bell R.E and Scambos T. (2014), Active lakes of Recovery Ice Stream, East Antarctica: a bedrock-controlled subglacial hydrological system. Journal of Glaciology, 60(223), 1015-1030. Mikucki J.A., Auken E., Tulaczyk S., Virginia R.A., Schamper C., Sørenson K.I., Doran P.T., Dugan H. and Foley N. (2015), Deep groundwater and potential subsurface habitats beneath an Antarctic dry valley. Nature Communicatoins, 6(6381). Kilner M., West L.J. and Murray T. (2005), Characterisation of glacial sediments using geophysical methods for groundwater source projects. Journal of Applied Geophysics, 57, 293-305. Kim K.Y., Lee J., Hong M.H., Hong J.K., Jin Y.K. and Shon H. (2010), Seismic and radar investigations of Fourcade Glacier on King George Island, Antarctica. Polar Research, 29, 298-310.
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Peters L.E., Anandakrishnan S., Holland C.W., Horgan, H.J., Blakenship D.D., Voigt D.E. (2008), Seismic detection of a subglacial lake near the South Pole, Antarctica. Geophysical Research Letters, 35(23), F02008. Reynolds J.M. (2011), An Introduction to Applied and Environmental Geophysics, 2nd Edition, Wiley. West L.J., Rippin D.M., Mader H.M. and Hubbard B. (2007), Dielectric permittivity measurements on ice cores: Implications for interpretation of radar to yield glacial unfrozen water content. Journal of Environmental and Engineering Geophysics, 12, 37-45.
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Constructing geochemical analogues of the asthenosphere: Earth’s largest geochemical reservoir Dr Jason Harvey (SEE) Dr Dan Morgan (SEE), Dr Thomas Mueller (SEE) Contact email:
[email protected], Project synopsis: It is becoming increasingly clear that the study of basalt composition does not always lead to an accurate assessment of the isotopic and chemical composition of its mantle source. Therefore, in order to understand mantle composition better, it is more prudent to collect direct evidence from the mantle itself. However, obtaining representative samples of the modern-day convecting mantle is problematic because it is overlain by the lithosphere, yet lithospheric mantle samples are the only examples of the mantle available. Drilled or dredged abyssal peridotite is often heavily altered by interaction with seawater and hydrothermal fluids. On the other hand, peridotite xenoliths, while exposed at the surface of the Earth, are often affected by metasomatism, melt-rock interaction and / or refertilization processes. This project will examine the various processes that have Figure 1. Serpentinized oceanic lithospheric affected the composition of peridotites mantle. But how representative is abyssal recovered from the Mid-Atlantic ridge, the peridotite like this of the convecting mantle Iberian passive margin and the Cape Verde as a whole, before it is hydrated? (from Bach et al., 2004)
Islands in order to (i) better understand the processes that can overprint the chemical and isotopic signatures preserved in the upper mantle at different tectonic settings, and (ii) assess peridotite from each of these provinces for its suitability as a chemical and isotopic proxy for the more voluminous, yet unreachable, asthenospheric mantle. Objectives: Two key questions can be posed: (i) Which of the chemical and isotopic heterogeneities observed in lithospheric mantle samples are representative of all of the mantle (i.e. the lithosphere and asthenosphere) and which are only present in the lithosphere (i.e. are unlikely to contribute to the
Figure 2. Peridotite from oceanic basins (abyssal
peridotite) while only recently separated from the asthenosphere is often extensively altered by seawater – peridotite interaction, obscuring the composition of primitive melt depleted mantle (from Bach et al., 2004)
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composition of asthenosphere-derived basalts) and (ii) in which source of lithospheric peridotite (abyssal peridotite vs. peridotite xenoliths) is it easiest to “see through” secondary chemical and isotopic “smudging” and provide a realistic representation of the composition of the asthenospheric mantle and the heterogeneities that are preserved within it? Background: For over half a century the composition of the Earth’s mantle has been determined largely by indirect methods. These range from geophysical measurements, which identify large volumes of the mantle that have distinctly different physical properties to their immediate surroundings, to the composition of basalts, which traditionally have been assumed to be representative of the mantle domains that produced them through partial melting. Apart from fortuitous instances where the mantle is exposed, for example in orogenic massifs, in ophiolites and as peridotite xenoliths (accidently transported to the surface in alkali basalts), the majority of what we think we know about the composition of the mantle has, to date, been derived indirectly. Experimental petrology and geochemical modelling tell us about the behaviour of incompatible trace elements during partial melting in the mantle, allowing the prediction that peridotite and the lavas that it produces have complimentary elemental compositions. Moreover, because isotope ratios of elements are not fractionated during high temperature processes, it is assumed that the isotopic fingerprint of a mantle reservoir is reproduced in the melts that it produces. However, there is increasing evidence to suggest that it is not necessarily safe to make the assumption that these fingerprints are faithfully transferred from mantle source to basalt – the fingerprint is somewhat “smudged” as there are domains of varying sizes (cm to km) in the mantle that do not correspond to the composition of basalts they are inferred to have produced (e.g. Alard et al., 2005; Warren et al., 2009; Burton et al., 2012; Warren and Shirey, 2012). Unfortunately, the mantle samples that we have to work with are exclusively from the lithospheric mantle – the asthenosphere, which contributes to the majority of intra-plate basalt petrogenesis is not accessible for direct sampling. Dredged or drilled abyssal peridotite is often heavily hydrated and its composition drastically altered through interaction with seawater and / or hydrothermal fluids, whereas peridotite xenoliths can be affected by cryptic metasomatism, melt infiltration and refertilization – all of which potentially obscure the primary nature of the original peridotite that it represented when melt was first extracted from it. Samples: For this project Mid-Atlantic Ridge peridotites will be sampled from the extensive collections available to Harvey at Woods Hole Oceanographic Institution, Massachusetts and Scripps Institution of Oceanography, California. In addition, peridotite samples from the Iberian passive margin (Ocean Drilling Program Leg 173) will be also be selected from the IODP core repository in Bremen. These will be supplemented by field sampling of peridotite xenoliths from the Cape Verde Islands, 8
Figure 3. Melt-rock interaction in peridotite from the sub-continental lithospheric mantle overprints primary geochemical and isotopic melt depletion signatures. Only by “looking through” these secondary processes can we gain insight into the composition of Earth’s asthenosphere (from Harvey et al. (2010).
where primitive basalts have erupted through similar oceanic mantle to the other sample suites. The majority of the project will concern high-precision isotope geochemistry to be performed in a clean laboratory environment at the University of Leeds. We are developing cutting edge techniques for the measurement of mantle sulphide grains and mantle minerals for e.g. Os and Pb isotopes in individual sulphide grains and high precision Sr and Nd isotope measurements on single clinopyroxene grains, and measurements of this nature will be a key element of the project. This work will complement ongoing work in this field already being undertaken at the University of Leeds by Harvey, Morgan and Mueller who have a strong track record in publishing high quality geochemistry research. Training: The successful applicant will be trained in a wide range of cutting edge geochemical techniques involving sample preparation, purification and analytical methods. The measurements made will make full use of the facilities available within the School of Earth and Environment (secondary electron microscope, electron probe microanalyser, inductively coupled plasma mass spectrometry (both with and without laser ablation) and thermal ionization mass spectrometry Potential for high impact outputs: Challenging the fundamental tenet that mantle composition cannot be accurately assessed through the composition of the basalts that it produces has enormous potential to yield high impact outputs. Over the last few years the lead supervisor and co-workers have demonstrated that the Earth’s mantle is chemically and isotopically heterogeneous at cm to km scales (Burton et al., 2012; Harvey et al., 2010, 2011, 2012; 2014a, 2014b, 2015; Warren et al., 2009) and that this variability is not always expressed in the composition of basalt. The next stage of this research is to unequivocally separate the processes that introduce heterogeneity into lithospheric mantle from those that are likely affecting the inaccessible asthenospheric mantle i.e. which processes are the real control on geochemical and isotopic signatures during basalt petrogenesis and which, although clearly present in the lithospheric mantle, simply mask asthenosphere composition. References: Alard et al. (2005) Nature 436, 1005-1008. Bach et al. (2004) Geochem, Geophys, Geosys 5 (9). Burton et al. (2012) Nat Geosci 5, 570-573. Harvey et al. (2010) Geochim Cosmochim Acta 74, 293-320. Harvey (2011) Geochim Cosmochim Acta 75, 55745596. Harvey et al. (2012) J. Petrol 53, 1709-1742. Harvey et al. (2014a) Geochim Cosmochim Acta 126, 30-48. Harvey et al. (2014b) Chem Geol 376, 20-30. Harvey et al. (2015) Geochim Cosmochim Acta 166, 210-233. Warren et al. (2009) J Geophys Res 114, B12203. Warren and Shirey (2012) Earth Planet Sci Lett 359-360, 279-293 Keywords: Earth history; Earth interior; Geochemistry; Mineralogy; Modelling.
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High-rate volcano deformation monitoring from space Prof. Andy Hooper (SEE), Prof Doug Parker (SEE) Contact email:
[email protected] The measurement of surface deformation on volcanoes is a key tool for probing magmatic processes occurring beneath the surface (Segall, 2010). This, in turn, is important both for the monitoring and forecasting of volcanic activity, and for furthering scientific understanding of the underlying physics controlling volcanic behaviour. This project involves both improving methods to measure deformation and applying the improved methods to active volcanoes, to address both the forecasting and scientific aspects of volcano monitoring. Deformation can be monitored by ground instrumentation (usually high-accuracy GPS), but the cost to install and maintain such a network is high. Radar interferometry (InSAR), on the other hand, is a spaceborne technique that can be applied to subaerial volcanoes worldwide. The temporal sampling is not as good as ground based techniques, but the spatial resolution of measurements is orders of magnitude better (Pinel et al, 2014).
Figure 1: Cumulative line-of-sight displacement over Eyjafjallajökull volcano, Iceland, from radar interferometry. The co-eruptive interval includes the flank eruption, during which there was very little deformation, and the later summit eruption, during which the volcano deflated. Background is shaded topography. The hole in the data is due to ice and snow cover. Black dots are earthquake epicentres for each epoch from the Icelandic Meteorological Office.
Radar interferometry works by detecting small changes in the phase of a reflected signal between overpasses at different times, due to the change in range between the satellite and the ground. Displacement of the ground is one cause of phase change (Figure 1), but changes in refractivity of the atmosphere also contribute. Attempts to reduce the influence of atmosphere, to date, have relied on atmospheric models, interpolating radiometric 10
measurements, determining empirical relationships, or filtering (Bekaert et al, 2015). However, all of these approaches have had limited success. The number of radar satellites has continued to increase in the last few years, and by combining measurements from multiple satellites, it is now possible to acquire measurements multiple times per day. This offers exciting new opportunities for analysis, a chief one being the possibility to model the atmospheric noise jointly with any deformation; such short repeat times mean that the atmospheric contribution to the phase is correlated between acquisitions, which is not the case when separated by many days. This project will focus on the development of new algorithms to achieve this aim, and the application of these algorithms to active volcanoes, to better image and interpret the deformation due to subsurface magmatic processes. Target volcanoes include Kilauea on Hawaii, and other volcanoes that show signs of activity during the PhD. Fieldwork will also be carried out, where necessary, for ground validation. Objectives: You will work with leading scientists at Leeds to: 1) Develop methods to jointly model the deformation and atmospheric signals present in radar interferometric measurements, 2) Apply these methods to a volcano, e.g., Kilauea in Hawaii, where there is an extensive archive of radar acquisitions and also an extensive ground-based network, 3) Apply these methods to one or more volcanoes showing signs of activity that are likely to lead to eruption, as part of the near-real-time monitoring, 4) Interpret the deformation at volcanoes where the techniques have been applied, to improve understanding of magmatic plumbing systems, 5) Carry out fieldwork on volcanoes to validate the new algorithms. Potential for high impact outcome: Volcanic eruptions pose a hazard to both proximal and distal populations, and also have the potential for high economic impact due to knock-on effects such as the grounding of aircraft. The improvement in our ability to measure and, potentially, forecast volcanic activity will lead to highly citable publications. The methods developed in this project will also be applied to at least one major eruption during the PhD, which we expect to lead to a high impact publication. In addition, we expect the new methods to be taken up by volcano observatories throughout the World. Training: You will work under the supervision of Prof. Andy Hooper and Prof. Doug Parker within both the Institute of Geophysics and Tectonics and the Institute of Climate and Atmospheric Sciences in the School of Earth and Environmental Sciences. You will also become a member of the Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), which brings together experts from the Universities of Oxford, Cambridge, Leeds, Bristol, Reading, Liverpool and Newcastle and University College London 11
(http://comet.nerc.ac.uk). Through COMET, you will have access to a range of training opportunities related to deformation monitoring and modelling, in addition to a broad spectrum of training workshops provided by the Faculty, from training in numerical modelling through to managing your degree and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). You will also be actively encouraged to present work at conferences and to publish papers. References
Bekaert, D. P. S., Walters, R. J., Wright, T. J., Hooper, A. J., and Parker, D. J. (2015). Statistical comparison of InSAR tropospheric correction techniques, Remote Sensing of Environment 170, 40-47. Pinel, V., Poland, M. P., and Hooper, A. (2014). Volcanology: Lessons learned from synthetic aperture radar imagery. Journal of Volcanology and Geothermal Research, 289, 81-113. Segall, P. Earthquake and volcano deformation. Princeton University Press, 2010.
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Volcanoes as a source of aerosol: experimental constraints and field observations Dr Evgenia Ilyinskaya (SEE), Dr Ian Burke(SEE), Dr Jim McQuaid (SEE), Dr Thomas Mueller (SEE), Dr Marie Edmonds (University of Cambridge) Contact email:
[email protected] Volcanism is a major natural source of aerosol particles; yet relatively little is known about how particles form within volcanic vents. The impacts of volcanic aerosol are most evident following large explosive eruptions (e.g., Krakatau, 1883; Mt Pinatubo, 1991), when enormous quantities of fine ash and sulphate aerosol are emplaced in the troposphere and stratosphere. However, given the infrequency of these very dramatic events and the short lifetime of particles in the atmosphere, a more enduring role (particularly on local scales) is played by the modest but persistent emissions from a large number of quiescently degassing volcanoes, such as Erebus (Antarctica), Etna (Sicily), Villarrica (Chile) and Masaya (Nicaragua) and various others (Fig 1). Almost all naturally occurring metals can be found in volcanic emissions, and these can have a great impact on the environment. The impact can be either adverse or stimulating for survival and growth of living organisms (e.g., Jones and Gislason 2008). However, there are many gaps in our Figure 1: Quiescent (non-eruptive) understanding of the behaviour of metals, and the emissions at Aso volcano in Japan. The estimates of global volcanic metal emissions are plume can be sampled from the crater few and highly variable (see review in Mather rim as shown here, but it has already 2015). undergone a significant amount of mixing with the background air
The state-of-knowledge on metal emissions from volcanoes is mostly based on field data, some thermochemical modelling and a few experiments on metal volatility in igneous melts. Common features include a concentration of aerosol mass in the 0.1 - 10 μm size range, high solubility in water, and strong associations between sulphate and certain metals. However, there is also a significant diversity, with some volcanic plumes dominated by sulphate (e.g., Masaya, Kilauea; e.g. Mather et al., 2003, 2012) and others by chloride (Erebus; Ilyinskaya et al., 2010, 2012), and variations in the number of size modes, their modal diameters and their compositions (in particular the metal speciation). It is not well understood whether these differences reflect differences in magmatic gas compositions, ambient atmospheric conditions or vent dynamics, or a mixture of all, as they cannot be easily constrained or quantified in the field (Fig 1). A fundamental uncertainty is whether the reported aerosol chemistry and aerosol size distributions from a given volcano are 13
persistent and stable, or whether sampling during short field campaigns offers an unrepresentative snapshot of the volcanic aerosol at each system. Metal speciation is particularly poorly understood, in spite of their importance for environmental impact. The current understanding, based on thermochemical modelling (Symonds et al. 1992) is that the majority of metals are volatilised directly from the magma during quiescent degassing and that their volatilisation is dependent on the chloride abundance in the magmatic gas, as metal-chloride compounds have high volatilities (Fig 1). This correlation between elevated abundances of Cl and metals has been supported by field measurements at a few volcanoes (e.g. Toutain et al. 1995). The role of other halogens (F, Br) is much more uncertain. Metal-sulphate particles have also been detected in volcanic plumes. Metals are not thought to be emitted from the magma as sulphate compounds although this has not been firmly established. Sulphur in the gas phase is believed to be initially in the form of SO2 gas. SO2 is converted to sulfate aerosol which is believed to replace the halogens in the aerosol, forming sulphate-metal particles and liberating halogen-bearing gas (Fig 2). This aspect of metal chemistry is also poorly understood. This project will address these uncertainties by analyzing aerosol emissions from a ‘volcano’ built in the lab under controlled conditions. The work will also include sampling of aerosol emission at actively degassing volcanoes to provide comparison with the experimental data. The focus is on analyzing the metal, halogen and sulfur pathways together, as they are released from the melt (Fig 2). Metal ratios combined with particle imaging will allow the tracking of aerosol formation, aerosol growth, metal fractionation and chemistry in the plume. This is an ambitious yet self-contained project that aims to provide a highly quantitative constraint on processes in volcanic plumes, which is why it has potential for enormous strides forward in our understanding of volcanic impact on the atmosphere and the environment. The findings will also have wide implications for health hazards arising from the characterisation of the concentrations of small particles in volcanic emissions (PM10 and PM2.5).
Figure 2: Metal degassing pathway from open-vent volcanoes. Metals (only a few examples shown here) are released from the magma as halogen compounds, but the halogens are later replaced by sulphate which forms from SO2 gas. This process is poorly understood as has not been directly observed.
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Objectives: The overarching aim is to create a robust chemical model for interpreting natural datasets. Volcanic degassing will be simulated under controlled conditions in order to eliminate the inherent uncertainties associated with field sampling. The experimental data will be compared to datasets from various active volcanoes. 1. Setting up experimental volcanic degassing by melting igneous rocks using a gasphase furnace (under controlled temperature, pressure and oxygen fugacity). The experiment will involve designing the capture of emitted components at different conditions. 2. Chemical analysis of the captured aerosol and gas phase. Chemical analysis will include ICP-MS, ion chromatography and isotopic analysis. Imaging will include SEM, TEM, synchrotron-based micro-focus XAS spectroscopy (and others techniques as appropriate). 3. Petrological analysis of the degassed melt for comparison with the emitted components. 4. Collection and analysis of novel field data for comparison with experimental data, at one or more persistently degassing volcanoes. 5. Review and consolidation of the available field data in the light of the experimental findings. Potential for high impact outcome: Natural aerosols account for 70% of the global aerosol loading and one of the main contributors is volcanic activity, in particular with regards to metal flux. Yet there are significant uncertainties regarding their emission, speciation and flux, as our understanding is mostly based on field data where changes in the sampling conditions are difficult to control and quantify. This project directly addresses these uncertainties through a highly novel, but well-defined and powerful experimental approach. The laboratory facilities at the University of Leeds are extremely well equipped to carry out the proposed research, especially due to the recent availability of a gas mixing furnace. The expertise of the supervisory team is wide-ranging in order to cover the interdisciplinary aspect of this project. We anticipate the project generating at least two to three papers for publication in high-impact journals. Training: The student will work under the supervision of Dr Evgenia Ilyinskaya in IGT, and have several co-supervisors. The panel of co-supervisors reflects the interdisciplinary nature of this novel project. Ian Burke (ESSI), supervision topics: particle capture and imaging; Thomas Mueller (IGT): furnace experiments; Jim McQuaid (ICAS), atmospheric chemistry and aerosol sampling techniques; Marie Edmonds (University of Cambridge): igneous petrology. The School of Earth and Environment at Leeds has all the lab facilities expected to be needed for this project. The student is also expected to undertake fieldwork at one or more actively degassing volcanoes.
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Specialist training will be provided in: (i) Experimental igneous petrology; (ii) Highly original experiments on high- and low-temperature aerosol chemistry; (iii) Analytical instruments including e.g. ICP-MS, IC, SEM, TEM, SIMS electron probe and LA-ICP-MS; (iv) Field sampling at active volcanic sites. The supervision will involve regular meetings with the primary supervisor, as well as scheduled regular meetings with the co-supervisors in Leeds and Cambridge. The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). References
Ilyinskaya E, C Oppenheimer, RS Martin (2012), Aerosol formation in basaltic lava fountaining – Eyjafjallajökull eruption 2010, JGR – Atmospheres and Solid Earth, special issue, 117, D00U27. Ilyinskaya, E., C. Oppenheimer, T.A. Mather, et al (2010), Size-resolved chemical composition of aerosol emitted by Erebus volcano, Antarctica, Geochemistry Geophysics Geosystems, 11, Q03017, doi:10.1029/2009GC002855 Jones, M. T. and S. R. Gislason (2008), Rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments, Geochimica et Cosmochimica Acta, 72, 3661–3680 Mather TA, MLI Witt, BM Quayle, A Aiuppa, E Bagnato, RS Martin, M Edmonds, AJ Sutton and E Ilyinskaya (2012) Halogens and trace metal emissions from the ongoing 2008 summit eruption of Kilauea volcano, Hawai'i, GCA, doi:10.1016/j.gca.2011.11.029. Mather, T. A., Volcanoes and the environment: Lessons for understanding Earth’s past and future from studies of present-day volcanic emissions, Journal of Volcanology and Geothermal Research (2015), doi: 10.1016/j.jvolgeores.2015.08.016 Mather, T.A., A.G. Allen, C. Oppenheimer, et al (2003), Size-resolved characterisation of soluble ions in the particles in the tropospheric plume of Masaya Volcano, Nicaragua: Origins and plume processing, Journal of Atmospheric Chemistry, 46, 207–237 Symonds, R.B., M. Reed and W. Rose (1991), Origin, speciation, and fluxes of trace-element gases at Augustine volcano, Alaska: Insights into magma degassing and fumarolic processes, Geochimica et Cosmochimica Acta, 56, 633-657 Toutain, J.-P., J.-P. Quisefit, P. Briole, et al (1995), Mineralogy and chemistry of solid aerosols emitted from Mount Etna, Geochemical Journal, 29, 163–173
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Excitement of torsional waves in Earth’s core Dr Phil Livermore (SEE), Dr Jon Mound (SEE), Prof. Rainer Hollerbach (Maths) Contact email:
[email protected] Background: The Earth’s internally generated magnetic field is a fundamental yet still poorly explained feature of our planet [see e.g. Stern]. It is truly a global scale feature: its energy source is the swirling motion of hot liquid iron, thousands of miles beneath our feet within the Earth’s core, yet its effects are felt both on the Earth’s surface (for example, its use in navigation) and out into near-Earth space, where it acts as a magnetic shield against harmful solar radiation. The dynamics of the liquid outer core, bounded between the solid inner core and overlying solid mantle (on which we all live) is one of the key outstanding questions of modern Earth science and physics. The internal geomagnetic field is not constant in time, but changes on a timescale of years to decades (Finlay and Jackson, 2015). Part of this signal is due to a special class of waves that travel inside the core: torsional waves (Cox and Brown, 2013). Due to the strong influence of rotational forces (caused by the Earth’s daily rotation), these waves travel as cylinders of fluid in a radial direction. All waves require a restoring force, and here it is “magnetic tension”, just as a plucked string on a guitar generates waves because the string is under tension. These waves are believed to traverse the core in around 6 years (Gillet et al, 2010), and are one of the few aspects of the interior dynamics of the core that we can observe (Zatman and Bloxham, 1997).
Figure 1: (left) Torsional waves form a pattern of nested cylinders of fluid that rotate in different directions. (right) Torsional waves traverse the core in about six years, perturbing the magnetic field and creating an observational signal.
Although many studies have focussed on torsional waves since the 1970’s, there are two fundamental aspects that we do not understand. First is why they are there at all – that is, we do not know what their excitation mechanism is. Various options have been suggested, such as deviations from an equilibrium, both global or localised on the tangent cylinder (the 17
imaginary cylinder of fig 1 that just touches the inner core), or fluctuations in physical quantities such as the magnetic field, the background convection, or inner-core rotation. Second is their direction of travel: simulations and observational-based studies disagree on whether the waves travel radially outwards, inwards or a combination of both. Objectives: This two-part project will involve both theoretical and computational aspects in attempting to realistically model the excitement and evolution of torsional waves. Initial conditions for the wave propagation will be motivated by theoretical considerations, such as - perturbations from an equilibrium (so-called) Taylor state (Taylor, 1963) - perturbations from shear flows on the tangent cylinder (e.g. Livermore and Hollerbach, 2012) - perturbations of inner-core rotation - random background perturbations to either the magnetic field and core-flow, both localised and global. 1) The evolution of the waves generated by the above mechanisms will be simulated using a computer code run on the Leeds university supercomputer ARC3. The student will need to adapt and modify existing computer code, to generate and visualise results. 2) Simulations of the waves will then be compared with satellite data, in particular the recent Swarm mission launched in 2013, to assess which models are compatible. Milestones: Year 1: Familiarisation with geomagnetic data, theory of field generation and simulations. Benchmarking against documented simulations of torsional waves. Year 2: Modelling and simulations of the various types of excitation mechanism. Visualisation of the dynamics. Year 3: Comparison of the simulations with observations. Potential for high impact outcome: Explaining important features in the Earth’s magnetic field is an international endeavour and of wide interest. Studies of waves in the core date back to the 1970's and have been published in high impact journals such as Nature and Science. An explanation for how the waves are excited would potentially be a paradigmchanging study. Training: The student will learn both the theory and computational techniques required to model the Earth’s core, and will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). The student will be a part of the deep Earth research group, a vibrant part of the Institute of Geophysics and Tectonics, currently comprising 8 staff members and several postdocs and
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PhD students. The deep Earth group has a strong portfolio of international collaborators which the student will benefit from. Although the project will be based at Leeds, there will be opportunities to attend international conferences (UK, Europe, US and elsewhere), and potentially collaborative visits within Europe. Requirements: We seek a highly motivated candidate with a strong background in mathematics, physics, computation, geophysics or another highly numerate discipline. Knowledge of geomagnetism is not required, and training will be given in all aspects of the PhD. Other opportunities: The Deep Earth Research Group in Leeds (http://www.see.leeds.ac.uk/research/igt/deep-earth-research/) is one of the largest groups of scientists studying the structure and dynamics of Earth’s core and mantle in the world. Research topics include the dynamics and structure of the Earth’s magnetic field and convection in the outer core, material properties under high pressure and temperature and Global Seismology. The Group collaborates closely with the Department of Applied Mathematics in Leeds and Deep Earth research groups worldwide. Dr Livermore is interested in the dynamics of the core and geomagnetism. Please contact him (
[email protected]) to discuss further PhD opportunities. References
Stern, D. A Millennium of Geomagnetism, online material: http://www.phy6.org/earthmag/mill_1.htm Cox and Brown (2013). Rapid dynamics of the Earth’s core. Astronomy and Geophysics, 54 (5): 5.32-5.37. doi: 10.1093/astrogeo/att167 Gillet et al. (2010). Fast torsional waves and strong magnetic field within the Earth’s core. Nature, 465 (74). doi:10.1038/nature09010 Jackson and Finlay (2015). Geomagnetic Secular Variation and Its Applications to the Core. Treatise on Geophysics, Vol 5.05, Elsevier. Livermore and Hollerbach (2012). Successive elimination of shear layers by a hierarchy of constraints in inviscid spherical-shell flows. J. Math. Phys. 53(073104). http://dx.doi.org/10.1063/1.4736990 Taylor (1963). The Magneto-Hydrodynamics of a Rotating Fluid and the Earth's Dynamo Problem, Proceedings of the Royal Society of London Series A-Mathematical Physical and Engineering Sciences, (274) pp 274–283. Zatman and Bloxham (1997). Torsional oscillations and the magnetic field within the Earth's core. Nature 388 (6644) pp 760 – 763.
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Hydration of the lower oceanic crust and upper mantle: implications for element fluxes at subduction zones Dr Andrew McCaig , Dr Ivan Savov and Dr Jason Harvey Contact e-mail for enquiries:
[email protected] Project synopsis: We propose an integrated study of alteration in the lower oceanic crust and upper mantle focussing on Hess Deep (Pacific Ocean), and the Oman ophiolite. We will combine traditional isotopic methods (Sr and O) with boron contents and boron isotopic analysis to assess the mechanisms and extent of hydration and the contribution that early alteration of the ocean floor may make to isotopic and elemental fluxes in subduction zones. The combination of Sr-O-B isotopes offer a wealth of opportunities for the unravelling of the extent and history of hydration of the lithosphere at a mid-ocean ridges and will provide invaluable information regarding the nature of material entering into subduction zones, and perhaps even further into the deep mantle. Background: It is often assumed that the upper mantle being subducted around the Pacific is highly serpentinised. Dehydration of this serpentine, and other minerals such as chlorite (Fig. 1), is thought to be an important control on melting in the mantle wedge above the subducting slab (eg. Schmidt and Poli, 1998; Till et al., 2011). If the upper mantle is hydrated, it is likely that the lower gabbroic crust is as well. The lower oceanic crust and uppermost mantle are extremely difficult to sample directly, particularly at fast-spreading ridges where topography is subdued and hence seafloor exposures of the lower crust extremely rare. One place where the lower oceanic crust and uppermost mantle of the Pacific is exposed is at Hess Deep, where it has been sampled by the Ocean Drilling Program during Expedition 147 and most recently Integrated Ocean Drilling Program Expedition 345, in 2012-13. 20
Figure. 1: Depth of dehydration reactions within a subducting slab (after Schmidt and Poli, 1998)
Figure. 2: Sample from IODP expedition 345 showing intense alteration zone containing chlorite and prehnite (middle; grey-green) overprinting serpentine-rich alteration in troctolite (dark), (345_U1415P_19G_50-58)
Preliminary work on Expedition 345 samples has revealed a heterogeneously developed greenschist to sub-greenschist facies alteration dominated by chlorite, prehnite and secondary clinopyroxene, which overprints less intense alteration to serpentine, talc and amphibole (Fig. 2). This alteration is localised by cataclastic fault zones and veins. Fault zones are probably the main way in which seawater can gain access to the upper mantle in conditions where serpentine can form (< 500 °C) - these rocks are our only current samples of fault zones in the lower ocean crust in the Pacific, and are therefore the best constraint we have on altered ocean crust entering subduction zones at trenches. Our initial analyses show that the altered rocks are significantly enriched in boron and have high δ11B and 87Sr/86Sr values. The earlier ODP leg 147 also sampled mantle rocks at Hess Deep – these are extensively serpentinised and contain rodingitised gabbroic intrusions, highly altered to prehnite and hydrogarnet. Work on small scale rodingitic assemblages in oceanic troctolites shows that chlorite is produced during reactions between plagioclase and olivine over a wide range of temperatures (Frost et al., 2008). Hence, one place where chlorite is introduced into mantle and lower crustal rocks is where mafic and ultramafic rocks interact chemically during alteration. Chlorite, in crustal assemblages in particular, can persist to high temperatures and pressures during subduction, releasing water at depths sufficient to significantly impact arc volcanism (Till et al., 2011). Because the lower crust has been relatively little studied, alteration at this level has been neglected. In particular the prehnite-chlorite alteration association has not previously been identified as significant in ocean floor alteration, yet it also occurs in rodingites in the upper mantle. In ophiolites and slow spreading crust, fault zones have been identified as important controls on mineral and isotopic alteration (Coogan et al., 2006; McCaig et al., 2007). We will use the Sr-Nd-O isotope approach with the addition of measuring boron abundance and isotope ratio, which are excellent tracers for alteration by seawater (cf. Pabst et al., 2012; Harvey et al., 2014). The student will undertake fieldwork in the Oman ophiolite in collaboration with Marie Python (Hokkaido) and Jurgen Koepke (Hannover). There may be the opportunity to be involved in forthcoming International Continental Drilling Programme in the Oman ophiolite. The student will also collect samples from drill core through visits to the core repository in College Station Texas. Work in Oman will focus on outcrop-scale sampling of altered fault zones cutting the lower crust (eg. Coogan et al., 2006), and rodingites in the mantle sequence (cf. Python et al., 2011). An initial suite of IODP samples is already available in Leeds and will be characterised using SEM and electron probe to assess the conditions of alteration and select domains for isotopic analysis (eg. Fig. 2). Sr/Nd isotope and ICPMS work will be undertaken at Leeds, oxygen isotopes at SURRC East Kilbride, and boron analysis in collaboration with Samuele Agostini (CNR B isotope facility, Pisa), and the Bristol University MC-ICPMS facility. This project will give the student a strong grounding in characterisation of alteration in mafic and ultramafic rocks in outcrop, hand specimen, optical microscope and SEM/electron probe, and in analytical methods such as Thermal Ionisation Mass Spectrometry (TIMS), MC-ICPMS and LAICPMS. The student will join the High Temperature Geochemistry Group. Details of group members and current projects are here: http://www.see.leeds.ac.uk/research/igt/high-temperature-geochemistry/ 21
References:
Coogan L. A., Howard, K.A., Gillis, K.M., Bickle M.J., Chapman, H. Boyce A.J., Jenkin G.R.T. & Wilson R.N. Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust (2006) A. J. Sci. 306, 389-427; Gillis, K.M., et al, (2013) Primitive layered gabbros from fast-spreading lower oceanic crust, Nature, 505, pp.204-207. doi: 10.1038/nature12778 Harvey J; Savov IP; Agostini S; Cliff R; Walshaw R (2014), Si-metasomatism in serpentinized peridotite: the effects of talc-alteration on strontium and boron isotopes in abyssal peridotites from Hole 1268a, ODP Leg 209. Geochimica et Cosmochimica Acta, 126, pp.30-48. doi: 10.1016/j.gca.2013.10.035; Kirchner, T.M. & Gillis, K.M, (2012) Mineralogical and strontium isotopic record of hydrothermal processes in the lower ocean crust at and near the East Pacific Rise. Contrib. Mineral. Petrol. 164: 123-141; Lécuyer, C., and Gruau, G., 1996. Oxygen and strontium isotope compositions of Hess Deep gabbros (Holes 894F and 894G): high-temperature interaction of seawater with the oceanic crust Layer 3 In Mével, C., Gillis, K.M., Allan, J.F., and Meyer, P.S. (Eds.), Proc. ODP, Sci. Results, 147: College Station, TX (Ocean Drilling Program), 227–234; McCaig AM; Cliff RA; Escartin J; Fallick AE; MacLeod CJ (2007) Oceanic detachment faults focus very large volumes of black smoker fluids. Geology, 35, pp.935-938; Maclennan, J., Hulme, T., and Singh, S.C., 2004. Thermal models of oceanic crustal accretion: linking geophysical, geological, and petrological observations. Geochem., Geophys., Geosyst., 5(2):Q02F25. doi:10.1029/2003GC000605 Pabst S; Zack T; Savov I; Ludwig T; Rost D; Tonarini S; Vicenzi E (2012) The fate of subducted oceanic slabs in the shallow mantle: Insights from boron isotopes and light element composition of metasomatized blueschists from the Mariana forearc, Lithos, pp.162-179. doi: 10.1016/j.lithos.2011.11.010; Python, M. Yoshikawa, M., Tomoyuki, S. and Arai, S. (2011) Diopsidites and Rodingites: Serpentinisation and Ca-Metasomatism in the Oman Ophiolite Mantle. In R.K. Srivastava (ed.), Dyke Swarms: Keys for Geodynamic Interpretation, DOI: 10.1007/978-3-642-12496-9_23 Schmidt, M.W. & Poli. S., Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation (1998) EPSL, 163: 361-379; Till, C.B., Grove, T.L. & Withers, A.C. The Beginnings of Hydrous Mantle Wedge Melting, (2011) Contrib. Mineral. Petrol., DOI 10.1007/s00410-011-0692-6
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Boundary Controls on Convection from the Top and Bottom of Earth’s Fluid Core Dr Jon Mound (SEE), Dr Chris Davies (SEE) Contact email:
[email protected] Convection within the Earth’s fluid core generates the planetary magnetic field; spatial and temporal variations of the geomagnetic field can thus be used to gain insight into the dynamics of this otherwise inaccessible region. Convection in the mantle controls both the rate at which heat leaves the core and the pattern of heat flow across the core-mantle boundary. Similarly, spatial variations in the rate of inner core growth will result in heterogeneous heat flux at the bottom of the fluid core. Non-axisymmetric structure in the Earth’s magnetic field, such as observed patches of anomalously strong magnetic field at high latitudes, that persist over long timescales almost certainly arises due to the influence of the heterogeneous boundary conditions imposed at the top and bottom of the fluid layer. Although considerable previous work has considered the influence on core convection of heterogeneous heat flux conditions imposed at the core-mantle boundary, relatively little work has been done investigating heterogeneous boundary conditions applied at the inner core boundary. Initial simulations of both non-magnetic convection and the geodynamo indicate that the inclusion of heterogeneous boundary conditions at the bottom of the fluid core can significantly reorganise the pattern of convection and hence impact on the structure and dynamcis of the geomagnetic field. This project will investigate both the general principles of how heterogeneous boundary conditions influence the dynamics of convection within a rotating fluid shell, and the potential of such boundary conditions to explain the non-axisymmetric components of the Earth’s magnetic field.
Figure 1: Core flow in the equatorial plane of the outer core from models of non-magnetic convection with strong heterogeneity at the core-mantle boundary (left), and inner core boundary (right). Eastward directed flow is red and westward directed flow is blue.
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This project will involve both theoretical and computational aspects of rotating convection. Simulations will be run using pre-existing code on the Leeds university supercomputer ARC2. Initial work will focus on the influence of heterogeneous heat flux boundary conditions in non-magnetic rotating convection. The PhD student will investigate scaling relations for heat transport varying typical control parameters (e.g. Rayleigh number, Ekman number, Prandtl number) and the patterns, amplitudes and relative orientations of the imposed boundary conditions. The results of the non-magnetic models will be used to direct investigation of the most significant effects in geodynamo models, with particular interest in those cases that might explain the observed non-axisymmetric structures of the Earth’s magnetic field. Project outline: Year 1: Familiarisation with geomagnetic data and field generation. Familiarisation of core dynamics and how they are simulated using the Leeds dynamo code. Benchmarking against documented simulations of boundary effects on core convection. Year 2: Non-magnetic simulations exploring a range of boundary forcing strengths and geometries at moderate control parameters. Visualisation and analysis of the dynamics of the core fluid. Year 3: Extension of selected model runs to more extreme control parameters and inclusion of magnetic field. Comparison of the simulations with existing geomagnetic and palaeomagnetic observations. Potential for high impact outcome: The Earth’s magnetic field is one of the fundamental features of the planet, and understanding its long-term behavior through observational, theoretical, and numerical investigations remains an international endeavor of wide interest. The findings of this project will be well situated to take advantage of new data sets currently under construction and may provide a new understanding of the importance of boundary control on the dynamics of the fluid core and hence the observed magnetic field. Training: The student will learn both the theory and computational techniques required to model the Earth’s core, and will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). The student will be a part of the deep Earth research group, a vibrant part of the Institute of Geophysics and Tectonics, currently comprising 8 staff members and several postdocs and PhD students. The deep Earth group has a strong portfolio of international collaborators that the student will benefit from.
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Although the project will be based at Leeds, there will be opportunities to attend international conferences (UK, Europe, US and elsewhere), and potentially collaborative visits within Europe. Related Papers
Aubert, J., Finlay, C. C., & Fournier, A. (2013). Bottom-up control of geomagnetic secular variation by the Earth’s inner core. Nature, 502(7470), 219–223. http://doi.org/10.1038/nature12574 Christensen, U. R., Aubert, J., & Hulot, G. (2010). Conditions for Earth-like geodynamo models. Earth and Planetary Science Letters, 296(3-4), 487–496. http://doi.org/10.1016/j.epsl.2010.06.009 Davies, C. J., & Constable, C. G. (2014). Insights from geodynamo simulations into longterm geomagnetic field behaviour. Earth and Planetary Science Letters, 404, 238–249. http://doi.org/10.1016/j.epsl.2014.07.042 Davies, C. J., Silva, L., & Mound, J. (2013). On the influence of a translating inner core in models of outer core convection. Physics of the Earth and Planetary Interiors, 214(C), 104– 114. http://doi.org/10.1016/j.pepi.2012.10.001 Mound, J., Davies, C., & Silva, L. (2015). Earth and Planetary Science Letters. Earth and Planetary Science Letters, 424(C), 148–157. http://doi.org/10.1016/j.epsl.2015.05.028
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Robustness of isotope signatures as proxies of formation conditions in Earth Sciences: experimental study on kinetic isotopic fractionation in carbonates Dr Thomas Mueller (SEE), Dr Jason Harvey (SEE), Dr Dan Morgan (SEE) Contact email:
[email protected] Project synopsis: Element transport in the Earth crust is one of the key parameters controlling geochemical cycles and is inherently linked to mineral reactions (Fig. 1). Carbonates are not only major constituents of the Earth crust but also play an important role in various physical, chemical and biological processes such as the effective storage of hydrocarbons (e.g., Warren, 2000 and references therein) and are therefore linked to changes in the atmosphere and biosphere. Dolomitization, for example, is one of the most important diagenetic processes but the reaction rate and timescale of dolomitization, i.e. replacement of calcite (CaCO3) by dolomite [CaMg(CO3)2] remain a topic of controversy (Jonas et al. 2015). In natural settings, carbonate replacement reactions are typcially linked to the presence of a fluid phase and may even lead to the formation of ore deposits of economic interest. A quantitative understanding of the mechanisms and rates of element and isotope incorporation into carbonates as well as isotope exchange between carbonates and fluid phases in particular, is therefore crucial to understand large scale geochemical cycles and mass transport in the Earth crust. Carbonates serve as archives that contain valuable information on mass transport and element cycles in different environments and on different scales. First, textures and compositional profiles of major and trace elements as well as stable Figure 1: Schematic representation of the carbon isotopes within carbonate grains serve cycle (after Bice, 2001). Mineral reactions involving as a proxy for the external conditions of carbonates are significant for mass transport in the crust and thus linked to processes in the formation. With this precedents in mind, we atmosphere and biosphere. A quantitative understanding of the kinetics of carbonate reactions propose to take a tested experimental is crucial to fully describe geochemical cycles. approach from our group (Jonas et al, 2015; Jonas et al. under review) to the next level and investigate C-Fe-Mg-Ca-Sr-Ba isotope diffusion and element/isotope exchange during carbonate mineral reactions to (1) quantify the relative diffusivities of different elements and isotopes, and (2) evaluate the effect of reaction kinetics on recorded isotope fractionation and compare the behaviour of both 26
major and trace elements during hydrothermal carbonate replacement processes. Quantitative and spatially resolved analysis of newly formed carbonate replacement products and reacted fluid samples resolves the time-dependent evolution of the isotopic composition as a function of the reaction progress and hence evaluates their potential and robustness as proxy recording conditions of formation. Background: Reading the element and isotope zoning record correctly allows the reconstruction of entire geological processes. For example, geothermometers such as the calcite-dolomite and the ankerite-siderite thermometer enable to estimate peak metamorphic temperatures and are based on the temperature-sensitive cation exchange between the two coexisting phases of the mineral pair (e.g., Anovitz and Essene, 1987). The spatial zoning of carbonate grains has been successfully used to reconstruct the diffusive element transport within the grains and thus allows to decipher the temporal evolution of contact metamorphic rocks (Mueller et al., 2008). Interestingly, experimentally determined diffusion data for various divalent cations in carbonates suggest that intracrystalline diffusion may play an important role even at relatively low temperatures (100-200 °C) typical for burial diagenetic conditions (Mueller et al, 2012). Hence, even low temperature processes such as dolomitization are potentially modifying element and isotope signatures recorded in the carbonates which, in turn, would largely affect the interpretation of the proxies. Similarly, magnesium isotopes in carbonate minerals were proposed as sensitive geochemical tracers for possible sources and sinks in low temperature aqueous systems (Higgins and Schrag, 2010) as well as potential tracers for seawater chemistry through the Earth’s history (Eisenhauer et al., 2009). Stable Mg-isotopes may also be a sensitive tracer of the diagenetic history of hydrothermal dolomites that form in the early diagenetic and deep burial environment (e.g., Lavoie et al., 2014; Geske et al., 2015; Li et al., 2015; Walter et al., 2015). However, due to the effects of dissolution, precipitation, diagenetic resetting and non-equilibrium fractionation processes, hydrothermal dolomites show a wide range of Mgisotopic values, and there is still a lack of experimental data on Mg-isotope fractionation behavior under reaction conditions representative of the burial diagenetic environment. In a recent study, Jonas et al. (2015) presented the first results investigating the replacement reaction of single rhombohedral calcite crystals to form Mg-carbonates in an MgCl2-solutions at 200 °C. These experiments are thus aimed to simulate dolomitization processes during burial diagenesis. Their experiments revealed that the replacement of calcite by multi-layered reaction rims of magnesite and dolomite takes place by a dissolution-precipitation mechanism. More importantly, the experimental results suggested that the limited transport of the major elements Mg and Ca through the inter-connected pore space of the reaction rim forms compositional gradients in the fluid phase that control both the rate of the overall replacement process and the chemical composition of the reaction product on a microscopic scale.
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Figure 2: BSE and schematic model of calcite replacement by layered reaction rims (Jonas et al., 2015). The replacement reaction mechanism is dissolution-precipitation, but the reaction progress is controlled by diffusion of elements in the pore fluid resulting in a compositional zoning of the reaction rim.
The transport-controlled nature of the reaction highlights that the quantification of fluidmediated transport, i.e. the determination of the effective element flux across the reaction rim, is of key relevance to understand element and isotope zoning developed during carbonate reactions. These findings were supported by a second experimental study on the replacement of biogenic and abiogenic aragonite materials (Jonas et al., under review) indicating that the permeability of the initial reactant governs the effective flux across the system and has a high influence on the reaction rate and the composition of newly developing reaction fronts. Objectives: It has been shown, that rates of carbonate reactions depend on the effective element flux and control both the phase petrology and the spatial chemical composition of the reaction product. The same experimental approach can now be used to test, whether isotope exchange is coupled to the reaction front or whether isotope exchange is independent of the reaction progress. Detailed knowledge on these parameters allows to predict the element and isotope zoning profiles developing in natural geological settings and allow thus to reconstruct the temporal evolution more accurately (see review of Müller et al., 2010 for details). We firmly believe that the next challenge to read the mineral record preserved in its spatial chemical composition is to quantitatively investigate the kinetically controlled incorporation and mass-dependent diffusion of isotope pairs in minerals.
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In this project, the student will work with experienced scientists at Leeds and possible collaborations with the Ruhr-Universität Bochum (GER) to experimentally investigate the relevant isotopes in carbonates. We aim to answer the following questions: 1. What are the mechanism and rates of element exchange and element/isotope diffusion of structure forming (Ca-Fe-Mg-Ba-C-O) and trace elements (Sr) in carbonates? 2. What is the effect of P, T, X, and fO2 on diffusion of structure and trace elements in carbonates under various conditions? 3. What is the best experimental strategy to optimize run products for analytical protocols on the nano-scale. 4. Which state-of-the-science analytics (SIMS, TIMS, LA-ICP-MS, RBS) is best suitable to extract compositional profiles in carbonates down to the nano-scale. 5. Which elements or isotopes can be used as proxies to determine the conditions during carbonate formation and which are rather useful to extract kinetic, i.e. time information of geological processes. The project provides possible interaction with the DFG funded “research priority group” CHARON working on mechanisms, rates and geological implications of carbonate proxies and alteration reactions. Experiments: The School of Earth and Environment at Leeds has all the lab facilities needed for this project and the proposed experiments will build on our previous studies on carbonate replacement (Jonas et al. 2015, Jonas et al., under review). This setup investigates mineral/rock-water interaction at diagenetic conditions using 1-atm furnaces (with or without controlled fO2) and rapid-quench cold seal apparatus. The setup has been successfully proven to allow for a temporal monitoring of the reaction progress combining analysis of the fluid and solid phases using various state-of-the-science techniques. These experiments will complement the quantitative description of the effective element flux during the carbonate replacement reactions, as the chemical gradient between the bulk solution and the fluid adjacent to the reaction interface is not only controlled by the permeability of the initial starting material, but also by the element and isotope composition of the initial reservoir fluid which will be varied within multiple experimental series. Potential for high impact outcome: Recent studies, partly driven by our group (Mueller et al. 2010, 2012, 2014; Jonas et al. 2015; Jonas et al., under review), emphasized the role of effective element fluxes on recorded compositional profiles. Likely, this will also affect isotope signatures which are typically used as environmental proxies. Consequently, reaction rates of minerals in the crust are, to a specific extent, directly linked to changes in the atmosphere and biosphere. This is in particular true for carbonates as they are a major 29
source and sink of the Earth CO2 budget. A quantitative understanding on the effect of reaction and exchange kinetics on isotope composition of carbonates is crucial to evaluate the robustness of those isotope signatures for its correct interpretation. The laboratory facilities at Leeds are extremely well equipped to carry out the proposed research, in particular due to the recent add-on of an experimental petrology laboratory. In combination with the superb analytical facilities of the low temperature geochemistry group (Cohen) and the high resolution TIMS lab, this infrastructure provided by the School of Earth and Environmental Sciences, makes the University of Leeds one of the unique places at which this project can be successfully achieved. Given the high interest of using stable isotope composition of carbonates as tracer of paleo-conditions we anticipate the project generating at least two or more papers being suitable for submission to a high impact journal with the potential of becoming 4* contributions. Training: The successful candidate will work under the supervision of Dr. Thomas Müller, Dr. Jason Harvey and Dr. Dan Morgan within the IGT research group. In addition, depending on the focus of interest of the PhD candidate the project offers and highly encourages the close collaboration with the Cohen research group being part of the ESSI within the School of Earth and Environmental Sciences. This project provides a high level of specialist scientific training in: (i) experimental techniques in diffusion studies including experiments with gasmixing furnaces, cold seal and piston cylinder apparatus and pulsed laser deposition (PLD); (ii) State-of-science analytical methods on the micro to nano-scale (SIMS, TIMS, LA-ICP-MS and RBS); (iii) numerical modelling (finite differences) in multiple programming environments. Part of the experimental and analytical work (sample preparation using PLD and RBS) will be performed under the co-supervision of Dr. Ralf Dohmen at the RuhrUniversität in Bochum. In addition, the successful PhD student will have access to a broad spectrum of training in various analytical and experimental techniques either by the Faculty or within collaborations in the UK and/or abroad. Moreover, research results will be presented at national & international meetings to integrate the student in the scientific community working in the field of reactive mass transport. References: Anovitz and Essene (1987) J.Pet., 28, 389-414. Bice (2001) J Geophys Res, 106, 11,529-11542. Eisenhauer et al. (2009) Elements, 5, 365-368. Geske et al. (2015) GCA, 149, 131-151. Higgins and Schrag (2010) GCA, 74, 5036-5063. Jonas et al. (2015) Geology, 43, 779-782. Jonas et al. (under review), submitted to GCA. Lavoie et al. (2015) Sediment Geol., 305, 58-68. Li et al. (2015) GCA, 157, 164-181. Mueller et al. (2008) AmMin, 93, 1245-1259. Mueller et al. (2010) RiMG, 72, 997-1038. Mueller et al. (2012) GCA, 84, 90-103. Mueller et al. (2014) GCA, 127, 57-66. Walter et al. (2015) Chem. Geol., 400, 87-105. Warren (2000) Earth-Sci. Rev., 52, 1-81. Keywords: kinetic isotope fractionation, mineral-water interaction, fluid-rock interaction, diffusion, reactive transport, carbonates, carbon cycle, Earth interior, geochemistry, mineralogy, modelling, experiments. 30
Using seismic and tilt measurements to forecast eruptions of silicic volcanoes Prof Jurgen Neuberg (SEE), Dr Mark Thomas (SEE) Contact email:
[email protected] Independent interpretations of seismic swarms and tilt measurement on active silicic volcanoes have been successfully used to assess their eruption potential. Swarms of lowfrequency seismic events have been associated with brittle failure or stick-slip motion of magma during ascent and have been used to estimate qualitatively the magma ascent rate which typically accelerates before lava dome collapses (Neuberg et al, 2006). Tilt signals are extremely sensitive indicators for volcano deformation and have been often modelled and interpreted as inflation or deflation of a shallow magma reservoir (Hautmann et al., 2009) This project aims to combine these two independent observations, seismicity and deformation, to design and implement a forecasting tool that can be deployed in volcano observatories on an operational level.
Figure 1 Rock-fall activity by night at Soufriere Hills volcano on Montserrat, West Indies, which showed continuing extrusion of magma closely linked to swarms of low-frequency seismicity and tilt.
Aims of the project: (i) Improving the modelling of tilt signals by taking the full stress tensor into account. So far tilt has been interpreted as caused by inflation or deflation of magma reservoirs, hence, as a volume or pressure change. These models often lead to unrealistically high pressures and fail to explain the observations (Fig 2). An alternative interpretation where tilt is caused by shear stress across the conduit wall during magma ascent acting as an ascent rate indicator, can explain the observations easily (Green et al, 2006). The amount of tilt caused by the ascending magma is controlled by its viscosity.
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Figure 2: Tilt on Tungurahua,Ecuador, drops significantly before an explosion. This cannot be explained by pressurization of a magma reservoir but by the decrease of shear stress as the ascending magma column comes to a grinding hold, pressurizes and explodes.
(ii) Quantifying magma ascent rates through seismic low-frequency swarms. Models linking magma ascent and seismicity have remained mainly conceptual, but latest laboratory experiments of magma properties, their shear strength and rheological properties allow an improved estimate of processes leading to the generation of seismicity during magma ascent. Advances in seismic moment tensor analysis together with better understanding of wave propagation of seismic low-frequency events enable us now to calibrate and quantify magma ascent rates from seismic records.
(iii) Combining seismicity and tilt. While the observation of seismicity and tilt are independent, their generating processes are not. Shear stress across the conduit wall can either lead to deformation of the edifice manifesting itself through tilt or, exceeding a certain threshold, can lead to the generation of seismicity, reducing the remaining shear stress that caused the tilt (Fig 3, Thomas & Neuberg, 2012). This interference between processes in the conduit leading to both seismicity and deformation will be exploited to design a technique to estimate whether magma ascent rates reach a critical value which can lead to eruptions. 32
Training: According to background and specific research interests, the student will be provided with training in analytical and numerical modelling techniques, applied to seismic wavefields and pressure variations in magma-gas mixtures, and will use and further develop tools in volcano seismic analysis, numerical magma flow modelling, and deformation models. Volcanic monitoring experience will be gained at Soufriere Hills volcano, on the Caribbean Island of Montserrat at the volcano observatory (MVO) as well as on Tungurahua and Cotopaxi with the Geophysical Institute (IG) in Quito, Ecuador (IG). We have maintained with both institutions very good links over many years and have an existing Memorandum of Understanding controlling data exchange and co-operation. Visits to both institutions will be necessary to implement forecasting tools at these observatories in co-operation with observatory staff. The student will be supervised by Prof Jurgen Neuberg and Dr Mark Thomas, and will be part of a colourful and multi-disciplinary group of scientists in the UK and abroad, due to the multi-national co-operation and research contacts of the Volcano Study Group at Leeds.
Figure 3: Tilt cycle (solid line) and seismicity (dots) on Montserrat, West Indies. The tilt signal goes through an inflection point (maximum/minimum of tilt derivative w.r.t. time, dotted line) as soon as seismicity starts, and again when seismicity ceases, indicating that part of the shear stress that causes the tilt is reduced by seismic slip at the conduit wall during magma ascent.
Impact of research: Safely living with active volcanoes has become a pressing issue for millions of people and scientist need to be able not only to predict a first onset of an eruption but also to identify the slightest change during volcanic activity in order to advise local authorities and civil defence agencies. The outcomes of this project will provide a major contribution to this aim, and through direct contact with relevant volcano observatories will develop a tool that can be used on an operational level where it is most needed.
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References:
Hautmann, S., Gottsmann, J., Sparks, R.S.J., Costa, A., Melnik, O., and Voight,B., 2009, Modelling ground deformation caused by oscillating overpressure in a dyke conduit at Soufriere Hills Volcano, Montserrat: Tectonophysics, v. 471, p. 87–95, doi:10.1016/j.tecto.2008.10.021. Green, D.N., Neuberg, J., and Cayol, V., 2006, Shear stress along the conduitwall as a plausible source of tilt at Soufriere Hills Volcano, Montserrat: Geophysical Research Letters, v. 33, L10306, doi:10.1029/2006GL025890. Neuberg, J. W., H. Tuffen, L. Collier, D. Green, T. Powell, and D. Dingwell (2006), The trigger mechanism of low-frequency earthquakes on Montserrat, J. Volcanol. Geotherm. Res., 153, 37–50, doi:10.1016/j.jvolgeores.2005.08.008. Thomas, M. E., and J. W. Neuberg (2012), What makes a volcano tick—A first explanation of deep multiple seismic sources in ascending magma, Geology, 40, 351– 354, doi:10.1130/G32868.1.
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Detecting microearthquakes in fault zones and geothermal fields—automatically Dr Andy Nowacki (SEE), Dr Doug Angus (SEE), Dr Sebastian Rost (SEE), Prof J-Michael Kendall (University of Bristol) Contact email:
[email protected] Overview Small earthquakes reveal a huge amount of information about current tectonic, hydrothermal and magmatic processes in many places on Earth—for instance, how continent-scale faulting occurs, where to drill for hot geothermal fluids, and how easily a hydrocarbon reservoir can be exploited. They show where stress is being released now and hence give an image of real-time faulting and fluid movement. All analysis of the results of seismic methods such as tomography and moment tensor inversion rely fundamentally on the initial detection and location of seismic events, usually done manually by seismic analysts. However, with an ever-increasing volume of seismic data, this approach cannot scale. This PhD project will build on and develop so-called ‘reverse time migration’ methods to automatically detect and locate seismic events, which sometimes are invisible in individual seismograms. Using current algorithms and developing new ones, you will ground-truth these methods against existing microearthquake catalogues in two initial settings: the North Anatolian Fault, Turkey (Poyraz et al., 2015), and the Aluto-Langano geothermal field in Ethiopia (Samrock et al., 2015). You will then find previously undetected low magnitude earthquakes, whose patterns can image fault structures and follow dynamics processes in the subsurface without detection bias. This more complete earthquake catalogue will improve our understanding of the tectonic regime of the faults and better characterise the geothermal fields by showing where energy is being released. Reverse time migration (RTM) constructs the predicted seismic arrivals for all possible combinations of event location and time—and can determine the focal mechanism of the earthquake—by combining the seismic data to test if an event matches this point in spacetime. Its advantages over manual inspection of seismograms are objectivity, speed and detection threshold. RTM has been successfully applied in both controlled settings like oil reservoirs (Chambers et al., 2014; Price et al., 2015), and active tectonic zones where magma is ascending (Drew et al., 2013; Langet et al., 2014). However, it is still a relatively immature discipline in regional tectonics. Applying this method to settings in Turkey and Ethiopia will deliver new insights into intra-continental transform and rift-extensional faulting from the patterns of seismicity we will uncover. A numerate physical scientist with a background in physics, Earth sciences or a related field would be suited to the project. The student will be an integral part of a school-wide group of researchers studying processes in the Earth from the core-mantle boundary to the near surface. They will receive specialist support from seismologists and wider input from
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geologists, geodynamicists and others in the School. Engagement and interaction with Turkish and Ethiopian scientists, as well the UK community, will be part of the project.
Figure 1: The two preliminary settings of the North Anatolian Fault (a,b) and the East African Rift (c). (a) Location of the seismic network. (b) Location of earthquakes. From Poyraz et al. (2015). (b) Location of the Aluto–Langano geothermal field (red triangle) in the Main Ethiopian Rift, with previously-located local event (blue circles).
Objectives With leading scientists in Leeds and Bristol, the student will work to increase our understanding of tectonics in fault zones of global scale and significance. According to their interests, they will: 1. Apply an RTM code to an existing seismic dataset to automatically detect and locate earthquakes, comparing these events with those identified in a hand-picked event catalogue to test the method. Test the method using state-of-the art 3D, finitefrequency synthetic seismograms for models of complex geology and topography such as exist on the Earth. 2. Examine the patterns of seismicity along the North Anatolian Fault and the East African Rift and interpret the tectonic and hydrothermal processes, incorporating other (e.g., magnetotelluric, geodetic, geological) data. 3. Extend or adjust the RTM method to generalise it to a wide range of local-scale seismic projects, potentially by incorporating more accurate physics (such as permitting general anisotropy or moving beyond ray theory), or improving the efficiency of the algorithm. 4. Release an open-source package for local RTM for the benefit of the global monitoring and academic community. Such a code would be the first to be open for the public’s benefit.
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Figure 2: The event migration method as employed by Chambers et al. (2014). The lower left volume shows the image stack power, with the individual moment tensor components’ intensity shown in the six volumes, upper right.
Potential for high impact outcome The automatic detection and location of small (and large) earthquakes is vital for many purposes, not least the monitoring of fault zones and volcanoes worldwide for seismic hazard, as well as hydraulically-stimulated wells and other sites of human intervention for integrity and safety. Routine, automatic event detection is also important to make quantitative use of the huge amount of data now available for research purposes. This project will be of wide importance for these purposes, potentially in the case of monitoring and regulation of natural and human activities. The creation of an open-source product based on this project is expected to be of great benefit to the global monitoring, academic and industrial communities, as current approaches are often limited by software availability and a reliance on simple travel time methods. Results will be published in high-impact journals. Training The student will work under the supervision of Drs Nowacki, Angus and Rost in Leeds, and with Prof J-Michael Kendall at the University of Bristol. They will receive a high level of specialist training in the processing of seismic data, development of processing codes, and interpretation of microseismic locations. They will be based in the Institutes for Geophysics and Tectonics, and Applied Geoscience, at the School of Earth and Environment, University of Leeds, which has an international reputation for the understanding of tectonics, volcanology, and applied and theoretical geophysics. However cooperation and collaboration with the School of Earth Sciences in Bristol will be important, opening up exposure to the additional expertise and datasets there. The student will also benefit from faculty- and university-wide training scheme via the EME Hub.
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References
Chambers, K, Dando, BDE, Jones, GA, Velasco, R, and Wilson, SA (2014). Moment tensor migration imaging. Geophysical Prospecting, 62, 879–896. doi:10.1111/1365-2478.12108 Drew, J, White, RS, Tilmann, F and Tarasewicz, J (2013). Coalescence microseismic mapping. Geophysical Journal International, 195, 1773–1785. doi:10.1093/gji/ggt331 Langet, N, Maggi, A, Michelini, A and Brenguier, F (2014). Continuous kurtosis-based migration for seismic event detection and location, with application to Piton de la Fournaise volcano, La Réunion. Bulletin of the Seismological Society of America, 104, 229– 246. doi:10.1785/0120130107 Poyraz, SA, Teoman, MU, Türkelli, N, Kahraman, M, Combaz, D, Mutlu, A, Rost, S, Houseman, GA, Thompson, DA, Cornwell, D, Utkucu, M and Gülen, L (2015). New constraints on micro-seismicity and stress state in the western part of the North Anatolian Fault Zone: Observations from a dense seismic array. Tectonophysics, 656, 190–201. doi:10.1016/j.tecto.2015.06.022 Price, D, Angus, DAC, Chambers, K, Jones, G (2015). Surface microseismic imaging in the presence of high-velocity lithological layers. Geophysics, 80, WC117–WC131. doi:10.1190/geo2015-0242.1 Samrock, F, Kuvshinov, A, Bakker, J, Jackson, A, and Fisseha, S (2015). 3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia. Geophysical Journal International, 202, 1923–1948. doi:10.1093/gji/ggv270
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Slab Graveyards and Plume Generation Zones: Mixing and melting in the Earth's lowermost mantle Dr Sebastian Rost (SEE), Dr Andy Nowacki (SEE) Contact email:
[email protected] The Earth's mantle is heterogeneous on multiple scales ranging from 10s of kilometres to thousands of kilometres as is evidenced in seismological and geochemical data [1]. Heterogeneity is continuously generated at mid-oceanic ridges when oceanic crust is extracted from the mantle leaving depleted material behind [2]. Convection and subduction reintroduce this heterogeneity back into the mantle (Figure 1). On the other hand, the Earth also shows large-scale heterogeneity that might represent remnants from processes such as the crystallization of the basal magma ocean or remaining primordial material. These heterogeneities are often interpreted to be large scale thermo-chemical piles (now often called Large-Low Shear Velocity Provinces - LLSVPs) located at the core-mantle boundary beneath Africa and the Pacific. Nonetheless, the origin of LLSVPs is unknown and models invoking a purely thermal origin or the sedimentation of subducted crustal material as possible processes to produce LLSVPs are also discussed. The interaction of the small-scale heterogeneity delivered to the deep mantle through subduction and the large-scale LLSVPs is the topic of this PGR project and will provide essential information on the origin and evolution of LLSVPs. While the large-scale heterogeneities of the LLSVPs can be easily imaged using global tomographic methods, the heterogeneities introduced into the mantle through the subduction process are well below the resolution of these methods. But recent studies show that the scattered seismic wavefield contains information about these heterogeneities which show scale-lengths on the order of the seismic wavelength, i.e. on the order of 10-km for the 1Hz teleseismic wavefield in good agreement with the expected scales of subducted crust. We expect that the cumulative effect of 4.5 Ga of mantle convection has left a distribution of heterogeneities that allows us to set constraints on mixing processes in the mantle and constrain if subducted crustal material might account for the generation of LLSVPs. Of particular interest is not just the distribution of heterogeneities along the coremantle boundary, but also their distribution throughout the mantle and their possible pathways to LLSVPs and their potential composition which can be derived from the inferred seismic velocities.
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Figure 1: Cartoon of the seismically detected structures of the Earth’s interior. We observe smallscale structure related to melting processes at plume roots especially at the edges of the large-scale thermo-chemical piles beneath Africa and the Pacific and the recycling processes connected to subduction. This project will contribute to our understanding of the connection between these processes. Image courtesy of Ed Garnero (Arizona State University).
This project will use both seismic data recorded at globally distributed arrays and stations, but will also look into improved methods to model the scattered seismic wavefield to better understand the imprint of the small-scale heterogeneities on the seismic wavefield. In particular, this project will use scattered energy related to the core-reflected phase PcP (Figure 2) which has so far rarely been used to study the Earth's small-scale structure [3]. The interaction of the PcP wavefield with heterogeneities in the lowermost mantle produces distinctive precursors arriving at the station earlier than the core reflection PcP (Figure 2). Using PcP scattered energy and common-mid point stacking approaches, common in the processing of active source data, will allow us to separate the deeper heterogeneities close to the CMB reflection point from heterogeneities in crust and lithosphere near source or receiver. Adaptive optics approaches, a method to improve the performance of the seismic system by removing wavefront distortions from heterogeneities close to the receiver, might also be applicable. Additionally, we will remove the impact of heterogeneities located in the mid-mantle along the PcP path using information extracted from scattering related to the phase PP [4], which will allow further characterization of the deep heterogeneities. Modelling of the interaction of the seismic wavefield with the small-scale heterogeneities will be important in understanding the structures that are being imaged. We will use hybrid wave-propagation methods combining 3D seismic wave propagation through homogeneous axi-symmetric velocity models with high-frequency 3D wave-propagation on regional scales through a heterogeneous medium. These codes are currently under development at the University of Oxford and we will collaborate closely with the group of Dr Tarje NissenMeyer. Using this hybrid approach will allow computational seismic wave propagation in 3D
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media with frequencies relevant for seismic scattering enhancing our imaging of the smallscale structures. Figure 2: Seismic wavefield simulation of seismic energy propagated through a 1D heterogeneous structure (with small-scale heterogeneities in the D” region). Grey-scale indicates amplitude of the seismic wavefield Synthetics are calculated using a Monte-Carlo phonon scattering approach [2]. Insert shows scattered energy related to the interaction of PcP with the heterogeneities close to the CMB arriving as precursors to the main phase. Lithospheric scattering produces most of the coda energy visible after the main arrivals and can be removed using adaptive optics approaches. Traveltimes of main arrivals calculated for a 1D Earth model (IASP91) are shown as red lines.
The combination of data and synthetic waveforms will provide unique insight into the smallscale structure of the deep Earth. The global image of the distribution of small-scale heterogeneities in the Earth's lowermost mantle that will be the result of this research project will provide important information on flow and mixing processes in the mantle. The characteristics of the heterogeneities and their distribution in relation to large-scale lower mantle features will allow further insight into the structure and evolution of the Earth's deep interior and will provide constraints on the origin of the LLSVPs in the lowermost mantle and the connection of subducted crust to mantle plumes. Objectives: In this project you will work with leading specialists in the areas of seismic wave propagation and seismic scattering to gain better insight into the distribution, origin and composition of small-scale recycling products in the Earth’s lowermost mantle. Depending on your research interests the studentship could involve
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1. Seismic data analysis of scattered energy related to the core reflected phase PcP and comparison with suggested 1D Earth models of small-scale heterogeneity. 2. Combination of PcP results with other seismic probes for Earth’s small-scale heterogeneities such as PKP, Pdiff and PKKP. 3. Application of an adaptive optics approach to remove the scattered energy from the lithosphere and the crust at source and receiver that is currently under development at the University of Leeds. 4. Seismic wave propagation through 1D and 3D heterogeneous structures on global scales using methods developed at the University of Leeds and Oxford University. 5. Inclusion of the structures derived from thermo-chemical convection models into the seismic wave propagation simulations Training: You will work under the supervision of Dr Rost and Dr Nowacki in the Deep Earth Research Group within the Institute for Geophysics and Tectonics. The Deep Earth Research Group in Leeds is one of the largest research groups interested in the structure of the deep Earth in the world and consists of researchers within seismology, mineral-physics, geodynamics and geomagnetism. The project provides a high-level of specialist scientific training in: (i) seismic time series analysis using massive seismic datasets collected from international data centres and temporary arrays and networks, (ii) 1D and 3D seismic wave propagation techniques for heterogeneous media, (iii) combination of the results of thermochemical convection models with novel hybrid seismic wave propagation techniques. . The successful PGR will have access to a broad spectrum of training workshops provided by the Faculty (http://www.emeskillstraining.leeds.ac.uk/) and the DTP (http://www.nercdtp.leeds.ac.uk/research-training/) that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva. References
Van Keken, P.E., Hauri, E.H., Ballentine, C.J., 2002. Mantle Mixing: The Generation, Preservation, and Destruction of Chemical Heterogeneity. Annu. Rev. Earth. Planet. Sci. 30, 493–525. doi:10.1146/annurev.earth.30.091201.141236 Shearer, P.M., 2007. Deep Earth Structure - Seismic Scattering in the Deep Earth, in: Treatise on Geophysics. Elsevier, Amsterdam, pp. 695–729. Braña, L., Helffrich, G., 2004. A scattering region near the core-mantle boundary under the North Atlantic. Geophys. J. Int. 158, 625–636. Bentham, H.L.M., Rost, S., 2014. Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle. Geophys. J. Int. 197, 1627– 1641. doi:10.1093/gji/ggu043
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Petrological & geochemical insights into subduction initiation- the case of Izu-Bonin-Mariana volcanic arc Dr. Ivan Savov (SEE) & Dr. Jason Harvey (SEE) Contact email:
[email protected] Convergent margins mark sites of plate destruction and are unique to Earth among the terrestrial planets. However, currently we lack clear understanding of why and how they are initiated. Among a number of hypotheses that have been proposed, the so-called “spontaneous subduction initiation” model appears particularly relevant (Stern, 2004; Arculus et al, 2015) to the initiation of one of the largest, nominally intra-oceanic subduction zones in the W. Pacific-the Izu-Bonin-Mariana (IBM) system. Spontaneous subduction initiation occurs when a change in plate motion allows the gravitationally unstable lithosphere to founder along an existing plate boundary. The IBM system (Fig. 1A) represents an example of arc initiation wherein subsidence of relatively old Pacific lithosphere began along a system of transform faults/fracture zones adjacent to relatively buoyant lithosphere. The initial record on the overriding plate during the spontaneous subduction commences with rifting, spreading, and formation of magmas such as highly depleted, low-K tholeiites and boninites (Hickey-Vargas et al., 2007; Ishizuka et al., 2011; Arculus et al., 2015).
Site IBM1
Decreasing importance of slab/sedime nt fluxes?
Site 120 1
Figure 1A (left): Regional geology of the IBM arc-basin system south of Japan. Currently active volcanic arc front (thick green solid line), back arc basin rifts (thick black solid lines) and remnant (early) arcs (thick green stippled lines) are shown. Filled circles are ODP drill sites, and crosses are DSDP drill sites. The sites from which fallout tephra is recovered are highlighted in blue. Also shown is the location of the proposed Site IBM-1 (red circle; this study). Figure 1B (right): 143Nd/144Nd trends for the IBM volcanics. The trend towards higher 143Nd/144Nd at the Izu Bonin VF (and maybe in the Mariana VF) is interpreted as reflecting variations of the mantle fertilization due to changes in the slab/sediment fluxes over time [Straub et al., 2004, 2009]. Note that for both arcs the early (Paleogene) records are almost unknown, although the limited samples available (Site 1201; Savov et al., 2006-see green and blue fields) show truly fascinating insights into the temporal evolution of the
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subarc mantle feeding arc volcanism. Data sources after Schmidt [2001]; Straub [2003, 2004]; Savov et al., [2006].
Following subduction initiation, further igneous development is fundamental to the creation of arc volcanic chains, which in turn is essential to the formation and evolution of continental crust. Testing models of subduction initiation and subsequent arc evolution requires identification and exploration of regions adjacent to an arc, where pre-arc crust (basement) is directly overlain by undisturbed arc-derived materials. The IBM system is globally important because we have clear evidence for the age (~52 Ma; Ishizuka et al., 2011) and exact site of inception, (Kyushu-Palau Ridge; Fig 1A), the duration of arc activity, and changes in magmatic composition and volumes through time through extensive drilling (ash & pyroclast records) and dredging (Fig. 1A). Dr. I.Savov sailed as shipboard scientist on IODP Exp. 351, which successfully drilled site IBM-1 at the Amami Sankaku Basin (ASB), just west of the KPR proto-arc (Fig. 1A). The Site penetrated the pre-arc IBM basement (Early Eocene) and the overlying 1400m of volcaniclastic sediments. Studying the sedimentary cover is crucial to decipher the geochemical and petrological evolution of the post-arc inception volcanism in the entire IBM region. The primary target of this PhD studentship is to study the arc- derived sediments directly overlaying the basement at Site IBM-1. In analogy to the previously drilled Site 1201 (where Dr. Savov also sailed as shipboard scientists and studied extensively- see Savov et al., 2006; Hickey-Vargas et al., 2007), these sediments should preserve the explosive ash and pyroclastic fragmental records for at least the first 25 M.yrs. of the developing nearby volcanic arc (KPR). To date, the geochemical data available for such materials recovered from the fore-arc regions of the IBM system (Site 782, see also Fig. 1A) are concentrated in the Neogene (e.g., Straub, 2003, 2004) and only limited Paleogene record of volcaniclastics and ash material that is well-dated AND appropriate for geochemical study is available from any of the existing DSDP/ODP Sites (incl. 291, 292, 294, 446-447 & 1201; Savov et al., 2006; Hickey-Vargas et al., 2007; Ishizuka et al., 2011; see Fig. 1A&B). Increasing the resolution of the post-IBM arc inception records will allow geochemical modelling aiding the quantification of the volumes of sediment, crust and mantle involved in the early arc volcanism. In detail, the PhD student will construct geochemical profiles across the entire Cenozoic, with emphasis on the Paleogene volcaniclastics. The student will analyse the volcanic ash and the most unaltered volcaniclasts for Sr, Nd, Pb and B isotopes in the TIMS Lab at the Univ. Leeds (http://www.see.leeds.ac.uk/business-and-consultation/facilities/geochronologyradiogenic-isotopes-laboratories/). These isotope systems, in addition to major and trace element variations, have been shown to be a powerful tool for resolving provenance (Pb, Sr), as well as uniquely trace the sedimentary and/or crustal (slab) and mantle contributions (Sr, B and Nd) to arcs (Straub, 2004; Hickey-Vargas et al, 2007 and ref. therein; see also Fig. 1B). Using a combination of elemental and isotope tracers will allow the PhD student to uniquely quantify the volumes of fluids/melts involved in the sources of the earliest volcanic eruptions that took place at the IBM (see Savov et al., 2006 and Hickey-Vargas et al., 2007). This task is critical for the subduction initiation modelling and also for better understanding of the causes of the modern along-IBM major element and isotope variations (Stern, 2004; Ishizuka et al., 2011). After vigorous examination under polarized microscope and with the help of the state-of-the-art SEM and EPMA Facility at Univ. Leeds, the student will be able to 44
identify up to 20 unaltered volcaniclasts and ash samples that are appropriate for Ar-Ar dating. In collaboration with the co-chief scientist O. Ishizuka (who manage Ar-Ar Lab at AIST, Japan) splits of these same samples will be dated. This, in addition to the ages derived from the nanofossil record (shipboard data), will allowthe student to constrain very precisely the temporal intervals that the selected IBM-1 sediments represent. Once this is known, the student will connect the anticipated results with the existing (mostly Neogene) ash and volcaniclasts datasets from other parts of the IBM (see Fig 1B; Bryant et al., 1999; Straub, 2003; Straub et al., 2004) and other W. Pacific sites (Savov et al., 2006; HickeyVargas et al., 2007, Ishizuka et al., 2011). Together, the combined age and isotope results are expected to be a major step toward the more complete understanding of the temporal chemical and petrological evolution of the arc volcanism after subduction initiation, which is an issue remaining largely unknown. Potential for high impact outcome: A recently published Nature Geoscience paper involving Dr.Savov (see Arculus et al., 2015) reported only the limited shipboard major and minor element geochemical data from Exp. 351. Building an insights from 3 additional radiogenic (Sr-Nd and Pb) and one novel stable isotope system (Boron) and their power to uniquely decipher high temperature processes such as magma sources and volumes, is an excellent sign that the contributions from this studentship will be able to deliver a publication in high ranked (4 star) paper. The processes of subduction zone initiation are identified as Challenge 11 of the Science Plan of IODP (www.iodp.org/ science-plan-for-2013-2023). As IODP is an international scientific effort and major investment (multi-million £) and since IBM has been repeatedly selected as a major target to understand subduction processess, the proposed studentship will surely be on the spotlight for new and unique insights derived from carefully selected tracers, which overall will be able to contribute to a major impact publication in the field of petrology, geochemistry and volcanology. Selected References:
Arculus et al., 2015; A record of spontaneous subduction initiation in the Izu–Bonin– Mariana arc, Nature Geoscience 8 (9), 728-733. Bryant et al., 1999. Laser ablation–inductively coupled plasma–mass spectrometry and tephras: a new approach to understanding arc-magma genesis. Geology, 27(12):1119–1122. Hickey-Vargas et al., 2007. Origin of diverse geochemical signatures in igneous rocks from the West Philippine Basin, AGU Geophysical Monograph Series, vol. 166, 287303. Ishizuka et al., 2011. Making and breaking an island arc: a new perspective from the Oligocene Kyushu-Palau arc, Philippine Sea. G-cubed, 12(5):Q05005. Savov et al., 2006. Petrology and geochemistry of West Philippine Basin basalts and early Palau-Kyushu arc volcanic clasts from ODP Leg 195, Site 1201D: Implications for the early history of the Izu- Bonin- Mariana arc, J. Petrology, 47(2): 277-299.
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Straub et al., 2004. Volcanic glasses at the Izu arc volcanic front: new perspectives on fluid and sediment melt recycling in subduction zones. G-cubed 5(1):Q01007.
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Quantifying Earth’s deep water cycle by combining seismology and mineral physics Dr Andrew Walker (SEE), Dr Andy Nowacki (SEE) and Prof Greg Houseman (SEE). Contact email:
[email protected] Earth’s hydrological cycle is not restricted to the atmosphere, oceans and surface, but involves the exchange of water between the surface and deep interior. Water reaches the surface environment during the volcanism that forms the oceanic and continental crust and is returned by subduction of old plates with water locked up inside hydrous minerals (e.g. Faccenda, 2014a). This deep segment of the water cycle is believed to be responsible for long-term variation in the volume of water at the Earth’s surface (e.g. Nishi, 2015) and may be responsible for the dramatic differences in the global scale tectonics of Earth and Venus, the two most similar planets in the solar system (Kaula, 1990, 1995). Although the deep water cycle is widely acknowledged to be critical to our understanding of the planet, measurement of the water flux between the surface environment and the deep interior, as well as quantification of the size of the deep reservoir, has proved elusive. In this project you will make use of new advances in seismology, mineral physics and geodynamics to quantify the downward flux of water carried deep into the Earth’s interior. The key advance will be to explore how water-bearing silicate minerals deform in subducting slabs at great depths. Whilst at relatively shallow depths, dehydration reactions lead to recycling of water and the formation of subduction-related volcanic arcs, it is believed some water is held in hydrous minerals into at least the mantle transition zone (>410 km). The deformation of hydrous minerals at this depth causes the slab to become seismically anisotropic and this can be measured by analysing the seismic signals recorded after deep earthquakes (Nowacki et al. 2015; Figure 1). The aim of this project is to measure and model the origin of these signals in order to constrain the present day flux of water into the deep mantle.
Figure 1: Global analysis of shear-wave splitting indicating that dense hydrous magnesium silicates are present in subduction zones (modified from Nowacki et al. 2015). a: core-traversing SKS phases are used to correct for anisotropy close to the seismic receiver (inverted triangle) and isolate the anisotropic signal near the source (orange circle). b: results are projected into a slab frame to allow comparison between different subduction zones. c: in common frame of reference source-side shear wave splitting observations are consistent with that expected from the deformation of the hydrous phase-D by down-dip compression. This suggests that water is carried deep into the Earth’s interior, and quantification of this is the key aim of the project.
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The project involves a wide range of different aspects of computational geoscience applied to the study of the Earth’s interior. In particular, you will need to model deformation of the dense hydrous magnesium silicate “alphabet phases” (e.g. Nishi et al. 2014) at the atomic scale using density functional theory, and apply results from these calculations to multiscale models of mantle deformation. Results from this study will, in turn, be used to study the development of seismic anisotropy in an around subducting slabs at depths beyond 250 km. By comparing these models with measurements of source-side shear wave splitting from broadband seismic observations of deep earthquakes, you will identify a range of plausible models of deformation style and composition of the subducting slab and this will allow an estimate of the volume of dense hydrous magnesium silicate and thus water being carried into the deep interior. This estimate will inform our understanding of the fluxes involved in Earth’s deep water cycle and provide basic constraints on the chemical makeup of the Earth.
Figure 2: Atomic scale model used to simulate the deformation of phase-D, one of the dense hydrous magnesium silicates to be studied as part of this project. The crystal structure, comprising sheets of silicate octahedra (silicon: blue; oxygen: red) separated by magnesium (yellow) and hydrogen (pink), is sheared along the horizontal red line. The excess energy associated with this distortion to the crystal structure allows larger scale models of polycrystal deformation to be constructed and used to predict seismic anisotropy.
Objectives: In this project, you will work with leading researchers at Leeds with backgrounds in mineral physics, seismology and geodynamics. For the first year you will be expected to engage in all three areas to create an initial model to begin to quantify the flux of water into the Earth’s deep mantle. Example of this kind of modelling applied to the lowermost mantle are given by Walker et al. (2011) and Nowacki et al. (2013). Further work will depend on your particular research interests, but may include: 1) Detailed modelling of how a range of dense hydrous magnesium silicates deform at the atomic scale in order to improve models of the deformation of the generation of 48
anisotropy in subducting slabs. See Walker et al. (2010) for a review of these methods. 2) Coupling simulations of the deformation of slabs with polycrystalline models of the deformation of dense hydrous magnesium silicates in order to study how details of the slab geometry alters the pattern of anisotropy. This would extend the type of analysis undertaken by Faccenda (2014b), to include slab hydration. 3) Simulation of seismic wave propagation using finite frequency full waveform techniques in order to extract improved estimates of the water flux from observations of shear-wave splitting. 4) Measurement of shear-wave splitting from global seismic data sets in order to broaden our observational database and better constrain the water flux into the deep mantle. Training: Beyond the essential transferable skills in research, communication, team-working and project management you will develop by engaging in graduate-level research as part of the “Spheres” Doctoral Training Programme, you will gain key skills in a wide range of computational modelling techniques with broad applications. For example, you will develop a high level of expertise in scientific programming and data analysis, you will learn to make use of national-scale high performance computing facilities, and will master a wide selection of modelling tools with applications in materials science and seismology. As a PhD student in the interdisciplinary deep Earth research group in the Institute of Geophysics and Tectonics, you will benefit from working with a large and active group of scientists tackling a wide range of problems. This project will run alongside the £8M NERC thematic programme Volatiles, Geodynamics & Solid Earth Controls on the Habitable Planet (http://www.deepvolatiles.org/) and you will be able to benefit from working alongside this large group of scientists, including a dedicated cohort of PhD students and postdoctoral researchers, who are based across the UK. References and further reading
Faccenda, M. (2014a) “Water in the slab: a trilogy”, Tectonophysics, 614, pp. 1 – 30. http://dx.doi.org/10.1016/j.tecto.2013.12.020 Faccenda, M. (2014b) “Mid mantle seismic anisotropy around subduction zones”, Physics of the Earth and Planetary Interiors, 227, pp. 1-19. http://dx.doi.org/10.1016/j.pepi.2013.11.015 Kaula, W. M. (1990) “Venus: A contrast in evolution to Earth”, Science, 247, pp. 1191 – 1196. http://dx.doi.org/10.1126/science.247.4947.1191 Kaula, W. M. (1995) “Venus reconsidered”, Science, 270, pp. 1460 – 1464. http://dx.doi.org/10.1126/science.270.5241.1460 Nishi, M., T. Irifune, J. Tsuchiya, Y. Tange, Y. Nishihara, K. Fujino and Y. Higo (2014) “Stability of hydrous silicate at high pressures and water transport to the deep lower mantle”, Nature Geosciences, 7, pp. 224 - 227. http://dx.doi.org/10.1038/ngeo2074 Nishi, M. (2015) “Deep water cycle: Mantle hydration”, Nature Geosciences, 8, pp. 9-10. http://dx.doi.org/10.1038/ngeo2326
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Nowacki, A., A. M. Walker, J. Wookey and J.-M. Kendall (2013) “Evaluating post-perovskite as a cause of D″ anisotropy in regions of palaeosubduction” Geophysical Journal International, 192, pp. 1085 – 1090. http://dx.doi.org/10.1093/gji/ggs068 Nowacki, A., J.-M. Kendall, J. Wookey and A. Pemberton (2015) “Mid-mantle anisotropy in subduction zones and deep water transport”, Geochemistry, Geophysics, Geosystems, 16, pp. 1 – 21. http://dx.doi.org/10.1002/2014GC005667 Walker, A. M., P. Carrez and P. Cordier (2010) “Atomic scale models of dislocation cores in minerals: progress and prospects” Mineralogical Magazine, 74, pp. 381 – 413. http://dx.doi.org/10.1180/minmag.2010.074.3.381 Walker, A. M., A. M. Forte, J. Wookey, A. Nowacki and J.-M. Kendall (2011) "Elastic anisotropy of D″ predicted from global models of mantle flow" Geochemistry, Geophysics, Geosystems, 12, art.no. Q10006. http://dx.doi.org/10.1029/2011GC003732
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What controls the magmatic plumbing systems of spreading centres in Afar? [Fully Funded NERC studentship] Supervisors: Professor Tim Wright (SEE), Professor Andy Hooper (SEE), Dr Juliet Biggs (Bristol) Contact email: t.j.wright(at)leeds.ac.uk Funding is provided by the UK Natural Environment Research Council (NERC) and is subject to standard eligibility regulations; normally UK nationals are eligible for a full scholarship, whilst EU candidates are awarded a Fees Only scholarship, although there are some exceptions. Please check with the administrator. Proposed start date October 2016. Background: Simple thermal models developed at mid-ocean ridges suggest that the depth of magma chambers at spreading centres is controlled primarily by spreading velocity and rates of magma supply (e.g. Phipps Morgan and Chen, 1993). In such models, isolated, deep magma chambers underneath central volcanoes are found at slow-spreading ridges such as the Mid-Atlantic Ridge, and elongated, shallow axial magma chambers are only found at fast-spreading ridges such as the East Pacific Rise. However, recent observations from the subaerial spreading centre in Afar (Pagli et al, 2012) of a shallow axial magmatic system under the slow-spreading Erta Ale magmatic segment suggest that existing models may need revision. This project will use existing and new InSAR observations of surface motions at volcanoes in Afar and elsewhere to test and constrain current models of magmatic systems at spreading centres. The studentship forms part of the NERC RiftVolc project, which aims to improve the understanding of rift volcanism past, present and future, largely focusing on the central Main Ethiopian Rift (MER). This region is of particular significance as it displays all the main features of rift volcanism, the volcanoes are a significant threat to the local population, and the potential for geothermal exploitation is enormous. This project will expand the existing reach of the RiftVolc project to the Afar region, which was the focus of the Afar Rift Consortium project, providing new constraints on magmatic systems that can be compared to those made in the MER. It would suit a numerate physical scientist with a background in Earth sciences, physics, or similar fields. They will be expected to interact with the RiftVolc project team, providing opportunities for fieldwork in both the MER and Afar regions. Objectives: The student will work with leading scientists at both Leeds and Bristol to improve understanding of deformation at Afar volcanic centres. In particular, they will:
make new, systematic observations of deformation (e.g. Wright et al., 2012) using the full 20 year archive of SAR data from ERS and Envisat, and new data acquired by Sentinel-1.
determine the depths and geometries of magmatic systems using elastic models, along with complementary constraints acquired during the Afar Rift Consortium project from GPS, seismicity, petrology, and magnetotellurics (e.g. Field et al., 2012).
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examine how the depth of the magmatic systems varies as a function of spreading velocity and rates of magmatic supply, and compare the observations with those made on mid-ocean ridges, and elsewhere on the East African Rift system.
use the data to test simple thermal models of magmatic systems at spreading centres.
Potential for high impact outcomes: The volcanoes of Afar have been very active over the last 20 years, and activity is expected to continue. As part of the collective RiftVolc project, the work has immediate relevance for a wide range of stakeholders with regard to the geohazard that active volcanism presents, including through the provision of practical advice and support to local communities, and research outputs and methodologies that will assist both policy makers and those working in natural hazard assessment and management more broadly. In scientific terms, the project will ensure that the outcomes of the Afar Rift Consortium are built upon so that the research community continues to benefit from improved understanding of this active and dramatic region.
Figure1: Data and model for the 2008 eruption on the Erta Ale range, from Pagli et al., 2012. (a) Plan view of the sill contraction, overlain by topographic contours and the outline of the lava flow; (b) cross section through the model geometry along line K-K’; (c,d,e) real, model, and residual interferograms. Data from the ALOS satellite; (f,g,h) cross sections through the data and model. 52
Training: The student will work under the supervision of Professors Wright and Hooper in Leeds, and Dr Biggs in Bristol. The project will provide a high level of specialist scientific training in the processing of satellite radar data, and the development of numerical models. Although based in the Institute for Geophysics and Tectonics in the School of Earth and Environment at Leeds, co-supervision will involve regular meetings between all partners and extended visits to Bristol’s School of Earth Sciences, where they will benefit from the expertise of members of the Volcanology and Geophysics groups. They will also interact with the wider RiftVolc team, specifically the geodetic and modelling aspects of the project, and be a member of NERC’s Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), a collaboration between scientists in Leeds, Oxford, Cambridge, Bristol, UCL, Glasgow, and Reading. Finally, the student will also have access to a broad range of Faculty- and University-led training via the EME Hub. References
Field L; Blundy J; Brooker RA; Wright TJ; Yirgu G (2012) Magma storage conditions beneath Dabbahu Volcano (Ethiopia) constrained by petrology, seismicity and satellite geodesy, Bulletin of Volcanology, 74, pp.981-1004.
Pagli C; Wright TJ; Cann JR; Ebinger CJ; Yun S-H; Barnie T; Ayele A (2012) Shallow axial magma chamber at the slowspreading Erta Ale Ridge, Nature Geoscience, 5, pp.284-288.
Phipps Morgan J; Chen YJ (1993) Dependence of ridge-axis morphology on magma supply and spreading rate, Nature 364, 706-708.
Wright TJ et al. (2012) Geophysical constraints on the dynamics of spreading centres from rifting episodes on land, Nature Geoscience, 5, pp.242-250.
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