Royal  Society  University  Research  Fellowship,  2015   Oliver  Thomas  Lord    

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The chemical architecture of the deep Earth The character of our planet was defined by its earliest experiences. The kinetic energy from giant impacts as it accreted by collision with smaller planetesimals, combined with the heat from radioactive decay caused repeated, wholesale melting of the Earth, the segregation of the metallic core from the silicate mantle, and ultimately, the formation of the moon1. The ensuing magma oceans, which may have extended all the way to the core mantle boundary, would have solidified within a few tens of millions of years at most2. Despite being completed within the first few per cent of Earth’s lifespan, these processes had effects on the geodynamics and habitability of Earth that were profound3 and long lasting: after 4.567 billion years of vigorous convection, driving plate tectonics, volcanism and the continual renewal of Earth’s surface, not all traces of this period have been obscured4. Evidence from the fields of geochemistry, geophysics and geodynamics strongly suggest that its signature has been written, indelibly, into the chemical architecture of the deep Earth. The overarching goal of this proposal is to harness technological advances in high-pressure mineral physics, both experimental and computational, to unpick this short but critical period in Earth’s history.

It is often assumed that the objects that accreted together to form the Earth are today represented by the most primitive, undifferentiated solar system materials: the chondritic meteorites. However, geochemists have struggled to reconcile this ‘chondritic Earth model’ with the rocks accessible for analysis; it is apparent that the accessible Earth is strongly depleted in many key trace elements3. Isotopic analysis of a variety of Earth rocks make this assertion practically indisputable: almost every lithology measured has a 142Nd/144Nd ratio that is 18±5 ppm higher than the Earth’s putative chondritic building blocks4. This result applies powerful constraints because 142Nd is formed by the radioactive decay of 146Sm and Sm and Nd are fractionated from one another during mantle crystallisation. Combined with a short half-life (68–103 Ma) and the fact that neither element is fractionated by volatile loss during accretion or alloying with liquid iron during core formation, this means that whatever caused the disparity must have occurred within the mantle and within the first 20-30 Ma of Earth history3. There are three possible explanations for this result: Earth wasn’t built from ordinary chondrites5, the Nd was fractionated into an early crust that was then lost to space, thus increasing the Sm/Nd and hence 142Nd/144Nd of the remaining material6, or perhaps there is a hidden reservoir in Earth’s deep mantle, with a composition that is complementary to the depleted, accessible mantle above.

This last theory has gained significant geophysical support in recent years, due of the discovery of enigmatic structures in the lowermost mantle that may represent the physical location of just such a hidden, enriched reservoir: the ultra-low velocity zones (ULVZ) and large low shear velocity provinces (LLSVP)7. The LLSVP are two antipodal piles of dense material, anchored to the core mantle boundary (CMB) and extending ~1000 km into the mantle, one below Africa and one below the Pacific. The ULVZ are discontinuous patches of even denser material at the CMB, tens of kilometres thick and seemingly associated with the edges of the LLSVP. An attractive solution exists that solves both the geochemists need for a hidden reservoir and the geophysicists desire to explain lower mantle structure: if the adiabatic gradient of the mantle intersects its liquidus at mid mantle depths, two magma oceans form, one at the base of the mantle and one at the surface, separated by a solid septum8. As the basal magma ocean cools and crystallises, the residual liquids are enriched both in trace elements and iron9 relative to the coexisting solids,

creating an enriched reservoir that is sufficiently dense to be gravitationally stable at the CMB, eventually evolving into the lower mantle structures we see today. However, there are problems with this scenario beyond the assumptions concerning the physics and chemistry of mantle crystallisation that are prerequisites for its feasibility. Firstly, geodynamic modelling suggests such a reservoir might not be stable against entrainment back into the convecting mantle10. Secondly, it is difficult to reconcile the chemistry of ocean island basalts, erupted at Earth’s surface and thought to be sourced from plumes rooted at the edges of the LLSVP with melting of an early enriched reservoir of the required composition11. Alternatively, these structures may represent graveyards of subducted oceanic crust12 or the products of reaction between the silicate mantle and iron core13, both likely modulated by pressure-induced changes in the physical properties of the constituent phases14.

Solving these complex problems which intertwine geochemistry, geophysics and geodynamics depends critically on the solutions to two key, outstanding questions concerning the physical and chemical processes of mantle crystallisation: Q1: Can magma ocean crystallisation lead to structures like those we observe in the mantle today? Q2: Is it possible to create an enriched reservoir with the correct chemistry to explain the depleted mantle we observe?

The resolution of these questions, which will only be

achieved through experiment, is a key challenge within the geosciences because of the significant implications their answers will have for our understanding of Earth’s composition, and thus its geodynamic history. The biggest implication concerns the distribution and abundance of the heat producing elements U, Th and K in the Earth. These elements exercise a significant controlling influence on the onset, style and evolution of plate tectonics through the ‘convective Urey ratio’ of radiogenic heating to total heat flux15. In turn, plate tectonics regulates climate through its modulation of the rate of volcanic emission of greenhouse gases and the rate of their drawdown by weathering of newly created crust3. The evolution of mantle convection also regulates the heat flux from the core; this in turn

Royal  Society  University  Research  Fellowship,  2015   Oliver  Thomas  Lord    

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determines the time of onset of core convection, the geodynamo and the geomagnetic field, which has been key in protecting our atmosphere from erosion by the solar wind16. Thus, the evolution of the habitability of Earth’s surface is firmly rooted in the chemical architecture of the deep Earth.

Unfortunately, the data we need to answer these questions with any degree of certainty is either of insufficient quality, or does not exist at all. I propose to fill this gap by combining cutting edge high-pressure experiments with nano-scale chemical analysis and ab initio molecular dynamics simulations. I have organised the work into two packages that will run concurrently and inform each other. Work Package 1: Creation & Survival To solve Q1, we need to know the identity, composition and physical properties of the solid and liquid phases that coexist within the crystallisation interval of a chondritic magma ocean up to the pressure of the CMB (136 GPa) where liquidus temperatures will likely exceed 4000 K. While some pioneering work on phase relations at these extreme conditions has been done9 the results are controversial and too limited to make anything other than broad-brush statements concerning the evolution of deep Earth structure. The reason for these difficulties is that the laser heated diamond anvil cell (LH-DAC), which is the only apparatus capable of reproducing such extreme conditions in a static, recoverable environment, suffers from two major drawbacks. Firstly, strong axial and radial temperature gradients are generated during laser heating, and secondly, samples can react with the medium used to thermally insulate them from the anvils. Both of these problems can lead to significant inaccuracies in phase relations and partitioning measurements.

A novel micro-fabrication process (Fig. 1), in which silicate samples are fully encapsulated in metal on a scale compatible with the LH-DAC solves both problems simultaneously: the high conductivity of the metal capsule leads to a thermal regime comparable to that found in a piston cylinder apparatus in which the sample is surrounded by a cylindrical furnace, minimizing temperature gradients in the sample. At the same time, the sample is chemically isolated from its surroundings. In brief, the method consists of drilling a grid of holes with a diameter of ~15 µm in the neck of a 10 µm thick Mo filament using UV laser ablation (Fig. 1-1) onto which is placed a powdered oxide mix of the desired composition. This is then heated resistively in an Ar atmosphere, melting the silicate, which flows into the holes (Fig. 1-2). Cutting the electrical power quenches the glass in the holes, which are then partially cut out (Fig. 1-3) and finally sputter coated with ~2 µm of Mo (Fig. 1-4). The

finished samples are then loaded into an LH-DAC sample chamber, between layers of thermal insulation. The samples are then laser heated from both sides using the state of the art system that already exists in the School of Earth Sciences17. The final drawback of the LH-DAC, that of small sample size, can be circumvented by employing the broad range of cutting edge, micro- and nano-beam analytical techniques now available – many at the University of Bristol.

Using these techniques, the first step in my research plan is to determine the melting phase relations in a simplified chondritic system. Recovered samples will be sectioned, mounted and polished axially, exposing a complete cross section of the sample within its capsule (Fig. 1-5 & 2). The samples will first be analysed using the field emission gun electron probe microanalyser (FEG-EPMA) at the School of Earth Sciences, which will provide sub-micron imaging of sample textures from which the phase relations can be determined, as well as quantitative chemical maps of the major element distributions (Mg, Si, Al, Fe, Ca) between the solid and liquid phases (Fig. 2). Inevitably, at either end of the crystallisation interval, the resolution of this technique will be insufficient for small melt or solid fractions. For that reason, a 1 µm thick wafer, extending into the sample ~10 µm at 90° to the initially exposed surface (the blue plane in Fig. 1-5) will be excavated using a focussed ion beam (FIB) mill either at the host institution’s Interface Analysis Centre, or with my

FIG.2| A recovered capsule. White = Mo, black = resin. The overlaid colour image is a map of Mg concentration; the red areas are Mg-Pv crystals, the blue areas, quenched melt. For reference, the interaction volume of an FEG-EPMA analysis is shown in blue and that for ATEM and nano-XRF in red – magnified 4-fold to make it visible.  

FIG.1| Sequence of steps for micro-fabrication and analysis of encapsulated samples compatible with the LH-DAC.  

Royal  Society  University  Research  Fellowship,  2015   Oliver  Thomas  Lord    

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collaborator Prof. Daniel Frost at the Bayerisches Geoinstitut (BGI), Germany. Textural and chemical analysis of these FIB sections will be performed at nano-scale resolution using analytical transmission electron microscopy (ATEM), also available at the host institution’s School of Chemistry and BGI.

This information will allow me to determine the crystallisation sequence of a cooling magma ocean. However, whether or not it is possible to create dense material that is gravitationally stable at the base of the mantle, also depends critically on the relative density of the solid and liquid phases involved as a function of pressure and temperature. What’s more, the long term survivability of such a dense body, whether solid, liquid or both also depends on other additional physical properties, notably viscosity10. Density usually decreases upon melting in an isochemical system, but this behaviour can be overwhelmed by compositional effects, notably the preferential partitioning of iron into the liquid9. Critically, my initial experiments give me this key compositional information. The densities of the solids can be reliably determined using published thermal equations of state, but determining the densities of silicate liquids experimentally is exceptionally difficult and little static data exists. Viscosity will also vary rapidly as a function of composition and temperature but there is no method for the direct determination of this property by experiment.

Thus, to provide these two final pieces of the puzzle, I will turn to ab initio molecular dynamics (AIMD) simulations using density functional theory, performed on the UK’s high performance computing facility, Archer, in collaboration with Prof. John Brodholt of UCL. Though some ab initio density18 and viscosity19 data exist for simple end member compounds, I will extend these methods to the more complex systems used in my experiments. In this way, the results of the experiments and simulations will be directly comparable. Simulations will be performed using density functional theory on systems in the NVT ensemble (fixed number of atoms N, volume V and temperature T) performed at a range of temperatures encompassing the crystallisation interval as determined from the experiments. The relationship of temperature with the computed thermodynamic properties pressure and internal energy will be used to constrain the thermal equation of state (EoS) of a given liquid composition. Once simulations are performed on a sufficient range of compositions, it will be possible, using these EoS, to estimate the density of the full range of liquids produced in our experiments. Viscosity will be determined in the same simulations using the Green-Kubo relation18 and thus can also be estimated for all experimentally measured liquid compositions.

These experimental and computational data will allow me to accurately model the likely crystallisation pathways of an early chondritic magma ocean and determine the feasibility of the contention that the LLSVP and ULVZ are vestiges of a basal magma ocean or whether alternative scenarios would have prevailed, requiring us to rethink our understanding of deep mantle structure. Work Package 2: Hidden Reservoirs However the mantle segregated, is it possible that any of the resulting reservoirs would be sufficiently enriched so as to be complementary to the depleted mantle we observe?

The solution to this question (Q2) of course depends on the results of the first work package, but requires additional knowledge: specifically, how the key trace elements, such as Sm, Nd, U and Th are partitioned between the various co-existing phases. Currently, no data of this kind exists beyond ~25 GPa, the pressure achievable in a large volume, multi-anvil press.

I will therefore perform experiments using the same methods and at the same conditions of pressure and temperature as described in the first work package, but on samples doped at the ~1 wt.% level with 2-3 elements out of U, Th, Nb, La, Nd, Sm, Lu, Hf and Ta. This will require several suites of experiments, but the experiments performed in the first work package can also be used for this purpose. The resulting FIB sections will be analysed chemically using another cutting edge nano-beam technique: synchrotron based X-ray fluorescence (XRF). Capable of measuring sub-weight per cent levels of elements 14 to 42 (Si to Mo) and 56 to 92 (Ba to U) at a spatial resolution on the order of 10 nm (Fig. 2), the resulting chemical maps will allow me to both visualise and quantify the distribution of the trace elements between the solid and liquid phases. Beamlines with this capability include P06 at the PETRA-III synchrotron at DESY lab, Hamburg, Germany and ID16B at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, while a similar beamline at the UK synchrotron facility, Diamond, is under construction. In the case of ID16B at the ESRF, X-ray diffraction (XRD) can be performed simultaneously with XRF allowing the identity of the solid phases to be determined while their chemistry is being probed.

Combined with the results of the first work package, these data will allow me to determine the feasibility of an early, enriched, and hidden geochemical reservoir in the mantle and thus constrain models of Earth’s composition, which has exercised a profound control on the geodynamic, tectonic and climatic evolution of our planet and ultimately, its habitability.

References [1] Nakajima, M., & Stevenson, D. J. (2015). Earth Planet. Sci. Lett., 427:286–295. 10.1016/j.epsl.2015.06.023 [2] Elkins-Tanton, L. T. (2008). Earth Planet. Sci. Lett., 271(1-4):181–191. 10.1016/j.epsl.2008.03.062. [3] Jellinek, A. M., & Jackson, M. G. (2015). Nat. Geo. 8:587–593, 10.1038/ngeo2488. [4] Caro, G. (2011). Ann. Rev. Earth Planet. Sci., 39(1):31–58. 10.1146/annurev-earth-040610-133400. [5] Campbell, I. H., & St C O'Neill, H. (2012). [6] Bonsor, A. et al. (2015). Icarus, 247:291–300. 10.1016/j.icarus.2014.10.019. [7] Garnero, E. J., & McNamara, A. K. (2008). Science, 320(5876):626–628. 10.1126/science.1148028. [8] Labrosse, S. et al., (2007). Nature, 450(7171):866–869. 10.1038/nature06355. [9] Andrault, D. et al. (2012). Nature, 487(7407):354–357. 10.1038/nature11294 [10] Jellinek, A. M., & Manga, M. (2004). Rev. Geophys. 42(3):RG3002. 10.1029/2003RG000144 [11] Andreasen, R. et al. (2008). Earth Planet. Sci. Lett., 266(1-2):14–28. 10.1016/j.epsl.2007.10.009 [12] Tackley, P. J. (2011). Phys. Earth Planet. Inter., 188(3-4):150–162. 10.1016/j.pepi.2011.04.013. [13] Hayden, L. A., & Watson, E. B. (2007). Nature, 450(7170):709–711. 10.1038/nature06380. [14] Huang, C. et al. (2015). Earth Planet. Sci. Lett., 423, 173–181. 10.1016/j.epsl.2015.05.006. [15] Korenaga, J. (2013). Ann. Rev. Earth Planet. Sci., 41(1):117–151. 10.1146/annurev-earth-050212-124208 [16] Tarduno, J. A. (2010). Science, 327(5970):1238–1240. 10.1126/science.1183445. [17] Lord, O. T. et al. (2014). Earth Planet. Sci. Lett., 408:226–236. 10.1016/j.epsl.2014.09.046 [18] Stixrude, L. et al. (2009). Earth Planet. Sci. Lett., 278(3-4):226–232. 10.1016/j.epsl.2008.12.006 [19] Karki, B. B., & Stixrude, L. P. (2010).