Royal Society Research Grants URF & DHF Appointed 2016 Oliver Thomas Lord Case for support 1 of 2 Modelling Mantle Mineralogy: A Novel Experimental Approach I propose to use a novel internal resistive heating method, interfaced with the diamond anvil cell and combined with Brillouin spectroscopy and synchrotron based X-ray scattering techniques to produce a new, internally consistent set of thermoelastic data for the major lower mantle mineral phases. These data will be used to improve our mineralogical models of the enigmatic structures observed by seismologists in the lowermost mantle. 1. Introduction Earth's lowermost mantle contains enigmatic seismically- detected structures including two vast, dense regions termed large low shear-wave velocity provinces (LLSVPs), one below the Pacific and one below Africa, as well as smaller, even denser patches of material at the core mantle boundary (CMB) called ultra-low velocity zones (ULVZs) 1 . One of the most prevalent hypotheses for the origin of this set of complex structures is that they represent the vestiges of the crystallization of one or more impact induced global magma ocean(s) present during the Hadean Eon 2 . Indeed, the primary questions I aim to answer as part of my URF are whether magma ocean crystallization could lead to such structures, whether they could survive to the present day and whether they have the correct geochemical characteristics to represent the locus of the 'hidden geochemical reservoir' required to explain a range of geochemical signals observed in rocks erupted at the surface. Alternatively, these structures may represent the remnants of dense, subducted oceanic crust that has accumulated at the CMB as a result of mantle convection with residence times anywhere from ~3 Ga 3 to only ~100s of Ma 4 . Many models also suggest that these structures must be significantly hotter than the surrounding mantle 5 and rely for their stability on structural and/or electronic phase transitions in a variety of lower mantle mineral phases 4 . In reality, a combination of these possibilities is likely 6 . Here, I propose to tackle the problem of complex lower mantle seismic structure from the opposite, modern-day perspective, by developing mineralogical models of the lowermost mantle by comparing its seismic response with the thermoelastic properties of the relevant mineral phases. We need to know at least three key parameters for each lower mantle phase to be able to do this: density (!), longitudinal sound velocity (" # ) and shear sound velocity (" $ ). In addition, knowledge of the directional anisotropy of " # and " $ is necessary because the mantle is seismically anisotropic as a result of convective flow generating crystal preferred orientation (CPO). Anisotropy is a key input for the new generation of geodynamic models that attempt to invert for both seismic velocity and anisotropy by including rheological properties 7 . The accuracy of our mineralogical models is of course dependent on the accuracy of our measurements. The LLSVPs extend to a maximum of ~1000 km above the CMB and so encompass pressures (P) of 85-135 GPa and temperatures (T) up to 2500 K (rising to ~4000 K in the thermal boundary layer that extends ~200 km above the CMB). The laser-heated diamond anvil cell (LH-DAC) is currently the only available technique capable of re-creating these conditions in a static environment. Unfortunately, LH- DAC experiments suffer from significant problems, most notably spatial temperature gradients (both radial and axial) of 100's of K μm -1 as well as temporal variations due to changes in the absorption characteristics of the sample during the experiment and power fluctuations in the incident laser beam 8 . These can lead to reductions in precision and accuracy when measuring the loci of phase boundaries and the physical properties of individual phases via micro-beam techniques such as X-ray diffraction that integrate these spatially and temporally varying thermodynamic states. In addition, LH-DAC samples are often in direct contact with a pressure medium chosen to reduce deviatoric stresses within the sample environment and act as thermal insulation between the sample and the anvils. While care is taken to choose materials that are unreactive with the sample, this may be impossible; even commonly used noble gases such as Ar have non-negligible solubilities in mantle silicates 9 that could impact the accuracy of thermoelastic measurements. 2. Methodology In my URF proposal, I described a novel technique to minimise these problems by employing laser heating to fully metal encapsulated samples. Since that proposal was submitted I have developed a new technique, potentially even more versatile, involving internal resistive heating (IRH; Fig. 1a) in wide opening angle BX-90 symmetric DACs. Briefly, the rhenium gasket is cut in half and then glued back together using a non-conductive adhesive such that the two sides are electrically isolated. A W filament, with a central hole containing the silicate sample and coated on both sides with 2-3 μm of W using magnetron sputtering (available at Bristol) is loaded into the sample chamber between form fitting discs of pressure media such that it bridges the two sides of the gasket. After compression to the target pressure (as determined by ruby fluorescence or Raman spectroscopy) the filament is heated by passing an FIG.1| a Exploded view of the proposed internal resistive heating setup. The dark line represents the non-conductive polymer that separates the two sides of the gasket. b Photomicrograph of internal resistive heating of a W filament in the DAC at 30 GPa and ~2500 K. White dashed lines represent the edges of the filament. a
Modelling Mantle Mineralogy: A Novel Experimental Approach
I propose to use a novel internal resistive heating method, interfaced with the diamond anvil cell and combined with Brillouin spectroscopy and synchrotron based X-ray scattering techniques to produce a new, internally consistent set of thermoelastic data for the major lower mantle mineral phases. These data will be used to improve our mineralogical models of the enigmatic structures observed by seismologists in the lowermost mantle. 1. Introduction Earth's lowermost mantle contains enigmatic seismically-detected structures including two vast, dense regions termed large low shear-wave velocity provinces (LLSVPs), one below the Pacific and one below Africa, as well as smaller, even denser patches of material at the core mantle boundary (CMB) called ultra-low velocity zones (ULVZs)1. One of the most prevalent hypotheses for the origin of this set of complex structures is that they represent the vestiges of the crystallization of one or more impact induced global magma ocean(s) present during the Hadean Eon2. Indeed, the primary questions I aim to answer as part of my URF are whether magma ocean crystallization could lead to such structures, whether they could survive to the present day and whether they have the correct geochemical characteristics to represent the locus of the 'hidden geochemical reservoir' required to explain a range of geochemical signals observed in rocks erupted at the surface. Alternatively, these structures may represent the remnants of dense, subducted oceanic crust that has accumulated at the CMB as a result of mantle convection with residence times anywhere from ~3 Ga3 to only ~100s of Ma4. Many models also suggest that these structures must be significantly hotter than the surrounding mantle5 and rely for their stability on structural and/or electronic phase transitions in a variety of lower mantle mineral phases4. In reality, a combination of these possibilities is likely6. Here, I propose to tackle the problem of complex lower mantle seismic structure from the opposite, modern-day perspective, by developing mineralogical models of the lowermost mantle by comparing its seismic response with the thermoelastic properties of the relevant mineral phases. We need to know at least three key parameters for each lower mantle phase to be able to do this: density (!), longitudinal sound velocity ("#) and shear sound velocity ("$). In addition, knowledge of the directional anisotropy of "# and "$ is necessary because the mantle is seismically anisotropic as a result of convective flow generating crystal preferred orientation (CPO). Anisotropy is a key input for the new generation of geodynamic models that attempt to invert for both seismic velocity and anisotropy by including rheological properties7. The accuracy of our mineralogical models is of course dependent on the accuracy of our measurements. The LLSVPs extend to a maximum of ~1000 km above the CMB and so encompass pressures (P) of 85-135 GPa and temperatures (T) up to 2500 K (rising to ~4000 K in the thermal boundary layer that extends ~200 km above the CMB). The laser-heated diamond anvil cell (LH-DAC) is currently the only available technique capable of re-creating these conditions in a static environment. Unfortunately, LH-DAC experiments suffer from significant problems, most notably spatial temperature gradients (both radial and axial) of 100's of K µm-1 as well as temporal variations due to changes in the absorption characteristics of the sample
during the experiment and power fluctuations in the incident laser beam8. These can lead to reductions in precision and accuracy when measuring the loci of phase boundaries and the physical properties of individual phases via micro-beam techniques such as X-ray diffraction that integrate these spatially and temporally varying thermodynamic states. In addition, LH-DAC samples are often in direct contact with a pressure medium chosen to reduce deviatoric stresses within the sample environment and act as thermal insulation between the sample and the anvils. While care is taken to choose materials that are unreactive with the sample, this may be impossible; even commonly used noble gases such as Ar have non-negligible solubilities in mantle silicates9 that could impact the accuracy of thermoelastic measurements.
2. Methodology In my URF proposal, I described a novel technique to minimise these problems by employing laser heating to fully metal encapsulated samples. Since that proposal was submitted I have developed a new technique, potentially even more versatile, involving internal resistive heating (IRH; Fig. 1a) in wide opening angle BX-90 symmetric DACs. Briefly, the rhenium gasket is cut in half and then glued back together using a non-conductive adhesive such that the two sides are electrically isolated. A W filament, with a central hole containing the silicate sample and coated on both sides with 2-3 µm of W using magnetron sputtering (available at Bristol) is loaded into the sample chamber between form fitting discs of pressure media such that it bridges the two sides of the gasket. After compression to the target pressure (as determined by ruby fluorescence or Raman spectroscopy) the filament is heated by passing an
FIG.1| a Exploded view of the proposed internal resistive heating setup. The dark line represents the non-conductive polymer that separates the two sides of the gasket. b Photomicrograph of internal resistive heating of a W filament in the DAC at 30 GPa and ~2500 K. White dashed lines represent the edges of the filament.
electrical current across it. I have already achieved temperatures of ~2500 K at a pressure of 30 GPa using this technique (Fig. 1b); the only impediment to extending this range is the necessity for further miniaturisation requiring very precise laser micromachining of the parts shown in Fig. 1a, such that they fit into a sample chamber ~80 µm in diameter. I have reached the limit of what can be achieved with the re-purposed, second-hand UV laser ablation tools I currently have at my disposal and so I have costed into this proposal 41% of the value of a new ns pulsed laser micromachining system (the remaining cost is guaranteed by the Faculty of Science and the School of Earth Sciences at the University of Bristol). In contrast to laser heating, axial and radial temperature gradients are minimised during IRH because heat is generated throughout the thickness of the filament. Sample temperature gradients depend solely on the diameter and thermal conductivity of the sample; numerical modelling underway now suggests that gradients in a 20 µm diameter silicate sample will be ~50 K, significantly smaller than hose routinely measured in LH-DAC experiments. In addition, IRH allows temperature to be varied in arbitrarily small steps and is much more stable than laser heating. These attributes will significantly reduce uncertainties in T and thus measured thermoelastic parameters relative to LH-DAC experiments. Of the sound velocity data that currently exists for lower-mantle phases, most does not extend beyond room temperature10. Even data relevant to the shallowest parts of the LLSVPs are rare11. While measurements of r based on thermal equations of state are much more common12 and have minimal uncertainties in P, what is required now is an internally consistent dataset of r, "# and "$ with reduced uncertainties in T. T will be measured using spectroradiometry above 1200 K, and either using pyrometry at lower temperatures or power vs. T functions calibrated at higher T where thermal emission is insufficient for spectroradiometry.
3. Research strategy First, in collaboration with Dr. James Drewitt (Bristol) I will miniaturise the IRH method ready for the PhD student to begin performing experiments at lower mantle conditions. The primary targets of this study will be the various polymorphs of the major compositional end members that describe the majority of the mode of peridotite (ambient mantle) and eclogite (subducted oceanic crust):
I. MgSiO3 bridgmanite (bm) / post-bridgmanite (p-bm) II. (Mg,Fe)O ferropericlase (fp)
III. CaSiO3 perovskite (Ca-Pv) IV. SiO2 post-stishovite (p-st) / seifertite (sf) V. Ca-ferrite structured NaAl-rich phase (NAL)
Polycrystalline samples of these phases will be used for X-ray diffraction (XRD) measurements, performed at beamline I15 of the UK synchrotron, Diamond to determine the Clapeyron slope of key structural transitions that are still controversial, notably the bm/p-bm transition13. The fine temperature control of IRH should allow a dramatic improvement in data density such that the subtle structural imprints of electronic transitions, such as the high-spin/low spin transition in fp should also be detectable. These data will also allow the construction of thermal equations of state for each phase providing the adiabatic bulk modulus (KS) and r
as a function of P and T. This is necessary but not sufficient to determine "# and "$ because "# = &$ + 4/3+$ and "$ =+$/! and thus knowledge of the adiabatic shear modulus
+$ is needed or "# and "$ must be determined directly. The next step therefore will be to synthesise single crystals of these phases (except p-bm and Ca-Pv which are unrecoverable from high PT conditions). These will be double polished, laser machined and fitted into IRH filaments and subjected to two additional analytical techniques: Brillouin scattering11 at the Bayreuth Geoinstitut, Germany, in collaboration with Prof. Dan Frost and inelastic X-ray scattering (IXS)14 at beamline ID28 of the European Synchrotron Research Facility (ESRF) in collaboration with Prof. James Badro (Institut de Physique du Globe de Paris). Brillouin scattering on single crystal samples is capable of measuring VP and VS directly as a function of crystal orientation thus also providing a direct comparison for seismic observations of seismic anisotropy. The disadvantages of Brillouin spectroscopy are that the visible light laser probe cannot penetrate the sample capsule, so the sample will have to be in direct contact with the pressure medium and the signal may be swamped by thermal emission at high T. These problems can potentially be overcome by IXS which is essentially a higher frequency version of Brillouin spectroscopy with the advantage that the X-ray probe can penetrate the metallic capsule so the sample can be separated from its environment. The IXS signal is also immune from being swamped by the thermal background encountered at high T. It may be challenging to preserve pre-synthesised single crystals loaded into IRH filaments to lower mantle conditions for all phases. Nevertheless, both Brillouin spectroscopy and IXS on polycrystalline samples can still provide useful aggregate sound velocity data. This proposal takes advantage of the ESRF II upgrade programme that will occur during the lifetime of the project; the order of magnitude increase in brightness and reduction in spot size will lead to dramatic improvements in IXS data quality. In parallel with the experimental part of the project, I will collaborate with Dr. James Wookey, a seismologist at Bristol with a background in studies of lower mantle structure15. Together we will use the new mineral physics results to create new mineralogical models designed to match the available radially averaged and global tomographic seismic data by varying mineral proportions and orientation.
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