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  1. Free, publicly-accessible full text available October 1, 2024
  2. The high-pressure structure and stability of the calcic amphibole tremolite (Ca2Mg5Si8O22(OH)2) was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C2/m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a/b and a/c, in conjunction with finite strain versus normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli (𝐾) of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0-5 GPa (regime I), 5-20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli (𝐾′) of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as 𝛽a : 𝛽b : 𝛽c = 2.22:1.0:0.78 (regime I), 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regimemore »at 5 GPa (I-II) transition is ascribed to stiffening along the crystallographic a-axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a-oriented surfaces of the structural I-beam from concave to convex. The II-III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite’s notable metastability range hinges on the calcium cation’s bonding environment. The 8-fold coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of diffraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C2/m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth.« less
    Free, publicly-accessible full text available October 1, 2023
  3. Abstract

    NASA's Mars 2020 and ESA's ExoMars will collect Raman measurements in dusty field conditions obscuring underlying rocks. This presents a challenge for remote Raman measurements at distances where mechanical or ablative sample cleaning is not straightforward. Historically, probing broad lithostratigraphic suites has been thwarted by the need for pristine targets and high‐quality spectra. We provide a means of identifying Raman spectra of common rock‐forming silicate, carbonate, and sulfate minerals under low signal‐to‐noise‐ratios, Mars‐like conditions using a convolutional neural network (CNN). The CNN was trained on the Machine Learning Raman Open Data set data set with 500,000+ Raman spectra of hand samples/powder mixtures (5,000+ spectra/mineral class). Diversity in sample microtopography, orientation, and crystallinity simulated varying laser focuses and spectral quality, and no traditional spectral preprocessing such as cosmic ray or baseline removal was employed. The CNN identified low‐intensity Raman scatterers (micas and amphiboles), mixed minerals, and distinguished between mineral endmembers with +99% success. We present among the first known implementations of “big data” machine learning using varied, high‐volume Raman spectral datasets. The pattern recognition abilities of CNNs can facilitate scientist Raman spectral interpretation on Earth and autonomous rover decision‐making on planets like Mars; increasing scientific yield, correcting human classification errors,more »reducing the need for thorough target dust removal during evaluative measurements, and streamlining the data communications pipeline—saving time and resources. This study examines an end‐to‐end development process for creating a deep learning algorithm sensitive to varieties of Raman spectra and provides guidelines for CNN model development at the interface of Raman spectroscopy, deep learning, and planetary science.

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  4. Abstract

    A comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H2O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2%more »could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.

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  5. Abstract

    Thermal pressure is an inevitable thermodynamic consequence of heating a volumetrically constrained sample in the diamond anvil cell. Its possible influences on experimentally determined density‐mineralogy correlations are widely appreciated, yet the effect itself has never been experimentally measured. We present here the first quantitative measurements of the spatial distribution of thermal pressure in a laser‐heated diamond anvil cell (LHDAC) in both olivine and AgI. The observed thermal pressure is strongly localized and closely follows the distribution of the laser hotspot. The magnitude of the thermal pressure is of the order of the thermodynamic thermal pressure (αKTΔT) with gradients between 0.5 and 1.0 GPa/10 μm. Remarkably, we measure a steep gradient in thermal pressure even in a sample that is heated close to its melting line. This generates consequences for pressure determinations in pressure‐volume‐temperature (PVT) equation of state measurements when using an LHDAC. We show that an incomplete account of thermal pressure in PVT experiments can lead to biases in the coveted depth versus mineralogy correlation. However, the ability to spatially resolve thermal pressure in an LHDAC opens avenues to measure difficult‐to‐constrain thermodynamic derivative properties, which are important for comprehensive thermodynamic descriptions of the interior of planets.

  6. Abstract

    The viscosity of iron alloy liquids is the key for the core dynamo and core‐mantle differentiation of terrestrial bodies. Here we measured the viscosity of Fe‐Ni‐C liquids up to 7 GPa using the floating sphere viscometry method and up to 330 GPa using first‐principles calculations. We found a viscosity increase at ∼3–5 GPa, coincident with a structural transition in the liquids. After the transition, the viscosity reaches ∼14–27 mPa·s, a factor of 2–4 higher than that of Fe and Fe‐S liquids. Our computational results from 5 to 330 GPa also indicate a high viscosity of the Fe‐Ni‐C liquids. For a carbon‐rich core in large terrestrial body, the level of turbulence in the outer core would be lessened approaching the inner core boundary. It is also anticipated that Fe‐Ni‐C liquids would percolate in Earth's deep silicate mantle at a much slower speed than Fe and Fe‐S liquids.