Antigorite Dehydration under Compression and Shear Loadings in a Rotational Diamond Anvil Cell
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Mineral dehydration in the subduction zone enormously affects Earth’s geodynamics and the global geochemical cycles of elements. This work uses Raman spectroscopy and X-ray diffraction to investigate the dehydration process of antigorite under compression and shear loading conditions in a rotational diamond anvil cell (RDAC) at room temperature. In order to compare the shear effects, T301 stainless steel and Kapton plastic are applied as the gasket materials. In the experiment using a high-strength T301 stainless steel gasket, two new broad OH-stretching peaks of H2O and H3O2− appear at 3303 and 3558 cm−1, respectively, at 1.7 GPa. The original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 remain, while the central pressure is increased to 8.0 GPa, and the largest pressure gradient is about 2.5 GPa in the sample chamber. In another experiment with a low-strength gasket of Kapton plastic, two new OH-stretching broad peaks of H2O and H3O2− also start to appear at 3303 and 3558 cm−1, respectively, at a lower pressure of 0.3 GPa, but the original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 almost completely vanish as the central pressure reaches 3.0 GPa, with the largest pressure gradient at around 4.8 GPa. The comparison between the two experiments shows that antigorite is easier to dehydrate in the chamber of a Kapton plastic gasket with a larger gradient of shear stress. However, its axial compression stress is lower. The high-pressure Raman spectra of MgO2(OH)4 octahedron and SiO4 tetrahedron in the low wavenumber zones (100–1200 cm−1) combined with the micro-beam X-ray diffraction spectrum of the recovered product strongly support the structural breakdown of antigorite. This investigation reveals that the water-bearing silicate minerals have strong shear dehydration in the cold subduction zone of the plate, which has important applications in predicting the physical and chemical properties of subduction zones and deducing the rate of plate subduction.Keywords:
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Kapton
Diamond anvil cell
We have developed techniques for high-pressure in situ structure measurement of low-Z noncrystalline materials with a diamond-anvil cell (DAC) by an x-ray diffraction method. Since the interaction between low-Z materials and x rays is small and the sample thickness in a DAC is also small, the incoherent scattering from the anvils overwhelms the coherent scattering from the sample at a high-Q range. By using a cubic boron nitride gasket to increase the sample thickness and the energy-dispersive x-ray diffraction method with a slit system to narrow the region from which detected x rays are scattered, we can reduce unfavorable effects of the incoherent scattering from the anvils and correct them accurately. We have successfully measured the structure factor of SiO(2) glass in a DAC over a relatively wide range of Q under high pressure.
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Mineral dehydration in the subduction zone enormously affects Earth’s geodynamics and the global geochemical cycles of elements. This work uses Raman spectroscopy and X-ray diffraction to investigate the dehydration process of antigorite under compression and shear loading conditions in a rotational diamond anvil cell (RDAC) at room temperature. In order to compare the shear effects, T301 stainless steel and Kapton plastic are applied as the gasket materials. In the experiment using a high-strength T301 stainless steel gasket, two new broad OH-stretching peaks of H2O and H3O2− appear at 3303 and 3558 cm−1, respectively, at 1.7 GPa. The original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 remain, while the central pressure is increased to 8.0 GPa, and the largest pressure gradient is about 2.5 GPa in the sample chamber. In another experiment with a low-strength gasket of Kapton plastic, two new OH-stretching broad peaks of H2O and H3O2− also start to appear at 3303 and 3558 cm−1, respectively, at a lower pressure of 0.3 GPa, but the original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 almost completely vanish as the central pressure reaches 3.0 GPa, with the largest pressure gradient at around 4.8 GPa. The comparison between the two experiments shows that antigorite is easier to dehydrate in the chamber of a Kapton plastic gasket with a larger gradient of shear stress. However, its axial compression stress is lower. The high-pressure Raman spectra of MgO2(OH)4 octahedron and SiO4 tetrahedron in the low wavenumber zones (100–1200 cm−1) combined with the micro-beam X-ray diffraction spectrum of the recovered product strongly support the structural breakdown of antigorite. This investigation reveals that the water-bearing silicate minerals have strong shear dehydration in the cold subduction zone of the plate, which has important applications in predicting the physical and chemical properties of subduction zones and deducing the rate of plate subduction.
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A gasket is an important constituent of a diamond anvil cell (DAC) assembly, responsible for the sample chamber stability at extreme conditions for x-ray diffraction studies. In this work, we studied the performance of gaskets made of metallic glass Fe0.79Si0.07B0.14 in a number of high-pressure x-ray diffraction (XRD) experiments in DACs equipped with conventional and toroidal-shape diamond anvils. The experiments were conducted in either uniaxial or radial geometry with x-ray beams of micron to sub-micron size. We report that the Fe0.79Si0.07B0.14 metallic glass gaskets offered stable sample environment under compression exceeding one megabar in all XRD experiments described here, even in those involving inter- or small-molecule gases (e.g. Ne, H2) used as pressure transmitting media or in those with laser heating in a DAC. These emphasize the material's importance for a great number of delicate experiments conducted under extreme conditions. Our results indicate that the application of Fe0.79Si0.07B0.14 metallic glass gaskets in XRD experiments of both uniaxial and radial geometries substantially improves the signal-to-noise ratio in comparison to that with conventional gaskets made of Re, W, steel or other crystalline metals.
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Abstract In this study, we determined the α–β quartz phase transition temperatures ( T tr s; up to ~924°C for a total of 23 obtained under 0.1 MPa and elevated pressures) in a hydrothermal diamond‐anvil cell (HDAC) at H 2 O pressures up to ~1310 MPa by using in situ Raman spectroscopy. When compared with the commonly used α‐quartz Raman band near 465 cm −1 (at 24°C and 0.1 MPa), the one near 128 cm −1 is much more sensitive for detecting T tr . The corresponding phase transition pressure ( P tr ) at each measured T tr was calculated from three equations of state (EoS) of pure H 2 O. The corresponding three α–β quartz phase boundaries were compared with those derived from previous experimental results. The best fitted one can be represented by: P tr (±5.6 MPa) = 0.0008· T tr 2 + 2.8056· T tr − 1877.5, where 574°C ≤ T tr ≤ 889°C with R 2 = 0.9998. Our results agree, within experimental uncertainties, with most of previous experimental data, and our newly determined α–β quartz P tr – T tr phase boundary can be used as a reliable pressure calibrant in HDAC. Furthermore, the experimental procedures used in this study, including the use of Raman spectroscopy criterion for T tr determination, can be easily applied to obtain improved EoS of geologically important aqueous solutions containing salt(s) at pressure–temperature conditions up to ~1.2 GPa–900°C.
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A new diamond-anvil cell for single-crystal x-ray diffraction structural studies is presented. By including a simple mechanism for adjusting the parallelism of the opposing diamond culets, the stability of the gasket is maintained to higher pressures than was achieved in earlier cells of a similar two-platen design. Equipped with 600 μm culet diamond anvils, the cell has been tested to a pressure of 25 GPa and has been used successfully for equation-of-state measurements and crystal structure determinations to 10 GPa.
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Pressure estimation using the frequency shift of the diamond Raman peak from the anvil culet is readily and widely used in diamond anvil cell experiments along with the conventional ruby fluorescence method. Here, we propose a modified diamond Raman scale particularly designed for pressure measurement below ~10 GPa. A series of experiments were conducted using a highly confocal Raman system and H2O, ethanol/methanol mixture and NaCl samples loaded in a rhenium gasket which was pre-indented to 40–60 or 100–110 μm thick. The result showed that the frequency of the diamond Raman peak from the anvil culet increases linearly with pressure between 1 and 13 GPa, when using a sufficiently pre-indented (40–60 μm thick) gasket. The frequency shifts are calibrated against the pressure determined by the ruby fluorescence method, which is an alternative pressure scale. In addition, a preliminary measurement at high temperature up to 575 K suggests the potential application of this method for high temperature experiments.
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The diamond anvil cell is a fundamental tool for investigating properties under extreme conditions. However, our knowledge of material behavior under high pressure has been limited by a lack of measurement capability in radial directions. In this study we introduce a gasketing technique based on a combination of Kapton, amorphous boron, and epoxy, that reliably solves this issue. We demonstrate how these gaskets allow precise imaging of stress, strain, and microscopic properties such as texture and lattice preferred orientations within the sample, in situ, up to pressures in the range of 65GPa.
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