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    Solid earth sciences

    201605201605

    Unit cell determination of coexisting post-perovskite and H-phase in (Mg,Fe)SiO3 using multigrain XRD: compositional variation across a laser heating spot at 119 GPa

    Zhang L, Meng Y, Mao H-k

    Lower mantle, Multigrain XRD, Diamond anvil cell, Post-perovskite, H-phase, Unit cell, Synchrotron X-ray

    Unit cell volumes of coexisting (Mg,Fe)SiO3 ppv and H-phase across a laser heating spot. The unit cell volumes of a (Mg,Fe)SiO3 ppv and b H-phase obtained using multigrain XRD are plotted as function of distance from the heating center to the margin at steps of 5 μm in a laser-heated sample at 119 GPa. The unit cell volumes of ppv and H-phase decrease by 0.16 and 0.54 %, respectively, from the center to 10 μm off the center, while the sample pressure remained the same in the Ne quasi-hydrostatic environment

    Multigrain X-ray diffraction (XRD) can be used to accurately calculate the unit cell parameters of individual mineral phases in a mineral assemblage contained in a diamond anvil cell (DAC). Coexisting post-perovskite (ppv) and H-phase were synthesized at 119 GPa and 2500 K from (Mg0.85Fe0.15)SiO3 in a laser-heated DAC. The unit cell parameters of the ppv and coexisting H-phase were determined using multigrain XRD with a 5 μm spatial resolution, close to the size of the X-ray beam, to understand compositional variations across the center area (20–30 μm) in a laser-heated sample. The ppv phase was Fe-depleted and the unit cell volume of ppv decreased by only 0.16 % (corresponding to ~3 % variation of FeSiO3) from the heating center to 10 μm off the center, while the sample pressure remained at 119 GPa in a Ne quasi-hydrostatic environment. The unit cell volume of the H-phase decreased by 0.54 % (~10 % variation of FeSiO3 content) over the same 10 μm distance. Both phases were more Fe-enriched in the slightly hotter center. This observation suggests that thermal diffusion may not be the major driver for the compositional variations of ppv and H-phase in the center portion of a laser-heated sample. Instead, these variations could be caused by a temperature effect on the partitioning between the ppv and H-phase over the small gradient.