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Geochemistry, Geophysics, Geosystems, 23, e2022GC010544, (2022) [doi: 10.1029/2022GC010544]

Textures induced by the coesite-stishovite transition and implications for the visibility of the X-discontinuity

M. Krug, M. Saki, E. Ledoux, J. P. Gay, J. Chantel, A. Pakhomova, R. Husband, A. Rohrbach, S. Klemme, C. Thomas, S. Merkel, C. Sanchez-Valle

The coesite-stishovite phase transition is considered the most plausible candidate to explain the X-discontinuity observed at around 300 km depth in a variety of tectonic settings. Here, we investigate the microstructure in SiO2 across the coesite-stishovite transition in uniaxial compression experiments. We apply the Multigrain Crystallography technique (MGC) in a laser-heated diamond-anvil cell (LH-DAC) to identify the seismic signature of the transition and the amount of SiO2 in the mantle. While coesite displays weak lattice-preferred orientations (LPO) before the transition, stishovite develops strong LPO characterized by the alignment of [112] axes parallel to the compression direction. However, LPO has little effect on the impedance contrast across the transition, which is up to 8.8 % for S-waves in a MORB composition at 300 km depth along a normal mantle geotherm, 10 GPa-1700 K. Therefore, 10 – 50 vol.% of a mid-ocean ridge basalt (MORB) component, corresponding to 0.6 – 3.2 vol.% SiO2, mechanically mixed with the pyrolytic mantle would be required to explain the range of impedance (and velocity) contrasts observed for the X-discontinuity. Based on the reflection coefficients computed for the coesite-stishovite transition, we show that the incidence angle or epicentral distance is critical for the detection of silica-containing lithologies in the upper mantle, with highest detection probabilities for small incidence angles. The intermittent visibility of the X-discontinuity may thus be explained by the seismic detectability of the coesite-stishovite transition rather than by absence of the transition or chemical heterogeneities in some specific tectonic settings.

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© Sébastien Merkel, Université de Lille, France

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