Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity

Abstract

AbstractThe 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 mid‐ocean ridge basalt (MORB) composition at 300 km depth along a normal mantle geotherm, 10 GPa‐1700 K. Therefore, 10–50 vol.% of a 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.