Towards a petrological explanation for the X-Discontinuity: The role of eclogite

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Combining petrological models with discontinuities in seismic wave velocities is the essential means for placing constraints on the structure and composition in the deep Earth. A local seismic feature observed at ~300 km depth is referred to as the X-discontinuity, but to date, no clear petrologic explanation for this discontinuity is generally accepted. The most recent intriguing explanation primarily involves the exsolution of stishovite from Ca-eskola-bearing (Ca0.5□0.5Si2O6) clinopyroxene (cpx) that is present in an eclogitic mineral assemblage. Our goal is to test this hypothesis using a two-step experimental approach: 1) determining the maximum Ca-eskola-component that can be incorporated in cpx at lower P in model and natural eclogitic compositions, and 2) re-equilibration of pre-synthesised assemblages at higher P within the stishovite stability field. Emphasis will be placed on defining the conditions where stishovite exsolves and monitoring changes in cpx composition and molar volume, as well as in phase distributions, microstructures and possible preferred orientation of the exsolved stishovite. A linkage between eclogite bodies and the X-discontinuity has very important implications for geodynamic modelling and for understanding the physical and chemical processes involved in recycling of oceanic crust into the mantle.