The seismic X-Discontinuity, occasionally observed at ~300 km depth, has been attributed partly to the exsolution of the calcium-Eskola (Ca0.50.5AlSi2O6, CaEs) component from eclogitic clinopyroxene upon reaching the coesite–stishovite transition. To test if this is a viable mechanism, we have undertaken high-pressure experiments between 4 and 11.5 GPa at 1000–1350 °C in order to further investigate the factors controlling CaEs incorporation in clinopyroxene at upper mantle conditions up to coesite–stishovite transition. Bulk compositions had pyroxene stoichiometry in the simple system CaO–MgO–Al2O3–SiO2 ± Na2O (CMAS ± Na), which restricts possible end-member components to diopside (di), calcium-Tschermaks (CaTs), calcium-Eskola (CaEs), and clinoenstatite (en), ±jadeite.

All run products had a typical eclogitic mineral assemblage with clinopyroxene always in equilibrium with garnet and coesite or stishovite, and in some runs kyanite. Clinopyroxene is non-stoichiometric due to the defect-bearing CaEs component. CaEs and CaTs contents vary systematically with P, T and bulk composition. CaEs reaches up to 0.22 mol % at 4 GPa and 1200 °C, and for a given bulk composition CaEs decreases continuously with pressure from 4 to 11.5 GPa. Our experiments, along with experiments performed with more complex compositions reveal different systematics when Na is present, since it is only accommodated in clinopyroxene. Incorporation of a jadeite component drives down the CaTs content and also influences the CaEs content. Generally, maximizing the CaEs content requires a coexisting free SiO2 phase and an elevated Al2O3 content, but it is not clear if kyanite must be part of the assemblage, or whether buffering of Al by garnet is adequate. At P > 9 GPa, the systematics of clinopyroxene composition change as garnet becomes majoritic and also begins to accommodate significant Na.

In both simple and complex bulk compositions CaEs content in clinopyroxene decreases steadily from ~4 GPa so that no sharp change occurs at pressures corresponding to the coesite–stishovite phase transition. Calculations reveal that only small (1–2 wt %) amounts of free stishovite can be exsolved from CaEs-bearing clinopyroxene, which are insufficient to produce a large enough impedance contrast to explain a significant contribution to the X-Discontinuity. If the X-Discontinuity is related to the coesite–stishovite phase transition in eclogite, as proposed in the literature, then free SiO2 must be already present in the mineral assemblage rather than being exsolved from CaEs-bearing clinopyroxene upon entering the stability field of stishovite.

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