Dynamic compression of diopside and salite to 200 GPa

Abstract

New Hugoniot data on single crystal diopside, CaMgSi_2O_6 (Di), suggest that transformation to a high-pressure thermomechanical state begins at ∼50 GPa and is complete above 100 GPa, in agreement with other pyroxenes and silicates of geophysical interest. Comparison of the new high pressure phase (HPP) data for Di and salite, CaMg_(0.82)Fe_(0.18)Si_2O_6 (Sa) with appropriate mixed oxide and perovskite models implies compatibility between either model and the data. Conversely, least-squares fits to the HPP Di data favor lower (3.6 - 3.9 Mg/m³) values of zero-pressure, room-temperature density than the models (4.0 - 4.1 Mg/m³). Similar comments apply to porosity-corrected HPP hedenbergite (Hd) data. The HPP Di, Sa, and Hd data also imply much larger density differences between these compositions in the HPP regime (e.g., ≈0.8 Mg/m³ between Di and Hd) than at STP (0.38 Mg/m³). This may represent the influence of multiple transition processes (e.g., polymorphism and Fe^(2+) high-low spin) as a function of Fe content across the Di-Hd series. The new HPP Sa data closely parallel (≈0.1 Mg/m³ less dense) the lower mantle density profile from ∼90 GPa to 136 GPa. Our results are consistent with the speculations of Jeanloz and Ahrens on the possibility of significant Ca in the lower mantle.

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