K in clinopyroxene at high pressure and temperature; an experimental study

American Museum of Natural History, Department of Earth and Planetary Sciences, New York, NY, United States

Clinopyroxene (Cpx) is a principal upper-mantle phase for concentrating large cations but has not been viewed as a major crystal-chemical reservoir for K because K (super +) is considered too large to enter the largest site, M2, in the pyroxene structure. Accumulating data from high-pressure conditions indicate this inference is incorrect, so multianvil experiments have been performed to evaluate maximal K solubility in Cpx at high pressure. End members and mixtures of diopside, jadeite, and kosmochlor have been mixed with K ₂ CO ₃ , KHCO ₃ or both in welded platinum capsules and heated typically for 24 h in the range of 5 to 14 GPa and 1200 to 1700 degrees C. These experiments produced K-rich Cpx in solid solutions by means of a fictive Kcpx component (KCrSi ₂ O ₆ or KAlSi ₂ O ₆ ). The maximum K ₂ O content obtained is 4.7 wt% in a Cpx (Di ₃₈ Ko ₃₉ Kcpx ₂₂ En ₁ ) formed from a 50:50 Di+Ko mixture at 10 GPa, 1400 degrees C. K uptake and partitioning is dependent positively on P, complexly on Cpx composition but not demonstrably on T. (super Cpx/liq) D _K2O is in the range of 0.03-0.1 and (super Cpx/liq) D _Na2O varies from 0.5 to 5, although the variations for each with Cpx composition are different. In diopside, Kcpx increases are always accompanied by increases in Nacpx, and cooperative Nacpx solution is necessary for Kcpx solution in the compositional systems examined. K appears to be accommodated in the M2 site of the Cpx structure by two types of spatial averaging: a large average M2 site, as in the case of Di, ameliorates the fit, but local accommodation by size averaging with a smaller M2 occupant, presumably Na, appears necessary suggesting that the polyhedral compressibility of Na and K are large in comparison with Ca. In application to Cpx inclusions in diamond, the data here imply that a chromium diopside with approximately 1 wt% K ₂ O forms in the presence of a C-rich melt with 15-28 wt% K ₂ O.

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