STATUS REPORT ON STABILITY OF K-RICH PHASES AT MANTLE CONDITIONS

Abstract

Experimental research on K-rich phases and observations from diamond inclusions, UHP metamorphic rocks, and xenoliths provide insights about the hosts for potassium at mantle conditions. K-rich clinopyroxene (Kcpx–KM3+Si2O6) can be an important component in clinopyroxenes at P>4 GPa, dependent upon coexisting K-bearing phases (solid or liquid) but not, apparently, upon temperature. Maximum Kcpx content can reach ∼25 mol%, with 17 mol% the highest reported in nature. Partitioning (K)D(cpx/liquid) above 7 GPa=0.1–0.2 require ultrapotassic liquids to form highly potassic cpx or critical solid reactions, e.g., between Kspar and Di. Phlogopite can be stable to about 8 GPa at 1250 °C where either amphibole or liquid forms. When fluorine is present, it generally increases in Phl upon increasing P (and probably T) to about 6 GPa, but reactions forming amphibole and/or KMgF3 limit F content between 6 and 8 GPa. The perovskite KMgF3 is stable up to 10 GPa and 1400 °C as subsolidus breakdown products of phlogopite upon increasing P. (M4)K-substituted potassic richterite (ideally K(KCa)Mg5Si8O22(OH,F)2) is produced in K-rich peridotites above 6 GPa and in Di+Phl from 6 to ∼13 GPa. K content of amphibole is positively correlated with P; Al and F content decrease with P. In the system 1Kspar+1H2O K-cymrite (hydrous hexasanidine–KAlSi3O8·nH2O–Kcym) is stable from 2.5 GPa at 400 to 1200 °C and 9 GPa; Kcym can be a supersolidus phase. Formation of Kcym is sensitive to water content, not forming within experiments with H2O<Kspar, and melt+aluminous phases forming with H2O>Kspar. Phase X, a potassium di-magnesium acid disilicate ((K1−x−n)2(Mg1−nMn3+)2Si2O7H2x), forms in mafic compositions at T=1150–1400 °C and P=9–17 GPa and is a potential host for K and H2O at mantle conditions with a low-T geotherm or in subducting slabs. The composition of phase-X is not fixed but actually represents a solid solution in the stoichiometries □2Mg2Si2O7H2–(K□)Mg2Si2O7H–K2Mg2Si2O7 (□=vacancy), apparently stable only near the central composition. K-hollandite, KAlSi3O8, is possibly the most important K-rich phase at very high pressure, as it appears to be stable to conditions near the core–mantle boundary, 95 GPa and 2300 °C. Other K-rich phases are considered.