The system NaAlSi.sub.2O.sub.6âCaMgSi.sub.2O.sub.6-CO.sub.2 at 3-6.5 GPa: implications for CO.sub.2 stability in the eclogitic suite at depths of 100-200 km

In this work, we studied the phase relationships in the NaAlSi.sub.2O.sub.6-CaMgSi.sub.2O.sub.6-CO.sub.2 system in the range of 3-6.5 GPa and 900-1500 â using a multianvil press. The duration of most experiments varied from 5 to 9 days. Equilibrium was examined from both sides using oxide-carbonate mixtures and diopside-jadeite glasses with Ag.sub.2C.sub.2O.sub.4 as starting materials. We found that the subsolidus assemblage is represented by clinopyroxene, coesite, dolomite, and CO.sub.2 fluid. The solidus of the system passes through 1015 â/3 GPa and 1095 â/6 GPa, and has a slope of 38 MPa/â. The near-solidus carbonate melt contains 1-4 mol% SiO.sub.2 and 1-4 mol% Na.sub.2O. We found that clinopyroxene can be in equilibrium with CO.sub.2 fluid under P-T conditions corresponding to the diamond stability field to at least 6-6.5 GPa and 1050-1100 â. Yet, the composition of such clinopyroxene corresponds to almost pure jadeite, Jd# 95-97. Thus, the infiltration of CO.sub.2 fluid into eclogite under conditions of the continental lithospheric mantle should be accompanied by partial carbonation of clinopyroxene with the formation of a low-Mg carbonate melt with moderate concentrations of sodium. As a result of the reaction, the compositions of clinopyroxenes of eclogites of Groups A and B should shift towards a high-jadeite composition corresponding to eclogites of Group C.