Geobarometry for mantle eclogites: Solubility of Ca-Tschermaks in clinopyroxene

A garnet-clinopyroxene barometer is proposed for mantle eclogites on the ba sis of the Ca-Tschermack (CaTs) solubility in clinopyroxene coexisting with garnet. This barometer permits estimation of P-T conditions for garnet-cli nopyroxene equilibration in the range 650 degrees C less than or equal to T less than or equal to 1700 degrees C and 20 less than or equal to P less t han or equal to 70 kbar, with a total uncertainty of 19%. This model is app licable to pyroxenes that are sodium rich (i.e., have high jadeite contents ). Application of this barometer results in high P-T estimates for Monaster y and Argyle diamond inclusions. Estimates also are lower for the host Yaku tian eclogites than for the diamonds included in them. This suggests differ ences in re-equilibration between the diamond inclusions and their host xen oliths. The results suggest that eclogite equilibration in the mantle occur s at depths from 60 to 230 km, characterized by two main types of geotherma l gradients, lying close to established peridotite geotherms (Boyd, 1973).