Melting experiments on the enstatite-diopside join at 70-224 kbar, including the melting of diopside

Melting relations on the enstatite-diopside (En, Mg2Si2O6-Di, CaMgSi2O6) join, including the compositions of crystalline phases and melts coexisting along the solidi, were experimentally determined in the pressure range 70-224 kbar with a split-sphere anvil apparatus (USSA-2000). Melting is peritectic in enstatite-rich compositions at 70-124 kbar (1840-2100 degrees C) and eutectic at higher pressures, while the diopside-rich clinopyroxene melts azeotropically at 70-165 kbar and up to 300 degrees C lower temperatures than the eutectic. Orthopyroxene is replaced with enstatite-rich clinopyroxene at 120 kbar and 2090 degrees C. First garnet with 17 mol% Di forms on the solidus at 158 kbar and 2100 degrees C. Two garnets coexist on the solidus at 165-183 kbar and 2100 degrees C, garnet coexists with CaSiO3 perovskite at 183-224 kbar (2100-2230 degrees C) and two coexisting perovskites are stable at higher pressures. The melting curve of diopside was determined at 80-170 kbar; the slope becomes negative at 140 kbar and 2155 degrees C. At 170 kbar and 2100 degrees C, diopside with 96% Di breaks down to garnet with 89% Di and CaSiO3 perovskite. The new data were used to calculate an improved temperature-pressure phase diagram for the CMAS system, which can be useful for estimating the mineralogy of the Earth's upper mantle.