Abstract
The monovariant reaction Opx+H2O⇄ Cum+Ol+Q and the Cum+Opx+Q stability field were studied under hydrothermal conditions at Ptotal=2940, 4900 bar and the oxygen fugacity of the QFM buffer. Under these conditions, the Opx lower stability brackets were 730°±10° and 740°±5° C, respectively. The kinetics of the reactions in the Cum+ Opx+Q mixture showed that there were only minor differences in the equilibrium compositions of the coexisting Opx and Cum over the 740°–780° C range. At T=780°, 760° and 740° C, the FeO/FeO+MgO ratio, in mol% was: Opx52.5−Cum49.5, Opx62−Cum57, Opx72−Cum66 (P=2940 bar) and Opx62−Cum58.5 Opx71.5−Cum66.5, Opx80−Cum75 (P= 4900 bar). The results are in good agreement with earlier studies in the Opx+Ol+Q and Cum+Ol+Q assemblages.
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