Clinoenstatite in a Volcanic Rock from the Cape Vogel Area, Papua

A porphyritic volcanic rock from Cape Vogel, Papua, contains abundant phenocrysts of multiply twinned clinoenstatite, and less common phenocrysts of orthopyroxene, set in a groundmass of pyroxene microlites, glass, and zeolites. The rock contains 54% SiO2, 13–16% MgO, and 6–7% FeO, but only 7–8% Al2O3, 4·5–5% CaO, and 0·6–0·8% Na2O. Microprobe analyses show that the clinoenstatite phenocrysts range from En92 to En82, and have very low Al2O3 and extremely low CaO contents. Their composition differs consistently from that of the orthopyroxene phenocrysts, which range from En87 to at least as Fe-rich as En78. The clinoenstatite phenocrysts are a metastable inversion-product from primary protoenstatite. The crystallization of protoenstatite as the liquidus phase is attributed directly to the unique magma composition.