Experimental duplication of conditions of precipitation of high-pressure phenocrysts in a basaltic magma

Abstract

An olivine basalt from the Auckland Islands contains partially resorbed “xenocrysts” of orthopyroxene, clinopyroxene and minor olivine. Electron probe microanalyses of these crystals confirm their similarity to near-liquidus crystals obtained experimentally in tholeiitic and alkali olivine basalts at high pressures. A high pressure experimental study of the host olivine basalt demonstrates that orthopyroxene and clinopyroxene are near-liquidus phases at 11–18 kb but the degree of solid solution between the pyroxenes crystallized from the dry magma is much greater than that observed in the natural pyroxenes. Addition of water to the experimental runs results in lowering of the liquidus of the basalt and in the appearance of orthopyroxene, clinopyroxene and olivine as near-liquidus phases at 13–18 kb, 1130–1230 °C. A close correspondence between chemical compositions of the natural “xenocrysts” and the experimental near-liquidus pyroxenes and olivine is obtained and the conditions of the precipitation of the “xenocrysts” from their host magma are inferred to be near 14–16 kb and 1200 °C. The host magma contained ∼2% H 2O at these conditions to produce the requisite depression of the liquidus. The “xenocrysts” are regarded as high pressure phenocrysts giving natural evidence of high pressure magmatic fractionation controlled largely by separation of orthopyroxene.