Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions

Abstract

The solidus of a model pyrolite composition is sensitively dependent on water content and has been determined experimentally up to 40 kb, for water-saturated (6% H 2O) and water-undersaturated (0.2% H 2O) conditions. Pargasitic hornblende is a major subsolidus phase to 29 kb and its breakdown at higher pressure has the effect of sharply depressing the solidus for (pyrolite+0.2% H 2O) from ∼1150°C to∼1020°C between 25 and 29 kb. Experiments have been carried out above the solidus to determine the nature of the partial melting process, particularly the nature and composition of the residual phases at a specific pressure, temperature and water content. The presence of siliceous (>58% SiO 2), low-magnesium glasses, broadly of andesitic or dacitic character, in experiments quenched at both 10 kb and 20 kb, is shown to be due to growth during quenching of olivine, clinopyroxene, amphibole and mica. However, it is possible in some experiments to use the compositions of the starting mix and analyzed residual phases to calculate the composition of the equilibrium liquid and degree of melting at the particular condition. High degrees of melting under water-saturated conditions at 10 kb yield magnesian, quartz-normative basaltic andesites ∼10%Qz, (1100°C, 28% melting) to quartz tholeiite magmas 5–7% Qz, (1200°C, 32.5% melting), and at 20 kb, yield olivine tholeiite magmas (1100°C, 27–30% melting). Andesitic or dacitic magmas are not products of equilibrium partial melting of pyrolite at P > 10kb under water-saturated conditions but may be derived from parental olivine-poor tholeiites, quartz tholeiites or basaltic andesites ( P < 10kb) by crystal fractionation. Parental magmas of the island arc tholeiitic magma series may originate by partial melting of upper mantle peridotite (pyrolite or residual peridotite of the lithosphere) under water-saturated conditions at ∼5–20kb.