Conditions of melting of basanite magma from garnet peridotite

Abstract

Experimental studies have been carried out at high pressures and temperatures, to determine the crystallization behaviour of olivine-rich basanite composition with various known water contents. Between 25 kbar and 30 kbar, in the temperature range from 1200°C to 1300°C and with water contents in the magma from 2% to 7% H 2 O, the olivine basanite has orthopyroxene, garnet, clinopyroxene and olivine on or near the liquidus. There is, at these conditions, a fractionation trend from higher-temperature olivine-rich basanite to lower-temperature olivine nephelinite produced by progressive crystallization (or, with increasing temperature, progressive melting) of the phases orthopyroxene, garnet, clinopyroxene and olivine. The analyzed compositions of near-liquidus phases in the olivine-rich basanite are closely similar to the residual phases present in pyrolite (+ 0.2% H 2 O) in a partially melted run at ∼ 30 kbar, 1250°C. The data are consistent with a specific model in which partial melting of pyrolite + (0.2−0.4)% H 2 O at 30 kbar, 1250°C yields 6% olivine-rich basanite magma. The residual peridotite contains 60% olivine, 18% orthopyroxene, 14% clinopyroxene and 18% garnet. It is concluded that magmas present in the low velocity zone are highly undersaturated and nephelinerich (olivine nephelinites to olivine melilitites).