NEAR-PRIMARY MELT INCLUSIONS IN ANORTHITE PHENOCRYSTS FROM THE GALAPAGOS PLATFORM

Partially crystalline melt inclusions in anorthite phenocrysts from a Galapagos seamount have been rebomogenized in a series of heating experiments in order to accurately determine their initial compositions. They are primitive tholeiites, high in CaO and Al2O3, and low in TiO2 and alkalies. We infer that they were entrapped at a temperature close to 1270-degrees-C. At this temperature, the trapped melt composition is multiply saturated with olivine, piagioclase and spinel at 1 atmosphere pressure, but it is also near-primary requiring addition of less than 5% olivine plus or minus plagioclase and spinel in order to be in equilibrium with mantle olivine. The inclusions occur in association with, and are potentially parental to, a suite of MORB-like pillow lavas which range in MgO content from 10 to 8 wt% and which could have been derived by up to 40% fractionation of olivine plus plagioclase from the inclusion composition. Although the host anorthites are remarkably homogeneous and unzoned and the inclusions themselves are uniform in major element composition, minor element contents vary by a factor of two or more. Unlike many other plagioclase megacrysts, these anorthites must have crystallized under conditions in which the major element compositions were buffered but minor (and presumably trace) elements were free to vary. Such conditions could be achieved by liquid-solid interactions either in the melting regime or, perhaps more likely, in a liquid-crystal mush within the oceanic crust. These melt inclusions are not unique. They appear to belong to a class of primitive, high CaO and Al2O3 MORB that occur in association with high-An plagioclase in a variety of oceanic settings, usually where magma supply is low. We propose that such magmas are derived from the shallowest, most depleted part of a mantle melt column, and that they constitute one end member of a spectrum of primary magmas which gives rise to the global array of MORB compositions.