The geochemical signature of fluid-saturated magma determined from silicate melt inclusions in ascension island granite xenoliths

Abstract

Silicate glasses in crystal-free to crystal-poor melt inclusions from two coarse-grained granite xenoliths in alkaline volcanic rocks of Ascension Island were analyzed for 29 major, minor, and trace elements. For most constituents, the glass compositions are similar to those of the volcanic whole rocks and the xenoliths; we interpret the glasses to be chemically representative of granite melt. The melt inclusions are silicic; alkaline; contain low S and P abundances; and are enriched in H 2O, Cl, F, and Na relative to K. Inclusions from one xenolith contain 1.3 wt.% F, on average, whereas those from the other xenolith contain half that amount. The melt inclusion compositions allow investigation of the means and extent of granite magma evolution. The presence of magmatic fluid inclusions in close proximity to melt inclusions in the phenocrysts indicates that the granite melt was saturated in one or more volatile phases (Roedder and Coombs, 1967). The Cl and H 2O contents of the melt inclusions are consistent with the exsolution of volatile phase(s) at pressures of 3000 to 2000 bars. The glasses also show trends involving the (Cl/H 2O) ratio and the incompatible trace elements in melt that are indicative of melt evolution via crystal fractionation of volatile phase-saturated melt. These trends should be useful for recognizing volatile phase saturation in other Cl-enriched melts.