Diffusional Gradients at the Crystal/Melt Interface and Their Effect on the Compositions of Melt Inclusions

Abstract

Melt inclusions are widely believed to represent melts from which host crystals grew. Melt inclusions represent melt adjacent to growing crystals, where compositional gradients exist due to preferential incorporation or exclusion of components by the crystallizing mineral. The possibility arises, therefore, that melt in inclusions may differ significantly from melt which was more remote from growing crystals at the time the crystals grew. We have tested this possibility by analyzing 45 major, minor, and trace components in 50 to 400 (µm diameter melt inclusions in phenocrysts from the rhyolitic Bishop Tuff, California. The following observations indicate that the effects of compositional gradients on chemical compositions of melt inclusions are negligible: (1) melt inclusions in quartz and sanidine phenocrysts have indistinguishable compositions; (2) no correlation is observed between sizes of melt inclusions and their chemical compositions; (3) ten melt inclusions in four quartz phenocrysts from a single...