OPEN SYSTEM BEHAVIOR OF OLIVINE-HOSTED MELT INCLUSIONS

Abstract

It is commonly assumed that an olivine-hosted melt inclusion is chemically isolated from the melt surrounding its host crystal. This assumption is probably valid for slow-diffusing, incompatible trace elements, but for readily exchanged major elements such as Fe and Mg, it is much more tenuous. We used numerical simulations constrained by experimental data on olivine–liquid equilibrium and interdiffusion rates of Fe and Mg in olivine to assess the extent to which post-entrapment processes alter the major element compositions of olivine-hosted melt inclusions. The results indicate that extensive diffusive communication between a melt inclusion and the melt surrounding its host olivine occurs at cooling rates as rapid as 1–2°C/yr. An included melt undergoes significant and irreversible compositional changes in order to maintain Fe–Mg exchange equilibrium with the fractionating external melt. When a magnesian olivine containing an exotic liquid, such as an ultra-depleted melt, becomes entrained in a basalt and equilibrates with the new external melt by diffusive exchange, the ensuing compositional changes to the included melt cannot be removed simply by adding olivine back into the final (measured) melt composition. Observable disequilibrium between an inclusion and its host olivine is likely to have been produced during cooling of the erupted lava. Simulations, supplemented by experiments carried out on inclusion-bearing olivines, demonstrate that laboratory heating can reverse the effects of syn-eruptive crystallization on melt inclusion composition, and the presence or absence of compositional zoning in the immediately adjacent olivine can provide an indication of how closely the experiment reproduced the pre-eruptive temperature.