Melt Inclusions in Olivine Phenocrysts: Using Diffusive Re-equilibration to Determine the Cooling History of a Crystal, with Implications for the Origin of Olivine-phyric Volcanic Rocks

Abstract

A technique is described for determining the cooling history of olivine phenocrysts. The technique is based on the analysis of the diffusive re-equilibration of melt inclusions trapped by olivine phenocrysts during crystallization. The mechanism of re-equilibration involves diffusion of Fe from and Mg into the initial volume of the inclusion. The technique applies to a single crystal, and thus the cooling history of different phenocrysts in a single erupted magma can be established. We show that melt inclusions in high-Fo olivine phenocrysts from mantle-derived magmas are typically partially re-equilibrated with their hosts at temperatures below trapping. Our analysis demonstrates that at a reasonable combination of factors such as (1) cooling interval before eruption ( 1000°C), and (3) inclusion size (<70 µm in radius), partial re-equilibration of up to 85% occurs within 3–5 months, corresponding to cooling rates faster than 1–2°/day. Short residence times of high-Fo phenocrysts suggest that if eruption does not happen within a few months after a primitive magma begins cooling and crystallization, olivines that crystallize from it are unlikely to be erupted as phenocrysts. This can be explained by efficient separation of olivine crystals from the melt, and their rapid incorporation into the cumulate layer of the chamber. These results also suggest that in most cases erupted high-Fo olivine phenocrysts retain their original composition, and thus compositions of melt inclusions in erupted high-Fo olivine phenocrysts do not suffer changes that cannot be reversed. Short residence times also imply that large unzoned cores of high-Fo phenocrysts cannot reflect diffusive re-equilibration of originally zoned phenocrysts. The unzoned cores are a result of fast efficient accumulation of olivines from the crystallizing magma, i.e. olivines are separated from the magma faster than melt changes its composition. Thus, the main source of high-Fo crystals in the erupted magmas is the cumulate layers of the magmatic system. In other words, olivine-phyric rocks represent mixtures of an evolved transporting magma (which forms the groundmass of the rock) with crystals that were formed during crystallization of more primitive melt(s). Unlike high-Fo olivine phenocrysts, the evolved magma may reside in the magmatic system for a long time. This reconciles long magma residence times estimated from the compositions of rocks with short residence times of high-Fo olivine phenocrysts.