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

changes that cannot be reversed. Short residence times also imply A technique is described for determining the cooling history of olivine that large unzoned cores of high-Fo phenocrysts cannot reflect diffusive phenocrysts. The technique is based on the analysis of the diffusive re-equilibration of originally zoned phenocrysts. The unzoned cores re-equilibration of melt inclusions trapped by olivine phenocrysts are a result of fast efficient accumulation of olivines from the during crystallization. The mechanism of re-equilibration involves crystallizing magma, i.e. olivines are separated from the magma diffusion of Fe from and Mg into the initial volume of the inclusion. faster than melt changes its composition. Thus, the main source of The technique applies to a single crystal, and thus the cooling high-Fo crystals in the erupted magmas is the cumulate layers of history of different phenocrysts in a single erupted magma can be the magmatic system. In other words, olivine-phyric rocks represent established. We show that melt inclusions in high-Fo olivine mixtures of an evolved transporting magma (which forms the phenocrysts from mantle-derived magmas are typically partially regroundmass of the rock) with crystals that were formed during equilibrated with their hosts at temperatures below trapping. Our crystallization of more primitive melt(s). Unlike high-Fo olivine analysis demonstrates that at a reasonable combination of factors phenocrysts, the evolved magma may reside in the magmatic system such as (1) cooling interval before eruption (<350°C), (2) eruption for a long time. This reconciles long magma residence times estimated temperatures (>1000°C), and (3) inclusion size (<70 m in from the compositions of rocks with short residence times of highradius), partial re-equilibration of up to 85% occurs within Fo olivine phenocrysts. 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