Melt inclusion formation during olivine recrystallization: Evidence from stable isotopes
Abstract
Melt inclusions are often used to infer melting processes or to determine source magmas that are usually overprinted in bulk rocks due to late stage mixing or near surface contamination. Here we present the first investigation of oxygen (O) isotope equilibrium between melt inclusions and their host olivines from arc samples. Olivines in all but one sample record either magma mixing or fractional crystallization. All six melt inclusions from Vulcano, 83% of seven from Sukumoyama, 44% of 21 from St Vincent, 37% of four from Iwate, and 21% of 13 from Aoba are not in isotopic equilibrium with their olivine host, despite the other major elements being in apparent equilibrium. A detailed study of some of the olivines shows that only a small volume around the melt inclusions is in equilibrium with its host. This strongly suggests that in these olivines melt inclusions are trapped in partly recrystallized olivines, highlighting the importance of magma mixing and crystal recycling in the magmatic plumbing system of these volcanoes. Oxygen isotope fractionation between melt inclusions and their host olivines, as well as phosphorus-δ18O systematics, could be used to better understand the formation of melt inclusions and crystal history. It could also provide valuable information to help characterize the magmatic plumbing system that the inclusions and their olivine hosts formed in (e.g., crystal rich-mush versus crystal poor melt lenses).
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