Crystallization processes of anorthoclase phenocrysts in the Mount Erebus magmatic system: Evidence from crystal composition, crystal size distributions and volatile contents of melt inclusions.

Mount Erebus, Antarctica, is a large, active, phonolitic volcano that contains an actively convecting and continuously degassing phonolite lava lake. Bombs emitted during eruptions contain strikingly large anorthoclase feldspar phenocrysts, which appear to be the product of a 2-stage growth process. Initial crystal growth produces a spongy core, containing abundant melt inclusions (MI), followed by deposition of fine laminae that form the crystal rims. Crystal size distributions suggest that these crystals have low nucleation rates, but nucleate and grow continuously through time. Trace and major element systematics, combined with Normarski imaging suggest that crystal cores are a result of rapid growth, whereas the rims are the result of slow growth coupled with repeated resorption episodes. The MI found in anorthoclase crystals are compositionally similar to matrix glass. The volatile contents of MI are low, suggesting that the crystals grew from degassed magma at depths of between 400 m and the surface. Degassing of the magma during ascent may trigger rapid, low-pressure crystallization.

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