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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