Shallow-level decompression crystallisation and deep magma supply at Shiveluch Volcano

Abstract

Recent petrological studies indicate that some crustal magma chambers may be built up slowly by the intermittent ascent and amalgamation of small packets of magma generated in a deep-seated source region. Despite having little effect on whole-rock compositions, this process should be detectable as variable melt trace element composition, preserved as melt inclusions trapped in phenocrysts. We studied trace element and H2O contents of plagioclase- and hornblende-hosted melt inclusions from andesite lavas and pumices of Shiveluch Volcano, Kamchatka. Melt inclusions are significantly more evolved than the whole rocks, indicating that the whole rocks contain a significant proportion of recycled foreign material. H2O concentrations indicate trapping at a wide range of pressures, consistent with shallow decompression-driven crystallisation. The variation of trace element concentrations indicates up to ∼30% decompression crystallisation, which accounts for crystallisation of the groundmass and rims on phenocrysts. Trace element scatter could be explained by episodic stalling during shallow magma ascent, allowing incompatible element concentrations to increase during isobaric crystallisation. Enrichment of Li at intermediate pH2O reflects influx and condensation of metal-rich vapours. A set of “exotic melts”, identified by their anomalous incompatible trace element characteristics, indicate variable source chemistry. This is consistent with evolution of individual magma batches with small differences in trace element chemistry, and intermittent ascent of magma pulses.