HR: 09:30h
AN: V11A-07    [Abstracts]
TI: Magma Evolution and Open System Processes at Shiveluch Volcano: Insights From Phenocryst Zoning and Melt Inclusions
AU: * Humphreys, M C
EM: madeleine.humphreys@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: Blundy, J D
V11A-07 AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: Sparks, S
V11A-07 AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ United Kingdom
AB: This study identifies open processes relating to magma evolution and mixing at Shiveluch Volcano, Kamchatka, from phenocryst zoning and melt inclusion chemistry. Shiveluch lavas are hornblende-plagioclase andesite with average pre-eruptive temperatures of 857 °C and high fO₂ (NNO+1.5 to NNO+2). Phenocryst zoning in plagioclase, hornblende and apatite is diverse, with oscillatory, simple, multiple and patchy zoning and sieve textures identified. Each phenocryst type records a distinct magmatic history, and different populations are spatially and temporally separated during crystallisation. In patchy plagioclase, melt inclusions are associated with low-An patches, while in patchy hornblende, they are associated with Al-rich patches. Ba, Sr systematics indicate that patchy phenocryst cores are derived from cumulate material. The patchy texture forms by resorption during H₂O-undersaturated decompression. No external change in melt chemistry occurs, but decreasing XAn in plagioclase results from decreasing pH₂O and drives the compositional changes in hornblende. Abundant evidence for magma mixing events is also observed. Mixing with hotter, mafic magma results in olivine and orthopyroxene xenocrysts with reaction rims and strong normal zoning. Mixing with cooler, felsic magma results in sieve plagioclase with Mg, Fe-rich rims. Cryptic mixing, between magmas of similar compositions, can result in phenocryst zoning if there are differences in volatile contents. Sulphur-rich magma batches release SO₂ and O₂ as anhydrite breaks down, oxidising the surrounding melt and causing simple zoning in hornblende and apatite. Cryptic mixing reflects the intermittent ascent and assimilation of small magma pulses, which coalesce to form larger subvolcanic magma bodies. At shallower levels, oscillatory plagioclase and rims on patchy plagioclase crystallise during ascent in the conduit. Crystallisation is accompanied by oxidation, and by a temperature increase of ~80 °C as latent heat is released.
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005