Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning

• Published in: Journal of Petrology
• Main Authors: HUMPHREYS, MADELEINE C. S., BLUNDY, JON D., SPARKS, R. STEPHEN J.
• Format: Text
• Language: English
• Published: Oxford University Press 2006
• Subjects: Article; Kamchatka
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/egl045v1; https://doi.org/10.1093/petrology/egl045

Phenocryst zoning patterns are used to identify open-system magmatic processes in the products of the 2001 eruption of Shiveluch Volcano, Kamchatka. The lavas and pumices studied are hornblende-plagioclase andesites with average pre-eruptive temperatures of ∼840°C and f O 2 of 1·5-2·1 log units above nickel-nickel oxide (NNO). Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Apatite microphenocrysts display normal and reverse zoning of sulphur. The textural similarity of patchy hornblende and plagioclase, together with Ba-Sr systematics in patchy plagioclase, indicate that the cores of these crystals were derived from cumulate material. Plagioclase-liquid equilibria suggest that the patchy texture develops by resorption during H 2 O-undersaturated decompression. When H 2 O-saturated crystallization recommences at lower pressure, reduced p H 2 O results in lower X An in plagioclase, causing more Al-rich hornblende to crystallize. Plagioclase cores with diffuse oscillatory zoning, and unzoned hornblende crystals, probably represent a population of crystals resident in the magma chamber for long periods of time. In contrast, oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization. Increasing Fe/Al in oscillatory zoned rims suggests oxidation as a result of degassing of H 2 O during decompression. A general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization. We present evidence that the host andesite has mixed with both more felsic and more mafic magmas. Olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt. Sieve-textured plagioclase resulted from mixing of a more felsic magma with the host andesite. The mineralogy and mineral compositions of a mafic andesite ...