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

• Main Authors: Madeleine C. S. Humphreys, Jon D. Blundy, R. Stephen, J. Sparks
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2006
• Subjects: Kamchatka
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.520.4001; http://petrology.oxfordjournals.org/content/47/12/2303.full.pdf

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 840C and fO2 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 H2O-undersaturated decompression. When H2O-saturated crystallization recommences at lower