Journal of Petrology Advance Access originally published online on August 16, 2007
Journal of Petrology 2007 48(9):1793-1812; doi:10.1093/petrology/egm038
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Carbonate Assimilation at Merapi Volcano, Java, Indonesia: Insights from Crystal Isotope Stratigraphy
1Department of Geology, the University of Dublin, Trinity College, Dublin 2, Ireland
2Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK
3Department of Mineralogy, University of Geneva, Rue Des Maraîchers, 13 CH-1205, Geneva, Switzerland
4School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
5School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK
6Geological Institute, Øster Voldgade 10, 1350 Copenhagen K, Denmark
RECEIVED JANUARY 29, 2007; ACCEPTED JULY 3, 2007
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Abstract |
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Recent basaltic andesite lavas from Merapi volcano contain abundant, complexly zoned, plagioclase phenocrysts, analysed here for their petrographic textures, major element composition and Sr isotope composition. Anorthite (An) content in individual crystals can vary by as much as 55 mol% (An40–95) across internal resorption surfaces with a negative correlation between high An mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analyses of zoned plagioclase phenocrysts show that the 87Sr/86Sr ratios of individual zones range from 0·70568 to 0·70627. The upper end of this range is notably more radiogenic than the host basaltic andesite whole-rocks (< 0·70574). Crystal zones with the highest An content have the highest 87Sr/86Sr values, requiring a source or melt with elevated radiogenic Sr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptive rocks contain abundant xenoliths, including metamorphosed volcanoclastic sediment and carbonate country rock (calc-silicate skarns) analysed here for petrographic textures, mineralogy, major element composition and Sr isotope composition. The xenoliths contain extremely calcic plagioclase (up to An100) and have whole-rock 87Sr/86Sr ratios of 0·70584 to 0·70786. The presence of these xenoliths and their mineralogy and geochemistry, coupled with the 87Sr/86Sr ratios observed in different zones of individual phenocrysts, indicate that magma–crust interaction at Merapi is potentially more significant than previously thought, as numerous crystal cores in the phenocrysts appear to be inherited from a metamorphosed sedimentary crustal source. This has potentially significant consequences for geochemical mass-balance calculations, volatile saturation and flux and eruptive behaviour at Merapi and similar island arc volcanic systems elsewhere.
KEY WORDS: assimilation; isotopes; Merapi; xenolith; calc-silicate
*Corresponding author. +35318962675. E-mail: chadwij{at}td.ie
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