HR: 1340h
AN: V13B-0530    [Abstracts]
TI: Xenocrystic Material in Arc Volcanics: Example from Recent Merapi Lavas
AU: * Chadwick, J P
EM: chadwij@tcd.ie
AF: The University of Dublin, School of Natural Sciences, Dept. of Geology, Trinity College, Dublin, 2 Ireland
AU: Troll, V R
EM: trollv@mail.tcd.ie
AF: The University of Dublin, School of Natural Sciences, Dept. of Geology, Trinity College, Dublin, 2 Ireland
AU: Ginibre, C
EM:
AF: University of Geneva, Department of Mineralogy, Rue des Maraichers, Geneva, 13 CH-1205 Switzerland
AU: Morgan, D
EM:
AF: University of Durham, Department of Earth Sciences, South Road, Durham, DH1 3LE United Kingdom
AU: Gertisser, R
EM:
AF: The Open University, Department of Earth Sciences, Milton Keynes, MK7 6AA United Kingdom
AU: Waight, T E
EM:
AF: Geological Institute/Danish Lithosphere Centre, Oster Voldgade 10, Copenhagen, 1350 K Denmark
AU: Davidson, J P
EM:
AF: University of Durham, Department of Earth Sciences, South Road, Durham, DH1 3LE United Kingdom
AB: Recent basaltic-andesite from Merapi volcano contains abundant complexly zoned plagioclase crystals, which we have analysed for petrographic textures, major element composition and Sr isotope composition. Electron microprobe traverses of these phenocrysts have revealed individual crystals with variations in An content between 40 and 95 mol% across resorption surfaces. In addition, there appears to be a negative correlation between An content and other indicators of magmatic evolution, such as MgO wt% and FeO wt%. In-situ Sr isotope analyses of these samples show Sr ratios of discrete zones in plagioclase phenocrysts that range from 0.70568 to 0.70627 (± 1). This range notably exceeds that seen in the recent host basaltic-andesite whole rocks (< 0.705737). Often, zones with the highest An content also have the highest radiogenic Sr values, and low MgO, which is indicative of a Ca-rich, MgO-poor, high radiogenic Sr contaminant. Recent Merapi deposits contain numerous xenoliths of metamorphosed limestone with feldspar of up to An95 and whole rock Sr ratios of 0.70584 to 0.70786 (± 2). The presence of these inclusions coupled with the Sr ratios observed in crystal zones of individual lava phenocrysts, indicates that magma crust interaction may be far more significant than previously thought, as numerous crystal cores in the lava appear to be inherited from a foreign source. This poses consequences for eruptive behaviour, mass-balance calculations, and hazard assessment at Merapi and similar island arc volcanic systems elsewhere.
DE: 1036 Magma chamber processes (3618)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005