The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, Sao Miguel, Azores |
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Authors: | SNYDER, DARIN C. WIDOM, ELISABETH PIETRUSZKA, AARON J. CARLSON, RICHARD W. |
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Affiliation: | 1 DEPARTMENT OF GEOLOGY, MIAMI UNIVERSITY, OXFORD, OH 45056-1300, USA 2 DEPARTMENT OF TERRESTRIAL MAGNETISM, CARNEGIE INSTITUTION OF WASHINGTON, 5241 BROAD BRANCH ROAD, NW, WASHINGTON, DC 20015, USA |
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Abstract: | The processes operating in the development of chemical zonationin silicic magma chambers have been addressed with a SrNdPbHfThisotope study of the chemically zoned trachyte pumice depositof the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variationis observed in whole rocks, glass separates and sanidine phenocrysts(whole-rock 87Sr/86Sr: 0·70490·7061; glass87Sr/86Sr: 0·70480·7052; sanidine 87Sr/86Sr:0·70480·7062). Thorium isotopic variationis observed in glass separates, with (230Th/232Th)o rangingfrom 0·8737 to 0·8841, and exhibiting a negativecorrelation with Sr isotopes. The Nd, Pb and Hf isotopic compositionsof the whole-rock trachytic pumices are invariant and indistinguishablefrom basalts flanking the volcano. The Sr isotope variationsin the whole rocks are proposed to be the result of three distinctprocesses: contamination of the Fogo A magma by assimilationof radiogenic seawater-altered syenite wall rock, to explainthe Sr and Th isotopic compositions of the glass separates;incorporation of xenocrysts into the trachytic magma, requiredto explain the range in feldspar Sr isotopic compositions; andpost-eruptive surface alteration. This study emphasizes theimportance of determining the isotopic composition of glassand mineral separates rather than whole rocks when pre-eruptivemagmatic processes are being investigated. KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers |
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Keywords: | : Azores open-system processes Sr isotopes trachytic pumices zoned magma chambers |
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