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Journal of Petrology | Volume 45 | Number 4 | Pages 723-738 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.

The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, São Miguel, Azores

DARIN C. SNYDER1,*, ELISABETH WIDOM1, AARON J. PIETRUSZKA2,{dagger} and RICHARD W. CARLSON2

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

* Corresponding author. Telephone: (513) 529-3217. Fax: (513) 529-1542. E-mail: snyderdc{at}muohio.edu

The processes operating in the development of chemical zonation in silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Th isotope study of the chemically zoned trachyte pumice deposit of the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variation is observed in whole rocks, glass separates and sanidine phenocrysts (whole-rock 87Sr/86Sr: 0·7049–0·7061; glass 87Sr/86Sr: 0·7048–0·7052; sanidine 87Sr/86Sr: 0·7048–0·7062). Thorium isotopic variation is observed in glass separates, with (230Th/232Th)o ranging from 0·8737 to 0·8841, and exhibiting a negative correlation with Sr isotopes. The Nd, Pb and Hf isotopic compositions of the whole-rock trachytic pumices are invariant and indistinguishable from basalts flanking the volcano. The Sr isotope variations in the whole rocks are proposed to be the result of three distinct processes: contamination of the Fogo A magma by assimilation of radiogenic seawater-altered syenite wall rock, to explain the Sr and Th isotopic compositions of the glass separates; incorporation of xenocrysts into the trachytic magma, required to explain the range in feldspar Sr isotopic compositions; and post-eruptive surface alteration. This study emphasizes the importance of determining the isotopic composition of glass and mineral separates rather than whole rocks when pre-eruptive magmatic processes are being investigated.

KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers


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