TY - JOUR
T1 - The role of open-system processes in the development of silicic magma chambers
T2 - A chemical and isotopic investigation of the Fogo A trachyte deposit, São Miguel, Azores
AU - Snyder, Darin C.
AU - Widom, Elisabeth
AU - Pietruszka, Aaron J.
AU - Carlson, Richard W.
N1 - Funding Information:
M. Horan and T. Mock are thanked for assistance with all isotope analyses at the Department of Terrestrial Magnetism. Mark Reagan, John Wolff and Dennis Geist provided thorough and insightful comments on an earlier draft. Financial support for this project was provided by the Geological Society of America (student research awards to D.S. 2000 and 2001), Sigma Xi (Grants-in-Aid of research awards to D.S. 2000 and 2001) and the National Science Foundation (NSF EAR 0207529 and EAR 9902956 awarded to E.W.)
PY - 2004/4
Y1 - 2004/4
N2 - 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 867Sr/86Sr: 0.7048-0.7062). Thorium isotopic variation is observed in glass separates, with (230Th/232Th)0 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 he the result of three distinct processes: contamination ofthe 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.
AB - 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 867Sr/86Sr: 0.7048-0.7062). Thorium isotopic variation is observed in glass separates, with (230Th/232Th)0 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 he the result of three distinct processes: contamination ofthe 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.
KW - Azores
KW - Open-system processes
KW - Sr isotopes
KW - Trachytic pumices
KW - Zoned magma chambers
UR - https://www.scopus.com/pages/publications/2142816733
U2 - 10.1093/petrology/egg104
DO - 10.1093/petrology/egg104
M3 - Article
AN - SCOPUS:2142816733
SN - 0022-3530
VL - 45
SP - 723
EP - 738
JO - Journal of Petrology
JF - Journal of Petrology
IS - 4
ER -