Abstract
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 Sr/Sr: 0 7049---0 7061; glass Sr/Sr: 0 7048---0 7052; sanidine Sr/Sr: 0 7048--0 7062). Thorium isotopic variation is observed in glass separates, with (Th/Th)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 posteruptive 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.
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