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

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-87/Sr-86: 0.7049-0.7061; glass Sr-87/Sr-86: 0.7048-0.7052; sanidine Sr-87/Sr-86: 0.7048-0.7062). Thorium isotopic variation is observed in glass separates, with (Th-230/Th-232)(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.