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 zonationin silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Thisotope 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: 0·7049–0·7061; glass⁸⁷Sr/⁸⁶Sr: 0·7048–0·7052; sanidine ⁸⁷Sr/⁸⁶Sr:0·7048–0·7062). 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     

Syenite Nodules as a Long-Term Record of Magmatic Activity in Agua de Pao Volcano, Sao Miguel, Azores

Syenite nodules ejected during the Fogo A and Fogo C trachyteeruptions of Agua de Pao volcano, Sao Miguel, Azores, representbulk liquid compositions resembling the more evolved Fogo Atrachytes. Alteration due to the influx of a hydrous fluid hasdepleted the nodules in P₂O₅, Rb, U, Ba, and Sr, and increasedthe SiO₂ content. The nodules can be broadly divided into threetextural groups: (I) fresh, friable syenite with open miaroliticcavities and homogeneous sanidine; (III) dense syenite withexsolved alkali feldspar and no void space; (II) intermediatebetween (I) and (III), with small amounts of void space. U-seriesmodel ages range from <10000 years to 200000 years, suggestingthat trachytic magmatism has occurred in Agua de Pao volcanofor >200 ka. The youngest nodules are from the textural groupI and are cogenetic with the Fogo A trachytes, whereas thosefrom groups II and III are xenoliths and possibly correlatewith older volcanic events. A strong correlation of age withtexture shows that with increasing age the nodules become cooler,lose their void space, and undergo granulation and recrystallization.Considerations of the fluid dynamic regimes in the magma chamberduring crystallization of the nodules suggest that the bulkliquid composition nodules formed at the roof and/or upper wallsof the magma chamber, where crystallization rates exceed theconvection rate of residual liquid. We envision an onion-skinarrangement, with older syenite on the outside and young, cogeneticsyenite on the inside in contact with the magma. Long coolinghistories, continuous flux with hydrous fluids, and successiveexplosive eruptions cause the older syenite to evolve in compositionand texture. Fragmentation of the roof and upper sidewalls duringeruption can explain the occurrence of a range of nodule ages. ^* Present address: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road N.W., Washington, DC, 20015