HR: 1400h
AN: V43A-06    [Abstracts]
TI: Processes and Timescales of the Five Most Recent Eruptions of Fogo Volcano, Sao Miguel, Azores
AU: * Watanabe, S
EM: watanas@muohio.edu
AF: Miami University, Dept. of Geology, 114 Shideler Hall, Oxford, OH 45056, United States
AU: Widom, E
EM: widome@muohio.edu
AF: Miami University, Dept. of Geology, 114 Shideler Hall, Oxford, OH 45056, United States
AB: Fogo Volcano is an active stratovolcano located on Sao Miguel, Azores. Fogo summit eruptions typically involve volatile-rich trachytic magmas that give rise to explosive eruptions. Within the past 5,000 years, Fogo has been the source of five eruptions: Fogo A, B, C, D, and 1563AD in chronological order (volumes from 0.05-0.7 km³ d.r.e.). In order to understand the petrogenetic processes and timescales over which magma evolves beneath Fogo volcano, a detailed petrographic, major and trace element, Sr-Nd-Pb isotope and U-series study is underway. Representative samples from the five eruptions are all trachytes, exhibiting relatively limited major element variations but large trace element variations. Element-element diagrams show that each deposit has a distinct chemical signature with the exception of the Fogo B and C deposits, which are similar to one another. Samples generally have low Sr and Ba contents and negative Eu anomalies that correlate with SiO₂, indicative of compositional control by fractional crystallization of a sanidine-dominated mineral assemblage as proposed previously for the Fogo A and 1563AD deposits [1, 2]. However, fractionation of trace phases is likely responsible for differences between the five deposits in their incompatible trace element signatures. Previous studies of the Fogo A deposit have attributed ⁸⁷Sr/⁸⁶Sr variations (0.70490-0.70619; [2]) to syenite wall rock assimilation. The Fogo D deposit also shows ⁸⁷Sr/⁸⁶Sr variations (0.70491-0.70527) suggesting that similar open system process may have played a role. In contrast, Sr, Nd and Pb isotope signatures of samples from the Fogo B, C and 1563AD deposits show no variation and are similar to the unradiogenic end member of the Fogo A trachytes, consistent with closed-system evolution of these magmas. The recent detailed U-series study on the Fogo A deposit has shown that the time scale of differentiation was approximately 4.7 ky [3]. The results of U-series measurements on the remaining four deposits will allow us to constrain the time scales of differentiation and the relationship to eruptive volume. References [1] Widom et al. (1992) Contrib. Mineral. Petrol. 111: 311-328. [2] Snyder et al. (2004) J. Petrol. 45: 723-738. [3] Snyder et al. (in press) Chem. Geol.
DE: 1036 Magma chamber processes (3618)
DE: 3618 Magma chamber processes (1036)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly