HR: 1340h
AN: V53B-1566    [Abstracts]
TI: Temporal Geochemical Variations in Lavas From Sakurajima Volcano, Japan
AU: * Nagle, A N
EM: ashley_nagle@brown.edu
AF: Brown Univ, Geology Dept, Providence, RI 02912
AU: Harpp, K S
EM: kharpp@mail.colgate.edu
AF: Colgate Univ, Geology Dept, Hamilton, NY 13346
AU: Geist, D J
EM: dgeist@uidaho.edu
AF: Geology Dept, Univ Idaho, Moscow, ID 83844
AB: In historic times, Sakurajima volcano has erupted effusively four times, in AD 1471, 1779, 1914, and 1946. Yanagi et al. (1991) examined the major element contents of these four flows and showed that from 1471 to 1946, lavas became progressively more mafic with time from an initial dacitic composition. On the basis of data from the historic eruptions alone, they proposed a two-chamber magmatic system in which a wedge-shaped plagioclase-pyroxene cumulate plug sinks between a dacitic upper chamber and a basaltic lower chamber to drive the mixing process. In our study, we collected samples from all exposed Sakurajima lavas, extending coverage to approximately 12,000 years of eruptive history. Major and trace element contents of the lavas determined by ICP-MS and XRF reveal that the trend observed in the historic eruptions is only the most recent part of a cyclic variation in magma composition. Major and trace element contents define sinusoidal curves with time, varying systematically between more mafic and more felsic compositions, in two cycles. The oldest prehistoric lavas exhibit increasingly mafic compositions with each subsequent eruption, ranging from 67 to 61 wt.% SiO₂. This pattern then temporarily reverses such that lavas become progressively more felsic prior to the 1471 eruption. After 1471, the mafic character of erupted material increases once again with time, as reported previously. Linear correlations of major and trace element compositions with SiO₂ concentration and disequilibrium phenocryst assemblages indicate that magma mixing is contributing to the chemical changes observed in Sakurajima lavas. Mass balance calculations reveal that Sakurajima lavas have incorporated up to 55% mafic material into a parental dacitic magma. The cumulate-plug-settling model is consistent with the historical trends but does not explain the cyclicity. Instead, we propose a model with a shallow chamber of dacitic magma that is repeatedly flushed with mafic magma from a deeper reservoir, resulting in progressively more mafic compositions. A quiescent period between injections of basaltic magma re-starts the system, and the magma chamber evolves towards the parental dacitic melt composition again.
DE: 1036 Magma chamber processes (3618)
DE: 1065 Major and trace element geochemistry
DE: 8400 VOLCANOLOGY
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005