HR: 11:50h
AN: V12A-07    [Abstracts]
TI: Melt Inclusion Evidence for Both Water-Fluxed and Decompression Melting at Galunggung, Indonesia
AU: * Kelley, K A
EM: kelley@dtm.ciw.edu
AF: Carnegie Institution of Washington, Dept. of Terrestrial Magnetism 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Hauri, E
EM: hauri@dtm.ciw.edu
AF: Carnegie Institution of Washington, Dept. of Terrestrial Magnetism 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Sisson, T W
EM: tsisson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: A water flux from the slab into the mantle wedge has long been cited as the major cause of melting beneath arc volcanoes, but evidence increasingly favors pressure-release melting as contributing significantly to arc magmatism. Galunggung, Indonesia, is a classic example of an arc-front volcano where decompression melting plays an important part. We present new SIMS measurements of volatile elements (H$_{2}$O, CO$_{2}$, S, Cl, F) in olivine-hosted melt inclusions (n=60) from two high-Mg basalt bombs from the 1982-83 Galunggung eruption. These inclusions are <100 æm dia. and each consists of naturally clear, brown glass with a single vapor bubble. A previous melt inclusion study using these bombs revealed H$_{2}$O-poor basaltic melts atypical of arc-front volcanoes (Sisson & Bronto, 1998), but the new data show two populations of magmatic volatile compositions at Galunggung. The dominant group comprises nearly dry (0.08-0.52 wt.% H$_{2}$O) melts similar to those reported by Sisson & Bronto (1998). The SIMS data are also the first to reveal a small group of melt inclusions in the Mg basalt (n=11) that are wet (1.4-2.5 wt.% H$_{2}$O) and more typical of pre-eruptive H$_{2}$O concentrations measured at mafic arc volcanoes. The CO$_{2}$, S, Cl and F contents, however, overlap between the wet and dry melts. CO$_{2}$ (850-0 ppm) and S (2700-70 ppm) co-vary along a trend consistent with simultaneous degassing, but Cl (1000-2000 ppm) and F (200-400 ppm) cluster together, suggesting the inclusions were trapped before Cl or F degassing initiated. Early stages of the 1982-83 Galunggung eruption yielded amphibole-bearing andesites, so the discovery of hydrous melts is not unexpected. The presence of discrete wet and dry melts at the same volcano (or within the same hand sample), however, indicates that decompression and water-fluxing both cause significant melting beneath this arc. Widespread mantle wedge upwelling may sufficiently drive pressure-release melting, but fluid infiltration is also prevalent, enhancing extents of melting and producing hydrous primary magmas. Portions of the upwelling wedge that escape fluid infiltration could yield dry melts that exploit the same shallow magma feeder systems as hydrous magmas.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting