HR: 16:15h
AN: V12H-02    [PDF]
TI: A Melt-Inclusion Study of Trace-Metal Behavior During Degassing of Basaltic Magma at Miyake-Jima Volcano (Izu-Bonin Arc, Japan)
AU: * de Hoog, C (
EM: cees-jan@gvc.gu.se
AF: Geology, Dept. of Earth Sciences, University of Gothenburg Box 460, G\"{o}teborg, 405 30 Sweden
AU: Hattori, K H
EM: khattori@uottawa.ca
AF: Dept. of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5 Canada
AB: Following its eruptions in the summer of 2000, Miyake-jima volcano discharged on average 40 kton SO$_{2}$/day for over a year, the highest SO$_{2}$ flux in the world at the time. We used juvenile pyroclastic fragments of the June 27 (submarine) and August 18 (subaerial near the summit) eruptions to study trace-element behavior during degassing. The fragments are medium-K calc-alkaline basalts (51-53 wt% SiO$_{2}$, 4% MgO, 9-11% CaO, 2.1-2.7% Na$_{2}$O) with high concentrations of chalcophile elements, most notably Cu. Sulfides have not been observed in these samples. Melt inclusions (5-300 $\mu$m) are common in plagioclase phenocrysts and consist of brown glass with occasionally vapor bubbles. They show little compositional variation (52 wt% SiO$_{2}$, 5.1% MgO, 9.5% CaO, 2.3% Na$_{2}$O) and no significant differences between subaerial and submarine samples. Sulfur concentrations in melt inclusions are high, $\sim$900 ppm, compared to those in groundmass glass, $\sim$70 ppm, indicating significant sulfur loss after the entrapment of melt inclusions. However, no decrease is observed for the concentrations of any trace elements, not even the chalcophile or volatile elements (such as Cu, Zn, As, Sb, and Pb), except Bi. We conclude that large-scale open-system degassing at Miyake-jima did not mobilize trace elements in significant amounts. Comparable K/Cl ratios of melt inclusions and groundmass glass imply that little or no chlorine was lost from the magma, in accordance with its high solubility in mafic melts at low pressures. High-{\it T} fumarole studies and thermodynamic modeling indicate that many metals are transported as volatile chloride-complexes, which may explain the limited mobility of trace metals reported here. Our findings indicate that, at magmatic temperatures, sulfur only plays a limited role in the transport of metals across the melt-vapor interface.
DE: 1065 Trace elements (3670)
DE: 8409 Atmospheric effects (0370)
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting