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Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract xxxxx-xx,
2003
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HR: 16:15h AN: V12H-02 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