HR: 16:00h
AN: V34A-01    [Abstracts]
TI: Volatile Contents in Mafic Magmas from two Aleutian volcanoes: Augustine and Makushin
AU: * Zimmer, M M
EM: mzimmer@bu.edu
AF: Dept. Earth Sciences, Boston University, Boston, MA 02215
AU: Plank, T
EM: tplank@bu.edu
AF: Dept. Earth Sciences, Boston University, Boston, MA 02215
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015
AU: Nye, C
EM: cnye@giseis.alaska.edu
AF: Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775
AU: Faust Larsen, J
EM: faust@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964
AB: There are several competing theories for the origin of tholeiitic (TH) vs. calc-alkaline (CA) fractionation trends in arc magmas. One relates to water (TH-dry magma, CA-wet magma), another to pressure (TH-low pressure crystallization, CA-high pressure), and a third to primary magma composition (TH-low Si/Fe\#, CA-hi Si/Fe\#) These theories have been difficult to test without quantitative measures of the water contents and pressures of crystallization of arc magmas. We are in the process of studying several Aleutian arc tephra suites (phenocrysts and melt inclusions) with the aim of obtaining volatile element concentrations (by SIMS), major and trace element concentrations and thermobarometric data (by EMP and laser-ICPMS). We report preliminary results on olivine-hosted melt inclusions from Augustine and Makushin volcanoes that support the role of water in calc-alkaline fractionation. Basaltic melt inclusions from Augustine, a low-K$_{2}$O, calc-alkaline volcano, are hosted in Fo$_{80-82}$ olivine. The inclusions yield high water contents, up to 5 wt%, and contain 60-90 ppm CO$_{2}$, 3000-4500 ppm S, and 3000-6000 ppm Cl. Inclusions record vapor-saturation pressures near 2 kbar. Cl/K$_{2}$O ratios in Augustine inclusions (ave. 1.9) are among the highest documented in an arc setting, and likely record a Cl- and H$_{2}$O- rich fluid from the subducting plate. High water contents in Augustine primary melts may have contributed to the strong calc-alkaline trend observed at this volcano. Basaltic melt inclusions from Pakushin, a medium-K$_{2}$O, tholeiitic cone on the flanks of Makushin volcano, are hosted in Fo$_{80-86}$ olivine. These inclusions have low water contents ($<$0.15 wt%) and low CO$_{2}$ contents ($<$125 ppm), and record shallow vapor saturation pressures ($<$300 bars). The high sulfur (2000-4000 ppm) and Cl ($>$2000 ppm) in Pakushin melt inclusions, however, indicate that degassing was minimal. The low water contents and low vapor saturation pressures recorded in Pakushin melt inclusions are consistent with development of its tholeiitic trend, but we cannot distinguish whether the low water contents at Pakushin reflect a difference in the primary magma, or in crustal stresses that favor low pressure fractionation and degassing. These questions will be addressed with further work on the trace element contents of the melt inclusions and on phenocryst-melt geobarometers.
DE: 3640 Igneous petrology
DE: 1030 Geochemical cycles (0330)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting