HR: 1340h
AN: DI33A-1118    [Abstracts]
TI: Water and the Oxidation State of Global Arc and MORB Magmas
AU: * Kelley, K A
EM: kelley@gso.uri.edu
AF: Grad. Sch. of Oceanogrpahy, Univ. of Rhode Island, Narragansett, RI 02882, United States
AU: Cottrell, E
EM: cottrelle@si.edu
AF: Dept. of Mineral Sciences, Smithsonian Inst., Washington, DC 20013, United States
AU: Fischer, R
EM: FischerRA@si.edu
AF: Dept. of Mineral Sciences, Smithsonian Inst., Washington, DC 20013, United States
AU: Fischer, R
EM: FischerRA@si.edu
AF: Northwestern Univ., 633 Clark St., Evanston, IL 60208, United States
AB: Subduction zone magmas may derive from sources that have been subject to hydrous, oxidizing fluids, either within the mantle wedge or via alteration of oceanic lithosphere. Despite its fundamental importance, oxidation state is a difficult parameter to constrain from bulk erupted lavas since the initial magmatic oxidation state may change during late-stage degassing and eruption. To investigate the relationship between water content and oxidation state, we present here μ-XANES analyses of Fe speciation (Fe³⁺/ΣFe) in natural basaltic glasses from submarine pillows and olivine-hosted melt inclusions (MI) from a variety of global tectonic settings. Melt inclusions, trapped before large-scale degassing and crystallization take place, bypass the pitfalls of conventional bulk-rock techniques (e.g., `auto-oxidation' due to H outgassing) by providing direct samples of undegassed, H₂O-rich glasses that are ideal for microbeam analysis. Samples capture a full range of natural, undegassed, magmatic H₂O contents from mid-ocean ridges (0.2-0.5 wt%), back-arc basins (~1 wt%), and global arc volcanoes (0.3-6.4 wt%). In the arc MI, these new data show Fe³⁺/ΣFe ratios consistently higher (>QFM) than in the global MORBs (Cottrell et al., this meeting), and a general increase in Fe³⁺/ΣFe with increasing dissolved H₂O content. Neither the effects of post-entrapment crystallization, nor H diffusion through the host olivine, can fully explain this global trend. The data do suggest that post-entrapment Fe diffusion might affect the Fe content of olivine-hosted MI. Some MI indicate ferrous Fe contents too low to be in equilibrium with their olivine hosts, suggesting up to 20% loss of ferrous Fe from some MI. Accounting for these losses, however, does not significantly change the observation that Fe³⁺/ΣFe correlates with magmatic H₂O. This global correlation suggests that melt oxidation state is directly related to magmatic H₂O content, and that the high H₂O fluxes through subduction zone mantle wedges may thus play a central role in oxidizing arc magmas.
DE: 1030 Geochemical cycles (0330)
DE: 1036 Magma chamber processes (3618)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3621 Mantle processes (1038)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting