HR: 0800h
AN: V41D-0808    [Abstracts]
TI: Across Arc Variation in Basaltic fO2: Influence of a Subduction Component in the Cascadia Subduction Zone
AU: * Rowe, M C
EM: michael-rowe@uiowa.edu
AF: Oregon State University, Department of Geosciences, Corvallis, 97331,
AU: * Rowe, M C
EM: michael-rowe@uiowa.edu
AF: University of Iowa, Department of Geoscience, Iowa City, 52242,
AU: Kent, A J
EM: adam.kent@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, 97331,
AU: Nielsen, R L
EM: nielsenr@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, 97331,
AB: Oxidation of the subarc mantle in subduction zones can greatly affect mineral phase equilibria, the speciation of volatiles, and the transfer of multivalent elements in basaltic magmas. While peridotite xenoliths provide the most direct approach to measuring mantle oxidation states, such xenoliths in continental arcs are rare. In this investigation, we applied an alternative method, the determination of sulfur speciation in olivine-hosted melt inclusions and chromite-olivine oxygen barometry. We present a first attempt to spatially correlate oxygen fugacity relative to the subduction zone in a continental arc. The overall range in oxygen fugacity, based on sulfur speciation measurements, is from <-0.25 log units to +1.9 log units (ΔFMQ). Sulfur oxidation and the concentration of fluid-mobile trace elements both generally increase from backarc to forearc. This correlation is interpreted to reflect a progressively greater proportion of fluid-rich, oxidized subduction component closer to the trench. Estimates of the amount of subduction component (up to ~6 wt%) required to generate the geochemical diversity based on flux melt modeling correlate with oxygen fugacity, with the exception of calc-alkaline basalts, with high oxygen fugacity and greater proportion subduction component closer to the trench. Two other important observations to come from the flux melt modeling are 1) as basalt oxygen fugacity increases, calculated mantle temperature decreases, and 2) shoshonitic basalts require a depleted mantle source, distinct from the more enriched mantle source of the low-K tholeiite, calc-alkaline and ocean island-like magmas. The potential mantle source for shoshonitic basalts has a predicted oxygen fugacity from +0.3 to +2.4 log units (ΔFMQ) while the mantle source for low-K tholeiite, calc-alkaline and ocean island-like basalts may range from -1.1 to +0.7 log units (ΔFMQ), consistent with estimates of oxidation state for oceanic lithosphere. Therefore, despite the volatile and fluid-mobile trace element enrichment, the subarc mantle need not be significantly oxidized relative to unmodified oceanic lithosphere to generate the diversity in Cascade arc basaltic magmas.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting