HR: 1340h
AN: V43A-1418    [Abstracts]
TI: {\it f}O$_{2}$ Determination by Sulfur K$\alpha$ Peak Shift and Olivine-Spinel Oxygen Barometry: Implications for Mantle Wedge Heterogeneities in the Cascadia Subduction Zone
AU: * Rowe, M C
EM: rowem@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97331 United States
AU: Nielsen, R L
EM: nielsenr@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97331 United States
AU: Kent, A J
EM: adam.kent@science.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97331 United States
AB: Oxidation states of primitive, subaerially-erupted basalts, in the Oregon segment of the Cascade volcanic arc, have been determined by S K$\alpha$ peak shift measurements and olivine-spinel oxygen barometry. Olivine-hosted melt inclusions, with sulfur concentrations ranging from 0.080 to 0.503 wt.% S, have oxidation states varying from -0.8 to +1.8 log units, relative to the QFM oxygen buffer. High-K basalts have sulfur concentrations from 0.27 to 0.503 wt.% S with corresponding oxygen fugacities of +1.2 to +1.8 log units ($\Delta$QFM). Calc-alkaline basalts and OIB-like basalts have a greater range in log {\it f}O$_{2}$ (-0.8 to +1.0 $\Delta$QFM), significantly lower than the high-K basalts. Olivine-spinel oxygen barometry corresponds reasonably well with {\it f}O$_{2}$ determinations by S K$\alpha$ peak shift measurements. Preliminary analysis of olivine-spinel pairs from another high-K forearc lava suggests oxidation states may reach values greater than +3 log units ($\Delta$QFM). Additionally, high oxidation states have been calculated for several olivine-spinel pairs from OIB-like lavas. Coupled with increased sulfur and chlorine concentrations in these lavas, this may suggest the influence of a more oxidized source region, and/or metasomatic agent, for the generation of these magmas. Although previous studies have indicated that the oxidation states of basalts may not be truly representative of the mantle oxidation state, the relative difference between oxygen fugacities of various basalts may correlate to heterogeneities in the source region. This is expressed, in terms of {\it f}O$_{2}$, as a dichotomy between the groups of lavas. The high {\it f}O$_{2}$ suggested for the Oregon Cascade forearc lavas may result from metasomatic enrichment of a depleted mantle source. This is similar to recent modeling presented for the forearc region of the Mexican Volcanic Arc, and supported by the presence of relatively oxidized harzburgite xenoliths from the southern Washington Cascade backarc, believed to result from metasomatic enrichment of previously depleted peridotite.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting