Subduction Influence on Oxygen Fugacity and Trace and Volatile Elements in Basalts Across the Cascade Volcanic Arc

Fluids or melts derived from a subducting plate are often cited as a mechanism for the oxidation of arc magmas. What remains unclear is the link between the fluid, oxygen fugacity, and other major and trace components, as well as the spatial distribution of the impact of those fluids. To test the potential effects of addition of a subduction-derived fluid or melt to the sub-arc mantle, olivine-hosted melt inclusions from primitive basaltic lavas sampled from across the central Oregon Cascades (43°-45°N) have been analyzed for major, trace and volatile elements and fO 2 . Oxygen fugacity was determined in melt inclusions from sulfur speciation determined by electron microprobe and from olivine-chromite oxygen geobarometry. The overall range in fO 2 based on sulfur speciation measurements is from <-0·25 log units to + 1·9 log units (ΔFMQ, where FMQ is fayalite-magnetite-quartz buffer). Oxygen fugacity is positively correlated with fluid-mobile trace element and light rare earth element contents in basalts generated by relatively low-degree partial melting. Establishing a further correlation between fO 2 and fluid-mobile trace element abundances with position along the arc requires the basalts to be subdivided into shoshonitic, calc-alkaline, low-K tholeiite and enriched intraplate basalt groups. Melt inclusions from enriched intraplate and shoshonitic lavas show increasing fO 2 and trace element abundances closer to the trench, whereas calc-alkaline melt inclusions exhibit no significant across-arc variations. Low-K tholeiitic melt inclusions record an increase in incompatible trace elements closer to the trench; however, there is no correlated increase in fO 2 . The correlation observed in enriched intraplate and shoshonitic melt inclusions is interpreted to reflect a progressively greater proportion of a fluid-rich, oxidized subduction component in magmas generated nearer the subduction zone. Significantly, calc-alkaline melt inclusions with high ratios of large ion lithophile elements to high field strength elements, characteristic of ‘typical’ arc magmas, have oxidation states indistinguishable from low-K tholeiite and enriched intraplate basalt melt inclusions. The lack of across-arc geochemical variation in calc-alkaline melt inclusions may suggest that these basalts are not necessarily the most appropriate magmas for examining recent addition of a subduction component to the sub-arc mantle. Flux and batch melt model results produce a wide range of predicted amounts of melting and subduction component added to the mantle source; however, general trends characterized by increased melting and proportion of the subduction component from enriched intraplate, to low-K tholeiite, to calc-alkaline are robust. The model results do not require enriched intraplate, low-K tholeiite and calc-alkaline magmas to be produced from the same more fertile mantle source. However, enriched intraplate magmas, in contrast to calc-alkaline and low-K tholeiite magmas, cannot be generated from a depleted mantle source. Flux or batch melting of either the more fertile or depleted mantle sources used to generate the low-K tholeiite, calc-alkaline, and enriched intraplate magmas cannot reproduce shoshonitic compositions, which require a significantly depleted mantle source strongly metasomatized by a subduction component. The potential mantle source for shoshonitic basalts has a predicted fO 2 (after oxidation) from + 0·3 to + 2·4 log units (ΔFMQ) whereas the mantle source for low-K tholeiite, calc-alkaline, and enriched intraplate magmas may range from -1·1 to + 0·7 log units (ΔFMQ).