Re and Os isotopes of the central Oregon Cascades and along the arc indicate variable homogenization and mafic growth in the deep crust

Abstract

The rhenium–osmium isotopic system has the potential to track the maturation of arc crust at lower crustal levels because the 187Os/188Os in magmas is sensitive to the age and composition of crust with which they interact. We here present Re and Os isotopic data for a suite of samples from the extensional central Oregon Cascade arc that includes diverse, primitive (high Mg) basalts and low- and medium-K basaltic andesites from North Sister and Little Brother volcanoes. The basaltic andesites contain higher 187Os/188Os than the two most common basalt types, calc-alkaline basalt and low-K tholeiites (0.17–0.19 vs. 0.14–0.15), indicating interaction with mafic crust. The evolution of 187Os/188Os in the deep crust depends on Re partitioning that in turn depends on sulfide and magnetite stability and oxygen fugacity. We estimate bulk mineral-melt partition coefficients for Re to be ∼4 at fO2≈QFM and ∼10 at fO2≈+1 to +3 ΔQFM in keeping with near liquidus phase equilibria at deep crustal conditions (Mercer and Johnston, 2008) and on fO2 of central Oregon primitive magmas (Rowe et al., 2009). Modeling fractional crystallization of the central Oregon primitive basalts indicates that the deep arc crust is likely to have 187Re/188Os similar to the range measured in North Sister basaltic andesites (∼55–260). The age of the mafic crustal contaminant in the central Oregon Cascades is then estimated to be 9–29Ma and is younger than the start of Cascade arc volcanism at ∼45Ma in central Oregon. The range in 187Os/188Os at the Cascade volcanic centers Mt. Lassen and Mt. Adams is greater than is observed for central Oregon arc magmas. We estimate a range in mafic crustal contaminant ages of 8–340Ma for Mt. Lassen and 0–70Ma for Mt. Adams and are older than suggested by previous studies (Hart et al., 2002, 2003). Mafic contaminant age ranges reflect the degree to which the crust is homogenized, or the extent to which the composition of the crust is a product of recent arc magmatism vs. pre-existing protoliths. Intra-arc extension in central Oregon facilitates homogenization and leads to assimilation of relatively young arc crust. Where extension is less narrowly focused, the range in Re and Os isotopes is greater and reflects isotopic heterogeneities, including variably aged crust at Mt. Lassen or variable enrichment of Re and Os in the lithosphere beneath Mt. Adams.