Re-Os isotopic systematics of primitive lavas from the Lassen region of the Cascade arc, California

Rhenium-osmium isotopic systematics of primitive calc-alkaline lavas from t he Lassen region appear to be controlled by mantle wedge processes. Lavas w ith a large proportion of slab component have relatively low Re and Os abun dances, and have radiogenic Os and mid ocean ridge basalt-like Sr and Pb is otopic compositions. Lavas with a small proportion of slab component have h igher Re and Os elemental abundances and display mantle-like Os, Sr, Nd, an d Pb isotopic compositions. Assimilation with fractional crystallization ca n only generate the Re-Os systematics of the Lassen lavas from a common par ent if the distribution coefficient for Re in sulfide is similar to 40-1100 times higher than most published estimates and if most incompatible elemen t abundances decrease during differentiation. High Re/Os ratios in mid ocea n ridge basalts makes subducted oceanic crust a potential source of radioge nic Os in Volcanic arcs. The slab beneath the southernmost Cascades is esti mated to have Os-187/Os-188 ratios as high as 1.4, Mixing between a slab co mponent and mantle wedge peridotite can generate the Os isotopic systematic s of the Lassen lavas provided the slab component has a Sr/Os ratio of simi lar to 7.5 x 10(5) and Os abundances that are 100-600 times higher than mid ocean ridge basalts. For this model to be correct, Os must be readily mobi lized and concentrated in the slab component, perhaps as a result of high w ater and HCl fugacities in this subduction environment. Another possible me chanism to account for the correlation between the magnitude of the subduct ion geochemical signature and Os isotopic composition involves increasing t he stability of an Os-bearing phase in mantle wedge peridotites as a result of fluxing with the slab component. Melting of such a source could yield l ow Os magmas that are more susceptible to crustal contamination, and hence have more radiogenic Os isotopic compositions, than magmas derived from sou rces with a smaller contribution from the slab. Thus, the addition of the s lab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic Os to are magmas. (C) 2000 Elsevier Scienc e B.V. All rights reserved.