Supereruptions of the Snake River Plain: Two-stage derivation of low-delta O-18 rhyolites from normal-delta O-18 crust as constrained by Archean xenoliths

Supereruptive volumes of low-delta O-18 rhyolites define the Snake River Plain-Yellowstone Plateau volcanic province, begging the question of the sources and the processes by which O-18-depleted magmas are generated. New analyses show that Archean crustal xenoliths from the central and eastern Snake River Plain have normal-delta O-18 signatures of 6 parts per thousand-9 parts per thousand, precluding them as a low-d18O source in the genesis of >10,000 km(3) of low-delta O-18 (delta O-18 < 6 parts per thousand) Snake River Plain rhyolites. Synthesis of O-Sr-Nd-Pb isotope data for Archean xenoliths and Snake River Plain magmas defines separate compositional fields of rhyolites and their crustal and mantle sources. Sr-O and Nd-O isotopic mixing models for the most recent volcanic fields in the eastern Snake River Plain, Yellowstone Plateau and Heise, show that normal-d18O rhyolites have variable crust (similar to 30%-50%) and mantle (similar to 50%-70%) proportions that are specific for each eruption. Low-delta O-18 rhyolites can be traced along a genetic array of mixing lines from normal-delta O-18 rhyolite end members to a low-delta O-18 (similar to-1 parts per thousand) source. The data support a two-stage magma genesis process, in which normal-delta O-18 rhyolites are generated by partial melting and hybridization of the crust by mantle-derived basalt, and low-delta O-18 rhyolites tap similar to 20%-80% of hydrothermally altered portions of normal-delta O-18 rhyolitic rocks. This two-stage magma genesis process may be applicable to other caldera systems around the world for which the characteristic O isotope depletions are either less pronounced or undiscovered.