Large-volume, low-δ18O rhyolites of the central Snake River Plain, Idaho, USA

The Miocene Bruneau-Jarbidge and adjacent volcanic fields of the central Snake River Plain, southwest Idaho, are dominated by high-temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km(3). Samples from units representing at least 50% of this volume are strongly depleted in O-18, with magmatic feldspar delta(18)O(VSMOW) (Vienna standard mean ocean water) values between -1.4 parts per thousand and 3.8 parts per thousand. The magnitude of the O-18 depletion and the complete lack of any rhyolites with normal values (7 parts per thousand-10 parts per thousand.) combine to suggest that assimilation or melting of a calldera block altered by near-contemporaneous hydrothermal activity is unlikely. Instead, we envisage generation of the high-temperature rhyolites by shallow melting of Idaho Batholith rocks, under the influence of the Yellowstone hotspot, affected by Eocene meteoric-hydrothermal events. The seeming worldwide scarcity of strongly O-18-depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths.