Large-volume, low-δ 18 O 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 18 O, with magmatic feldspar δ 18 O VSMOW (Vienna standard mean ocean water) values between −1.4‰ and 3.8‰. The magnitude of the 18 O depletion and the complete lack of any rhyolites with normal values (7‰–10‰) combine to suggest that assimilation or melting of a caldera 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 18 O-depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths.