OXYGEN ISOTOPE STUDY OF ARCHEAN XENOLITHS CONSTRAINS THE SOURCE OF SUPERVOLCANIC RHYOLITES IN THE SNAKE RIVER PLAIN-YELLOWSTONE PLATEAU VOLCANIC PROVINCE

Extreme “supervolcanic” volumes of low-δ¹⁸O rhyolitic magmas characterize the Snake River Plain-Yellowstone Plateau volcanic province, yet constraining the oxygen isotope composition of the crustal source of these rhyolites has relied upon deduction from analysis of volcanic rocks rather than direct evidence of the crust at depth. Here we present oxygen isotope compositions of 20 Archean crustal xenoliths from the central and eastern Snake River Plain to constrain the oxygen isotope signature of the underlying cratonic North American crust. All xenolith samples possess normal-δ¹⁸O signatures of 6-9‰, precluding a low-δ¹⁸O crustal source in the genesis of >11,000km³ of low-δ¹⁸O (δ¹⁸O<6‰) Snake River Plain rhyolites. We combine the new oxygen isotope data reported here with published strontium and neodymium isotope data to define compositional fields of Snake River Plain magmas and their crustal and mantle sources. Using isotopic data for two of the most recent and well-studied volcanic fields in the Snake River Plain, Yellowstone-Plateau and Heise, we constructed isotopic mixing models of Sr, Nd, and O. These indicate ~70-80‰ crust and ~20-30‰ mantle contribution in the genesis of normal-δ¹⁸O rhyolites. Low-δ¹⁸O rhyolites can be traced along a genetic array of mixing lines from normal-δ¹⁸O rhyolite endmembers to a common -1.5‰ low-δ¹⁸O source. Low-δ¹⁸O rhyolites of the Bruneau-Jarbidge volcanic field also fit the Yellowstone-Heise trend. We advocate a two stage magma genesis process, in which normal-δ¹⁸O rhyolites are generated by partial melts of the crust and plume, and low-δ¹⁸O rhyolites tap ~20-80‰ of hydrothermally altered portions of normal-δ¹⁸O rhyolitic rocks.