HOW HOT IS A HOTSPOT? EVALUATION OF TEMPERATURE AND DISSOLVED H2O CONTENTS OF THE YELLOWSTONE AND SNAKE RIVER PLAIN BASALTS USING OLIVINE-MELT THERMOMETRY AND HYGROMETRY AT THE LIQUIDUS

The mantle source for Quaternary volcanism in the Basin and Range province, which formed due to the extension and decompression of the underlying mantle, can be split into (1) subduction modified lithosphere and (2) underlying asthenosphere. A third source of voluminous Quaternary volcanism is the Yellowstone (YS) mantle plume and the associated Snake River Plain (SRP) basalts, which reflect a combination of a thermal plume and an enriched subduction modified lithospheric mantle. The goal of this study is to apply olivine-melt thermometry (based on D_Mg^oliv/liq and D_Ni^oliv/liq) and hygrometry to evaluate temperature and dissolved water content at the liquidus. These results are then used to determine if the Yellowstone and Snake River Plain basalts are unusually hot and/or hydrous compared to other localities within the Basin and Range province. Results indicate the YS/SRP basalts have onset temperatures between 1204-1133°C and ∆T (=T_Mg – T_Ni) values that correspond to an average minimum melt water content of 0.9 ± 0.6 wt% H₂O. There is no observable difference in dissolved H₂O content, melt fO_2, or temperature between the YS and SRP basalts. Calculated temperatures are not anomalously hot compared to other asthenospheric melts including MORBs and Dish Hill (Mojave, CA). All YS/SRP basalts are shifted to significantly lower whole-rock Mg# compared to the Mexican arc, Big Pine Volcanic Field, and the Long Valley basalts, and show a narrow range in T_Ni over a comparably large range in Mg#. The low Mg# and hydrous nature of the YS/SRP basalts suggests that the YS plume is more fertile compared to the surrounding asthenosphere. The hydrous nature of the YS/SRP basalts and low Mg# over a similar temperature range may readily explain why these basalts erupt much more voluminously compared to other localities in the Basin and Range province.