POTENTIAL SOURCE CONTRIBUTIONS FOR LOW δ18O RHYOLITES IN THE CENTRAL SNAKE RIVER PLAIN

This study is intended to assess potential source materials for the ~8-14 Ma, low d¹⁸O rhyolites of the Central Snake River Plain (CSRP) in southern Idaho, USA. These voluminous deposits of rhyolitic ignimbrites and lavas represent the middle phase of volcanism associated with the Columbia-Snake-Yellowstone melting anomaly (COSYMA), and are considered to be the result of mantle derived basalts intruding into the continental crust. The CSRP rhyolites represent the largest known low d¹⁸O province (>10,000 km³), and their extremely low magmatic d¹⁸O values (-1.5‰ to 3.8‰) appear to require a large volume of low d¹⁸O source material. At present, the only known regional example of such a source is hosted within the Idaho batholith granites as fossil hydrothermal systems associated with Eocene volcanic activity.

In order to determine the possible mass contributions of crustal and mantle material in the final eruptive products, simple isotope mixing models were constructed using existing and new radiogenic isotope data (⁸⁷Sr/⁸⁶Sr = 0.709 - 0.711; ¹⁴³Nd/¹⁴⁴Nd = 0.5121 - 0.5123) and assessed for consistency with O isotopes within the CSRP rhyolites.

Isotopic values for the mafic mixing component were chosen from a variety of basaltic liquids associated with COSYMA volcanism. These include the Steens/Picture Gorge/Columbia River basalts and the younger post-rhyolitic Snake River Plain basalts. Isotopic values for the crustal mixing components were chosen from averages of published data for Cretaceous/Eocene granites in the Idaho batholith. Though any mixing model is strongly dependent on the choice of sources, our results suggest that the Idaho batholith cannot be ruled out as the dominant source component for the CSRP rhyolites. The more radiogenic, younger basalts from the SRP require mixing proportions too high (≥ 50%) to satisfy the d¹⁸O values observed in the CSRP rhyolites, but 10-20% fractions of primitive basaltic liquids associated with COSYMA volcanism (e.g. Steens basalts) can be be mixed with granitic fractions as high as 80-90% to produce isotopic values for O, Sr, and Nd consistent with CSRP rhyolites.