A GEOCHEMICAL COMPARISON OF THE DEATH VALLEY AND CRATER FLAT VOLCANIC FIELDS, GREAT BASIN, USA

Volcanism is an important issue for characterization of the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. Due to recent legal decisions that now require DOE to evaluate hazards over both 10,000 year and 1,000,000 year compliance periods, the definition of the area of interest for calculation of disruption probability and a knowledge of the volcanic process have become more important. New geochemical data for the Death Valley volcanic field in the Greenwater Range in Inyo County, California indicate that the Death Valley field and the Crater Flat field near Yucca Mountain are parts of the same volcanic field. The Death Valley field is just 35 km south of Yucca Mountain. Trace elements for both areas show a negative Nb anomaly, but differ in that Death Valley basalt has lower La (70 vs. 130 ppm). Isotopic ratios are remarkably similar and strongly support a link between the Death Valley and Crater Flat fields. For Death Valley and Crater Flat, respectively, epsilon-Nd is -11.88 to -3.26 and -13.17to -5.48; ⁸⁷Sr/⁸⁶Sr is 0.706322 to 0.707600 and 0.706221 to 0.707851; ²⁰⁶Pb/²⁰⁴Pb is 17.725 to 18.509 and 18.066 to 18.706; ²⁰⁷Pb/²⁰⁴Pb is 15.512 to 15.587 and 15.488 to 15.564; and ²⁰⁸Pb/²⁰⁴Pb is 38.237 to 38.854 and 38.143 to 38.709. Depths of melting calculated using the silica geobarometer (Lee et al., in press) indicate that basalt magma was generated at depths and temperatures of 42-63 km and 1295-1350^oC beneath Death Valley and 80-90 km and 1388-1415^oC for Crater Flat. This indicates asthenospheric melting for both areas. The Death Valley centers are 73% hypersthene normative and 27% nepheline normative, whereas the Crater Flat centers are mostly hypersthene normative. Combining the Death Valley and Crater Flat areas into a single volcanic field increases the area of interest for probability calculations by over 1/3 and increases the number of volcanic events by 24. The increased size of the volcanic field and number of volcanoes may result in an increase in probability of disruption of the repository by an igneous event.