SIMULATING THE PROLONGED COOLING HISTORY OF THE SHALLOW VOLCANIC SECTION AT YUCCA MOUNTAIN, NEVADA

The depositional history of secondary minerals in the unsaturated zone at Yucca Mountain, Nevada, recorded by fluid inclusion homogenization temperatures and geochronologic studies (Whelan et al., 2008, Appl Geochem 23, 1041), indicated a prolonged period of cooling followed emplacement of igneous intrusions beneath the Timber Mountain caldera complex extending to 8 km from the northern part of Yucca Mountain. Modeling the thermal history using the HEAT3D code (Wohletz et al., 1999, J Vol Geotherm Res 91, 381) attempted to match the known history, utilizing the geologic structure and history of the region. Initial results (Marshall and Whelan, 2001, GSA Abstr Prog 33, A-375) from purely conductive models showed that the general conceptual model was valid, but the sharp decrease in thermal gradients outside the caldera complex led to speculation concerning the exact locus of the magma chamber. Subsequent systematic modeling efforts were carried out, each model testing various changes in the geometry and especially in the locus and timing of hydrothermal flow outside the caldera. More than 20 simulations were tested, and modeling included the effect of erosion on the thermal gradients. The model results show that the thermal perturbation associated with the Timber Mountain magma chambers is adequate to account for the thermal history of the unsaturated zone at Yucca Mountain (Whelan et al., 2008), and provide an integrated thermal regime for modeling feldspar weathering rates within the unsaturated zone (Bryan et al., 2009, Appl Geochem 24, 2133). Other researchers, using both the same code, as well as other codes typically using different parameters, have claimed that the conceptual model is not valid (Dublyansky and Polyansky, 2007, JGR 112, B09201). This presentation provides a detailed examination of the systematic modeling effort that was completed, and demonstrates the ability of the simulations to match what is known of the thermal history of Yucca Mountain.