COOLING HISTORY OF THE TOPOPAH SPRING TUFF AT YUCCA MOUNTAIN, NEVADA

The 12.8-Ma Topopah Spring Tuff (Tpt) is about 350 m thick and almost entirely densely welded and crystallized near the central part of Yucca Mountain, Nevada, and is the host rock for the proposed geologic repository for spent nuclear fuel and radioactive waste. Such a thick ignimbrite had a protracted period of cooling from depositional to ambient temperatures, and processes of compaction, welding, lithophysae formation, crystallization, vapor-phase corrosion and mineralization, and fracturing determined most lithostratigraphic, hydrogeologic, and engineering properties. The compaction and heat-flow model of Riehle et al. (Bull. Volc., 1995) was used to constrain the timing and temperature of depositional units, compaction, early vapor pressure, and cooling of the ignimbrite. Initial values were determined by simulating a density profile from a less complex, 60-m-thick, vitric, nonlithophysal, mostly densely welded deposit near the Lathrop Wells cone. The two sites are compositionally zoned Tpt about 20 km apart along a radial flow path from the source. Subtle inflections in the 60-m thick density profile are interpreted as cooling breaks, and although corroborating depositional features such as bedding were not identified, changes in size and amount of pumice (or lithic) clasts have been described elsewhere as different flow units. The density profile was simulated by three units deposited tens of days apart, one 690°C crystal-poor tuff and two 720° to 730°C crystal-rich tuffs. The simulation for the 350-m-thick deposit included four 680°C tuffs and two 710° to 720°C tuffs each deposited 10 days apart, and resulted in behavior as a simple cooling unit. Compaction was completed in a few months, too rapid to allow for complete degassing throughout the deposit. Some initial vapor remained trapped and likely contributed to growth of lithophysal cavities. Subsequently, crystallization released additional vapor along with an exothermic reaction that increased gas pressure and enhanced inflation of lithophysae. The top and bottom of the deposit cooled rapidly, but the center required about 7,000 years to cool to 20°C. This preliminary model for cooling of the 350-m-thick Tpt provides general constraints on timing and duration of many petrologic processes that occurred during cooling of the ignimbrite.