SECONDARY ALTERATION MINERALS IN NUCLEAR MELT GLASS, NEVADA TEST SITE, NEVADA

A majority of the 828 underground nuclear tests conducted at the Nevada Test Site (NTS) were detonated in rhyolitic volcanic rocks or tuffaceous alluvium. Energy released from these tests produced an estimated 7 x 105 kg of high-silica melt glass per kiloton of yield. For tests conducted below the water table, ground water re-infiltrates the test cavity at rates determined by the hydrologic properties of the surrounding geologic media. In low permeability environments, advective fluid transport of residual heat from the explosion is minimal, and elevated cavity temperatures can persist for decades after detonation. These conditions are conducive for accelerated water/glass reactions similar to hydrothermal and diagenetic alteration found in natural volcanic glasses. We recently examined samples of melt glass from the flooded cavity of a test conducted more than two decades ago. Groundwater temperatures at the time of sampling were ~75°C. This study detected the zeolites clinoptilolite, erionite and/or mordenite formed on vesicle walls of melt glass that exhibited dissolution textures and discolored reaction rims. The dissolution textures are similar to those observed in partially perlitized obsidian, and the zeolites are similar to those observed on rhyolitic glass at nearby Yucca Mountain. Clay minerals (illite, smectite, and hectorite) were observed coating melt glass that exhibited no dissolution textures or reaction rims and are interpreted as coatings derived from drilling mud, although diagenetic formation of clay cannot be discounted. We conclude these zeolites formed from reactions with the melt glass at temperatures greater than 75° C, and published thermodynamic data indicate the temperature of formation may have been as high as 125° C. This work was funded by the Underground Test Area Project and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-Eng-48.