HYDROTHERMALLY ALTERED FELDSPARS IN TUFFS SURROUNDING A BASALTIC SILL: IMPLICATIONS FOR YUCCA MOUNTAIN

Since the 1997 identification by Matyskiela of significant tuff alteration at the Papoose Lake Sill (Geology, v. 25) , the Department of Energy (DOE) has done little to account for the possibility of significant heat related alteration to tuffs at Yucca Mountain. DOE plans to emplace 70,000 tonnes of heat producing nuclear wastes in tunnels there. That 1997 paper focused on changes to fracture-margin characteristics. Later abstracts (Packer & Matyskiela, GSA annual meetings 1999 and 2000), focused on fundamental physical changes to matrix permeability, density and mineralogy of tuffs in hydrothermal environments. Here we report on intrusion-heat related changes in matrix feldspar abundance and structure (devitrified welded tuffs of the Ammonia Tanks and vitric, non-welded tuffs of the Paintbrush) at the Papoose Lake Sill, 40 km northeast of Yucca Mountain.

We used x-ray diffraction of vitric and devitrified tuff to correlate host-rock temperature rise due to the intrusive sill with changes in abundance of high-temperature disordered feldspar. Prior to sill intrusion, the devitrified tuff contained moderately disordered feldspars generated during devitrication, as well as some from the volcanic eruption, whereas vitric tuff matrix samples had very little primary feldspar. We found that even the intermediate feldspar in devitrified tuff changed to a more ordered state when exposed to the heat from the intrusive sill. Vitric tuff feldspar abundance and disordering correlate with increasing sill related temperatures. This data suggests that the high-temperature thermodynamically unstable feldspars in the Topopah Springs welded devitrified tuff at Yucca Mountain will dissolve more readily (Helgeson et al, 1978, Amer. Jour. Sci., v 278A) than predicted by the conventional kinetic parameters used in DOE's performance assessment models, which are based on dissolution of well-ordered, slowly cooled feldspars (Hardin, LLNL,1996). Such accelerated feldspar dissolution rates can possibly explain the contradictions between early DOE lab experiments showing rapid tuff dissolution (Knauss et al, UCRL 53645, 1985), and the current minor tuff dissolution projected by repository safety analyses (Bodvarsson, NWTRB presentation, June, 2001).