Physical processes of magmatism and effects on the potential repository: Synthesis of technical work through Fiscal Year 1995

Abstract

This report was prepared as an account of work spoasond by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, mommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. EXECUTIVE SUMMARY PHYSICAL PROCESSES OF MAGMATISM AND EFFECTS ON THE POTENTIAL REPOSITORY: SYNTHESIS OF TECHNICAL WORK THROUGH FISCAL YEAR 95 Principal Investigator: G.A. ValentineThis chapter summarizes data collection and model calculations through Ey 95 under Study Plan 8.3.1.8.1.2 Physical Processes of Magmatism and Effects on the Potential Repository. The focus of this study plan is to gather information that ultimately constrains the consequences of smallvolume, basaltic magmatic activity at or near a potential repository. This is then combined with event probability estimates, described elsewhere in this synthesis report, to yield a magmatic risk assessment.There are two basic classes of effects of magmatism that are considered here: (1) Eruptive eflects, whereby rising magma intersects a potential repository, entrains radioactive waste, and erupts it onto the earth's surface. (2) Subsugace eflects, which includes a wide range of processes such as hydrothermal flow, alteration of mineral assemblages in the potential repository system, and alteration of hydrologic flow properties of the rocks surrounding a potential repository.Eruptive (direct) effects -We present data on the possible entrainment of waste into eruptive magma. These data were collected from analog volcanoes and represent a range of eruption mechanisms including Strombolian, Hawaiian, and hydrovolcanic. Strombolian and Hawaiian eruptions generally entrain little wall rock material, and are unlikely to have a strong influence on total system performance assessments. Hydrovolcanic eruptions entrain 103 -104 times more wall rock debris and could influence performance assessments, depending mainly on the probability of such eruptions.Subsurface (indirect) effects -We present field data from analog sites where the plumbing of small basaltic volcanoes is exposed, and where the volcanoes penetrated silicic tuffs similar to those at Yucca Mountain. Geometry of shallow intrusions is determined by a complicated interplay between pre-existing structures, shallow stress field variations, and rock mechanical properties. The intrusion geometry in turn determines the alteration of the host rocks. Chemi...

Features, Events, and Processes in UZ and Transport

“…A period of 200 to 300 years to cool to 500°C was estimated by Ratcliff et al (1994 [DIRS 106634] for the 150-m-thick Paiute Ridge lopolith, but thinner intrusions would cool much faster. Contact metamorphism resulting from dikes at a vadose-zone analogue site, Paiute Ridge, is confined to distances of a few meters from the dike (Valentine et al 1998 [DIRS 119132], Chapter 5). With significant thermal perturbations limited to less than 100 years and alteration limited to zones of a few meters around the dike, the effects of basaltic magmatism on transport pathways, advective velocities, and sorption coefficients (K d s) is negligible.…”

A Natural Analogue for Thermal-Hydrological-Chemical Coupled Processes at the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada

“…The intrusive complex is located in a fault-bounded valley surrounded by Paiute Ridge, Slanted Buttes, and Carbonate Ridge. The geology of the region was mapped by Byers and Barnes (1967) and remapped and studied in greater detail by Goff andothers in 1995 (Goff, 1995;Valentine et al, 1998) (Figure 1). (Sawyer et al, 199A Goff, 1995;Warren, 1995;Valentine et al, 1998).…”

“…The geology of the region was mapped by Byers and Barnes (1967) and remapped and studied in greater detail by Goff andothers in 1995 (Goff, 1995;Valentine et al, 1998) (Figure 1). (Sawyer et al, 199A Goff, 1995;Warren, 1995;Valentine et al, 1998). Many of these units are found at Yucca Mountain, 40 km to the southwest.…”

“…The geologic map by Byers and Barnes (1967) delineates tuff stratigraphy and intrusion geometries and attempts to identify areas of "alteration" (associated with the intrusions) and "zeolitization" (of any origin). However, these characterizations must be reevaluated in light of the revised stratigraphic definitions of the study area developed by Goff and others (Goff, 1995;Valentine et al, 1998) and Warren (1995). This newer mapping redefines several parts of the study are% reassigning strata termed "Paintbrush Tuff-Undivided" and "Altered Tuff" into older units (e.g., Calico Hills Formation, Wahmonie Formation, Tunnel Formation, Volcanics of Oak Spring Butte) with distinctly different petrographic and alteration characteristics.…”