Background:

Most of our studies of magmatic processes that could affect a potential repository at Yucca Mountain are aimed at understanding the range of subsurface processes that may significantly damage the repository system (this includes the repository itself and the surrounding natural barriers). Subsurface processes can be divided into a) those that occur during the thermal perturbation associated with a magmatic intrusion (short term) and b) those that occur over the long term as the hydrologic system is re-established with altered hydrologic and mineralogical properties. The long term effects include the possibility of perched water near low-permeability intrusive bodies (e.g., dikes), possible fast paths along intrusion-induced fractures, and reduced chemical retardation properties of the country rock resulting from hydrothermal alteration. Initial theoretical studies of hydrothermal flow processes in unsaturated rocks are presented separately. Here we describe field studies that are being carried out in conjunction with modeling. These field studies to date are focused on understanding the alteration processes around small, shallow basaltic intrusions in silicic tuffs, analogous to what would happen if a basaltic magma were emplaced in or near Yucca Mountain. We are currently focusing on two analog sites, like the zeolitized and unaltered (vitric) tuffs of Yucca Mountain. In addition to addressing the magmatic hazards issue, these studies are providing information that will be used to understand the effects of heat from decay of radioactive waste in the potential repository. Because modeling and field approaches are closely tied in this study, we will be able to develop our ability to model other processes with similar physics (e.g., the man-made hydrothermal system around a repository).

Natural Analogs:

One of the analog sites is Paiute Ridge, located about 40 km northeast of Yucca Mountain. Paiute Ridge is characterized by dikes, small sills, and conduit plugs, representing the shallow (100-200 m depth) intrusive plumbing of a small volume volcanic center. Due to erosion and faulting, only a small part of the eruptive lavas and scoria remain. The intrusions are 8.6 Ma old alkali basalts that were emplaced into country rocks consisting of bedded, variably zeolitized silicic tuffs. Although these tuffs are mapped as part of the same formation as the tuffs that may eventually host the repository, they are most analogous in their characteristics to units such as the Calico Hills formation, one of the main geochemically retarding units beneath the potential repository horizon at Yucca Mountain. We are carrying out geologic, mineralogical, and geochemical investigations to characterize the nature and extent of alteration of tuffs around the basaltic intrusions. Zeolite-rich tuffs are characterized by low-temperature alteration minerals. For example, the upper stability limit for alteration minerals such as clinoptilolite, smectite, and opal-CT that are common in zeolite-rich altered tuffs is generally about 100 degrees C. The effect of the dike and sill intrusions on the zeolite-rich tuffs will be investigated using petrographic, mineralogical, and geochemical studies.

The second analog site is Grants Ridge, in western New Mexico, a basaltic cinder cone that erupted through a thick sequence of unaltered rhyolite tuffs and volcaniclastic sediments. Since formation of the cinder cone, c. 2.6 Ma, erosion processes have dissected the cone so that a natural cross section is now exposed in a canyon wall. The exposure not only includes the interior of the cone itself, but the lava plug that formed in the feeder conduit, from the original (pre-basalt) land surface down to about 100 m depth. We are characterizing the rhyolite tuff sequence by sampling at regular distances away from the intrusive plug. Thermal and degassing effects are generally more pronounced along the contact zone. Alteration minerals will be described and identified using petrographic methods and X-ray diffraction. Thermal and degassing effects on the host rock at proximal and distal points will be compared and contrasted to determine the degree of geochemical mobilization during hydrothermal processes related to the basaltic intrusion.

Significance:

This study will address important questions concerning: 1) volatile contents of basaltic intrusions like dikes and sills in altered and unaltered tuffs. Volatile contents generally crystallize as vapor phase minerals in host rock fractures, cavities, and joints. 2) Thermal and degassing effects are generally preserved as alteration halos along the contact zone within the host rock depending on the size and duration of the intrusion. Comparison of petrographic, mineralogical, and geochemical results of samples from unaltered and zeolite-rich altered tuffs intruded by basaltic dikes and sills is expected to provide important information on subsurface processes that can occur in a repository environment.

For more information, contact Giday WoldeGabriel (505-667-8749 or wgiday@lanl.gov).