New Mexico Geological Society Annual Spring Meeting — Abstracts

A shallow basaltic intrusion into chemically homogenous silicic tephra at depths of <200 m was investigated as part of a natural analog study on the effects subsurface disposal of high-level radioactive waste. This is an attempt to understand the effects of intrusions into unsaturated zeolitic tuffs that will potentially host high-level radioactive waste. Moreover, the extent of man-made hydrothermal system that could develop from radioactive decaying in a shallow repository is evaluated.

A localized aureole up to 10 m wide developed around a 150-m wide, 2.6 Ma basaltic plug at Grants Ridge, New Mexico. The plug intruded into nonwelded, pumice-rich compositionally homogenous tuff and volcaniclastic sediments of similar age (3 .3 Ma). Color variation (pinkish to orange), strong local contact welding, brecciation, and stoping characterize the host rock within the contact zone. Despite the high temperature basaltic intrusion, there is no indication for extensive convective heat transfer and pervasive hydrothermal circulation and alteration. The proportion of volcanic glass, loss on ignition (LOI), fluorine, iron, and some trace and rare earth element contents in the
host rocks are depleted near the intrusion. Conversely, the degree of devitrification and the potassium content are higher along the contact. Vapor-phase expulsion of elemental species as fluoride, chloride, hydroxide, sulfide, and carbon oxide complexing may have been responsible for the depletion ofthe elements during the devitrification of silicic glass at near-solidus temperature related to the basaltic intrusion.

The results of finite-difference numerical modeling of the intrusion as a dry, conduction-dominated system agree well with geochemical and mineralogical data. Contact welding of the host rocks apparently occurred at temperatures >700°C under a density-driven lateral load of approximately 1 MPa, corresponding to the observed depth below the former ground surface of <200 m. Other physical changes in the first 10 m of host rock, represented by partial devitrification and color changes, apparently occurred at temperatures of 500-600°C, which probably persisted for up to 55 years after the emplacement of the basaltic plug. Devitrification is generally enhanced by the presence of aqueous fluids; however, the abundance of volcanic glass within a short distance (~10 m) from the plug is consistent with our inference that the limited water-rock interaction in the nonwelded ignimbrite and fallout was caused by intrusion into unsaturated environment. Our study appears to suggest that basaltic intrusion into nonwelded I unsaturated tuffs has localized contact metamorphic effects and insignificant hydrothermal alteration.