Strontium Isotopic Study of Fracture Filling Minerals in the Grande Ronde Basalt, Washington

An important issue for assessing the feasibility of a nuclear waste repository located in basalt at the Hanford Site, Washington, is whether or not waters can percolate vertically through the rocks at the BWIP site. Flow in well defined horizontal, interbed units is well known, but vertical flow has not been fully investigated. Because vertical fractures are common, it is possible that some waters could penetrate the basalts and move normal to the horizontal flow layers. To investigate this, we have chosen to examine the chemistry and isotopics of minerals found in the fractures. These minerals may have originated earlier in the basalt history, i.e. deuteric, in which case their chemistry and isotopic composition should reflect that of the basalt. Alternately, they may have formed late in the basalt's history, in which case their chemistry may be a reflection of material deposited from percolating waters. Thirdly, they may represent a mixing of the basalt and groundwater reservoirs. We examined this issue by determining the strontium isotopic composition of the various reservoirs, supported by other chemical studies. In this fashion we have carefully investigated the origin of the strontium and other trace elements in the fracture filling minerals. To date, waters from surface or subsurface reservoirs at Hanford have not been provided for isotopic analysis, although we plan to run these as soon as they are available. Our conclusions on the strontium isotopic work are therefore qualitative to semi-quantitative at best, but the results can be directly applied to the source of the strontium in the fracture filling minerals.