NEW CHEMICAL RELATIONSHIPS FROM DRILL CORE IN THE SNAKE RIVER PLAIN, IDAHO

Scientific and exploratory test wells and core holes dot the landscape of the Snake River Plain (SRP). Over the last 25 years, this has been mainly carried out at Idaho National Laboratory (INL). Unlike the extensive drilling at INL, the stratigraphy and geochemistry of subsurface lava flows from the margins of the SRP and the CSRP has been less reported. We report whole rock major and trace elements from the rhyolite section of Sugar City geothermal test well, near Rexburg, ID and major and trace elements in olivine from the Wendell (RASA) hydrologic test well, near Wendell, ID.

The Sugar City geothermal test well, near Rexburg, ID, cored basalts, rhyolite lava flows, and tuffs from the western margin of the Heise caldera complex. Analyses from the basalt section have been previously presented (Jean and Shervais, 2010). This new work focuses on the rhyolite and welded tuff section of this core, which constitutes 520 meters of core. Our new work demonstrates a large increase in LOI for depths between 500-600 meters, coupled with changes in major elements, i.e., decrease in SiO₂ and Na₂O and increase in Fe₂O₃, MgO, and Al₂O₃. While REE composition for our rhyolite samples is still being analyzed, lanthanum and cerium show relatively large ranges.

The Wendell (RASA) hydrologic test well penetrated an upper basalt unit from 0.305 to 122.8 m (Snake River Group) and a lower basalt unit from 179.8 to 327.1 m (Idaho Group). Average olivine concentrations include; SiO₂=37.48 wt.%; TiO₂=0.02 wt.%; Al₂O₃=0.05 wt.%; FeO=21.50 wt.%; MgO=40.00 wt.%; CaO=0.10 wt.%. Magnesium decreases with depth, while SiO₂ increases with depth. These relationships suggest the possible occurrence of magma recharge and fractionation. Investigating REE’s by the use of LA-ICP-MS shows promise in determining similarities in REE concentration to other standards, such as NMORB, primitive mantle, and OIB. Although this phase of the study is still currently in process, some preliminary correlations are suggested. The general trend for REE concentration appears to be an increase in REE content with depth. Our data shows a spike in Gadolinium (Gd) when normalized to all standards with an enrichment factor of up to 30.8 when normalized to C1-Chondrite. The frequently appearing spike in Gd suggests that a more careful approach to analyzing REE concentrations is needed.