PROJECT HOTSPOT: PRELIMINARY PETROGRAPHY AND GEOCHEMICAL RESULTS ON BASALTS OF THE KIMBERLY DRILL HOLE, SNAKE RIVER PLAIN, IDAHO

Project HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic history of the SRP, which represents the surface expression of this hotspot.

Project HOTSPOT has completed three drill holes. The Kimama site located along the central volcanic axis of the SRP; the Kimberly site located near the margin of the central plain; the Mountain Home site located in the western plain. We report here on our initial petrography and geochemical results for Kimberly site.

Kimberly drilling reached a total depth of 1958m. This deep hole is dominated by massive rhyolite lava and welded ash flow tuffs, with basalt/sediment intercalations at 241m to 424m depth, and thin altered ash interbeds around 610m depth.

Our data includes the 33m upper basaltic flow encountered at 230m downhole and the 65m lower basaltic flow encountered at 300m downhole. The flows are separated by 15m of sediments.

Thin sections showed the basalts contain equigranular textures consisting of olivines, plagioclases, clinopyroxenes, and oxides. Preliminary phase compositions based on electron mircoprobe and SEM results indicate the olivines’ Fo range of 74.7 to 47.4. Some olivines show an increase in Fa component going from the core to the rim of an individual crystals. Plagioclase An ranges from 76.6 to 53.2 with little to no zonation. Clinopyroxene endmembers (Wo En Fs) range from Wo9.6 En56.4 Fs34.0 to Wo45.2 En35.6 Fs19.1. The clinopyroxenes are found as groundmass only phases. Oxides compositions are that of mostly ilmenite.

Whole rock major element compositions shows the upper basalt flow has a limited Mg# range of 49.4 – 56.9, wt% SiO2 46.2-48.0, wt% TiO2 2.1-2.8, and wt% P2O5 0.06-0.45. The lower basalt flow has a wider Mg# range of 47.7 -62.6, wt% SiO2 46.6-49.5, wt% TiO2 1.55-3.08, and wt% P2O5 0.03-0.25. Major elements suggest that the lower basalt flow has undergone crystal fractionation while the upper basalt flow has undergone little if any crystal fractionation. Very preliminary REE data suggests that both the upper and lower basalt flows are LREE enriched.