NEW INSIGHTS INTO SNAKE RIVER PLAIN VOLCANIC PROVINCE FROM PROJECT HOTSPOT

Project Hotspot is an international collaborative effort to understand the origin and evolution of the Yellowstone-Snake River Plain volcanic province, which may represent the track of a deep-seated mantle plume. Project Hotspot completed three drill holes: Kimama (1912 m), located on the axial volcanic zone; Kimberly (1958 m), located near the margin of the plain; and Mountain Home (1828 m) in the western SRP graben. Kimama and Kimberlywere selected to sample a composite section of basalt and rhyolite. Mountain Home was selected to sample the Plio-Pliestocene transition in lacustrine sediments, and to sample older basalts that underlie the lake sediments. Over 5.4 km of core was collected, along with an array of borehole and surface geophysical data.

The Kimama core consists almost entirely of basalt, with minor sediment layers near the top and bottom. The basalts are mostly olivine tholeiites, with several flows of evolved Fe-Ti and high-K basalt intercalated. The oldest basalts are ~6.3 Ma, which overlaps in age the Shoshone Falls rhyolite found at the top of the Kimberly section; thus the Kimama-Kimberly pair represents a composite section of early rhyolite and later basalt (Potter et al, submitted). The Kimberly core samples an upper section of basalts and fluvial sediments (early caldera fill?) overlying 1300 m of welded ash flow tuffs. Mountain Home comprises an upper basalt section (213 m), middle lacustrine section (625 m), and lower section (850 m to TD) of altered basalt, basaltic sands, and hyaloclastite breccias. The lacustrine section captures the Pliocene-Pleistocene transition, marking the onset of glacial conditions in North America. Detailed studies of the core are in progress, but preliminary data suggest a basaltic flux along the plume track of some 10⁵ km³over 5.5 Ma. This is consistent with definitions of “Large Igneous Province” and does not account for magmatic flux of the rhyolites. We suggest that this flux requires a deep source to produce.