HAFNIUM ISOTOPE CONSTRAINTS ON THE GEOCHEMICAL EVOLUTION OF THE WYOMING CRATONIC SUB-LITHOSPHRIC MANTLE

Cenozoic volcanism on the Archean Wyoming craton began with a magmatic pulse at about 55-40 Ma with eruption of high-K lavas represented by the Absaroka volcanics, the Smoky Butte lamproites at 27 Ma, the Leucite Hills lamproites at 3.0 to 0.89 Ma, and the Snake River Plain basalts at 17 Ma to recent. These examples of Wyoming Cenozoic volcanism all erupted through the Archean Wyoming craton. Hafnium isotope data from Smoky Butte, and new data from the Leucite Hills and the Snake River plain place constraints on the evolution and nature of the Wyoming sub-lithospheric mantle. In Hf and Pb isotopic space the Smoky Butte lamproites, Leucite Hills lamproites, and the Snake River Plain basalts all fall along a linear-like trend that intersects the MORB array at high ²⁰⁶Pb/²⁰⁴Pb, at the isotopic composition inferred for the Yellowstone plume. At 55 Ma the North American Continent had not yet overridden the Yellowstone plume. At about 17 Ma the eruption of the Steens Mountain basalts marked the location of the plume beneath the continent. The Snake River Plain basalts of Idaho and Wyoming represent the track of the North American Continent over the Yellowstone plume. The Hf and Pb isotope data suggest that the sub-lithospheric mantle was already polluted by plume material in the Early Cenozoic and that the pollution increased proportionally as the Wyoming craton passed over the plume and the distance from the plume to the Snake River eruptions at Yellowstone decreased.