HELIUM AND NEON ISOTOPES IN THE IMNAHA BASALT, COLUMBIA RIVER BASALT GROUP - EVIDENCE FOR A YELLOWSTONE PLUME SOURCE

Helium, neon, and argon isotopic compositions were measured in two flo ws of the Columbia River flood basalt. The Imnaha Basalt has a He-3/He -4 ratio of 11.4 times atmospheric and Ne-20/Ne-22 and Ne-21/Ne-22 rat ios characteristic of a plume component. The measured He-3/He-4 is a l ower limit, due to possible preferential He-3 loss and/or addition of radiogenic He-4. A Wanapum Basalt flow, erupted approximately 2 Ma lat er in the waning stages of volcanism, has more MORB-like noble gases. The He, Nd and Sr isotopic compositions of these lavas suggest that th e Columbia River basalts were derived from the Yellowstone plume head which contained both 'high-helium' plume material and entrained deplet ed mantle. As the eruptions progressed the plume component in the melt ing region was gradually diluted or replaced. (C) 1997 Elsevier Scienc e B.V.