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

Abstract Helium, neon, and argon isotopic compositions were measured in two flows of the Columbia River flood basalt. The Imnaha Basalt has a 3 He/ 4 He ratio of 11.4 times atmospheric and 20 Ne/ 22 Ne and 21 Ne/ 22 Ne ratios characteristic of a plume component. The measured 3 He/ 4 He is a lower limit, due to possible preferential 3 He loss and/or addition of radiogenic 4 He. A Wanapum Basalt flow, erupted approximately 2 Ma later in the waning stages of volcanism, has more MORB-like noble gases. The He, Nd and Sr isotopic compositions of these lavas suggest that the Columbia River basalts were derived from the Yellowstone plume head which contained both ‘high-helium’ plume material and entrained depleted mantle. As the eruptions progressed the plume component in the melting region was gradually diluted or replaced.

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