Cosmogenic 3 He in terrestrial rocks: The summit lavas of Maui

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Abstract
We have identified terrestrial cosmic rayproduced 3 He in three lava flows on the crest of Haleakala Volcano on Maui, 3 km above sea level, and ≈0.5 million years old. Although these lavas, like all oceanic basalts, contain primordial 3 He from the mantle, the “cosmogenic” component ( 3 He C ) can be identified unambiguously because it is extractable only by high-temperature vacuum fusion. In contrast, a large fraction of the mantle helium resides in fluid inclusions and can be extracted by vacuum crushing, leaving a residual component with 3 He/ 4 He ratios as high as 75× those in the atmosphere, which can be liberated by melting the crushed grains. Cosmogenic 3 He is present in both olivines and clinopyroxenes at 0.8-1.2 × 10 -12 ml(STP)/g and constitutes 75% ± 5% of the total 3 He present. The observed 3 He C levels require a cosmic ray exposure age of only some 64,000 years, much less than the actual age of the lavas, if there is no erosion. Using a model that includes effects of uplift or submergence as well as erosion, we calculate an apparent “erosion rate” of the order of 8.5 m/10 6 years for the western rim of the summit crater, as an example of the application of measurements of cosmogenic rare gases to terrestrial geological problems.