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Cosmogenic 3He in Terrestrial Rocks: The Summit Lavas of Maui

H. Craig and R. J. Poreda
Proceedings of the National Academy of Sciences of the United States of America
Vol. 83, No. 7 (Apr. 1, 1986), pp. 1970-1974
Stable URL: http://www.jstor.org/stable/27119
Page Count: 5
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Cosmogenic 3He in Terrestrial Rocks: The Summit Lavas of Maui
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Abstract

We have identified terrestrial cosmic ray-produced 3He 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 3He from the mantle, the ``cosmogenic'' component (3HeC) 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 3He/4He ratios as high as 75× those in the atmosphere, which can be liberated by melting the crushed grains. Cosmogenic 3He is present in both olivines and clinopyroxenes at 0.8-1.2 × 10-12 ml(STP)/g and constitutes 75% ± 5% of the total 3He present. The observed 3HeC 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/106 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.

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