Collaborative Research: Cosmogenic 3He Dating of Accessory Minerals

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Cosmogenic isotopes such as 10Be and 26Al are widely used to determine the ageand exposure histories of landscape features. Such information is critical tounderstanding the timing and rate of geologic processes such as active fault ruptures,glacial advances, soil formation and erosion of mountain belts. However, the utility ofthese isotopes is limited by the high cost and intense labor associated with producing asingle measurement. This project proposes development of cosmogenic 3He as a datingtool in common accessory mineral phases such as apatite, sphene and zircon. Ifsuccessful, this technique will be unique in several respects. First, it will allow rapid andinexpensive cosmogenic dating of a wide range of rock types, thus increasing the numberof samples that can be dated in a single study. Second, because 3He is stable and has avery high production rate, it may prove useful in combination with other cosmogenicnuclides to address problems such as paleoaltimetry, ancient exposure and burial histories(100 my time scales), and dating of archaeological sites and artifacts.Several technical challenges need to be overcome before 3He in apatite, spheneand zircon can be used as a cosmogenic dating tool. First, it is necessary to developanalytical procedures such that the measurement of small amounts of 3He in the presenceof large quantities of radiogenic 4He is not hampered by the "tailing" of low energy 4Heions onto the 3He peak, or by space charge effects at high 4He abundances. The formeris likely not a problem based on the demonstrated abundance sensitivity at mass 3, andthe latter will be addressed using a spike of standard gas inlet during the measurementof an unknown to calibrate the instrument at that He pressure. A second challenge is toverify the production rate of 3He, which will be done using samples of known exposureage from glacial moraines in the Sierra Nevada and Nepal Himalaya. Implicit in this taskis identifying and correcting for non-cosmogenic sources of 3He such as inheritance fromfluid inclusions and production via 6Li(n, alpha) + time - 3He.