Collaborative Research: Cosmogenic 3He Dating of Accessory Minerals

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Technical DescriptionCosmogenic isotopes such as 10Be and 26Al are widely used to determine the age andexposure histories of landscape features. Such information is critical to understandingthe timing and rate of geologic processes such as active fault ruptures, glacial advances,soil formation and erosion of mountain belts. However, the utility of these isotopes islimited by the high cost and intense labor associated with producing a singlemeasurement. This project proposes development of cosmogenic 3He as a dating tool incommon accessory mineral phases such as apatite, sphene and zircon. If successful, thistechnique will be unique in several respects. First, it will allow rapid and inexpensivecosmogenic dating of a wide range of rock types, thus increasing the number of samplesthat can be dated in a single study. Second, because 3He is stable and has a very highproduction rate, it may prove useful in combination with other cosmogenic nuclides toaddress problems such as paleoaltimetry, ancient exposure and burial histories (100 mytime scales), and dating of archaeological sites and artifacts.Several technical challenges need to be overcome before 3He in apatite, sphene andzircon 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 pressures. The former islikely not a problem based on the demonstrated abundance sensitivity at mass 3, and thelatter will be addressed using a "spike" of standard gas inlet during the measurement ofan 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.Non-Technical ExplanationDetermining when rocks first appear at the Earth's surface is an important question inmany Earth Science disciplines. For example, the boulders deposited when a glacierretreats provide an important record of past climate change. The "surface exposure age"of these boulders, the time when they were first deposited, can be determined bymeasuring the buildup of unusual isotopes produced by cosmic rays interacting with theelements of the uppermost tens of centimeters of the rock. This proposal seeks to improveexisting methods of "surface exposure dating", using the appearance of a rare isotope ofhelium in several common minerals. The overarching goal is to develop a technique thatis broadly applicable and inexpensive and which may lead to improved understanding ofthe timing and rates of processes that modify Earth's surface.