Collaborative Research: Cosmogenic 3He Dating of Accessory Minerals

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Technical Description Cosmogenic isotopes such as 10Be and 26Al are widely used to determine the age and exposure histories of landscape features. Such information is critical to understanding the 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 is limited by the high cost and intense labor associated with producing a single measurement. This project proposes development of cosmogenic 3He as a dating tool in common accessory mineral phases such as apatite, sphene and zircon. If successful, this technique will be unique in several respects. First, it will allow rapid and inexpensive cosmogenic dating of a wide range of rock types, thus increasing the number of samples that can be dated in a single study. Second, because 3He is stable and has a very high production rate, it may prove useful in combination with other cosmogenic nuclides 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, sphene and zircon can be used as a cosmogenic dating tool. First, it is necessary to develop analytical procedures such that the measurement of small amounts of 3He in the presence of large quantities of radiogenic 4He is not hampered by the "tailing" of low energy 4He ions onto the 3He peak, or by space charge effects at high 4He pressures. The former is likely not a problem based on the demonstrated abundance sensitivity at mass 3, and the latter will be addressed using a "spike" of standard gas inlet during the measurement of an unknown to calibrate the instrument at that He pressure. A second challenge is to verify the production rate of 3He, which will be done using samples of known exposure age from glacial moraines in the Sierra Nevada and Nepal Himalaya. Implicit in this task is identifying and correcting for non-cosmogenic sources of 3He such as inheritance from fluid inclusions and production via 6Li(n, alpha) + time -> 3He. Non-Technical Explanation Determining when rocks first appear at the Earth's surface is an important question in many Earth Science disciplines. For example, the boulders deposited when a glacier retreats 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 by measuring the buildup of unusual isotopes produced by cosmic rays interacting with the elements of the uppermost tens of centimeters of the rock. This proposal seeks to improve existing methods of "surface exposure dating", using the appearance of a rare isotope of helium in several common minerals. The overarching goal is to develop a technique that is broadly applicable and inexpensive and which may lead to improved understanding of the timing and rates of processes that modify Earth's surface.