New Techniques for Surface Exposure Dating
Surface exposure dating provides critical information on a range of Earth science problems including ages of glacial features, erosion rates, and rates of fault motion. The technique relies on the production of rare isotopes produced by interactions of cosmic rays with target nuclei in rocks within ~ 1 meter of Earth's surface. The most commonly used "cosmogenic isotopes" are radionuclides (10Be, 26Al, 36Cl) that require extensive chemical purification and accelerator mass spectrometry for analysis. In contrast cosmogenic noble gases, especially 3He, require far less labor intensive purification and a simple sector-field mass spectrometer. Thus cosmogenic noble gases offer the advantage of faster and less expensive data acquisition. However cosmogenic 3He is now routinely analyzed only in olivine and pyroxene because only these minerals are known to retain 3He against diffusive loss and have tolerably low non-cosmogenic 3He background levels. We recently began work to develop cosmogenic 3He dating of additional minerals which are known to retain He at Earth surface conditions, especially the common accessory phases apatite, sphene, and zircon. The challenge of working with these minerals is their small grain size, which causes a variety of nuclear effects associated with the long stopping range of 3He in minerals. In addition the presence of high concentrations of 4He in these uranium rich minerals demands high abundance sensitivity for the 3He measurement. Despite these complications we have obtained excellent results in some settings. For example, this image shows cosmogenic 3He in the accessory phases separated from a tuff in Bolivia. This work was done in collaboration with Julie Libarkin at Ohio University. Work continues with Doug Burbank at UCSB and Caltech graduate student Willy Amidon.