Cosmogenic ³He and ²¹Ne Dating in Accessory Mineral Phases

Citation

Amidon, William Henry (2010) Cosmogenic ³He and ²¹Ne Dating in Accessory Mineral Phases. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/R6JB-BW27. https://resolver.caltech.edu/CaltechTHESIS:05192010-102315391

Abstract

The development of terrestrial cosmogenic nuclide dating has led to applications as varied as the dating of glacial moraines, establishing slip-rates on faults, measuring the erosion rates of basins, and measuring rates of soil formation. Studies in many of these fields could greatly benefit from analysis of far more samples than can be easily dated using ¹⁰Be, ²⁶Al, and ³⁶Cl. The rapid preparation and analysis of samples for cosmogenic ³He often allows a greater number of samples to be analyzed, but has so far been applied primarily to olivine and pyroxene in mafic rocks. Because ³He is produced in all mineral phases, it can potentially be applied in almost any lithology. The goals of this thesis is to expand the range of target lithologies suitable for cosmogenic ³He dating by calibrating production rates of cosmogenic ³He in accessory mineral phases such as apatite, zircon, and garnet. Results are presented from three calibration studies: glacial moraine boulders in the Nepal Himalaya, young rhyolite surfaces from California’s Coso volcanic field, and rhyolite surfaces scoured by the Bonneville flood near Twin Falls, Idaho. Both the Nepal and Coso studies compare ³He in zircon, apatite, and garnet against ¹⁰Be in quartz, finding that higher than expected ³He concentrations are likely due to anamolous elevation scaling in the Himalaya, and to production of ³He via neutron capture on ⁶Li at Coso. The Idaho calibration study is unique in that it is calibrated against the age of the Bonneville outburst flood (known by ¹⁴C dating), and uses a shielded sample to definitively document Li-produced ³He components in the deep sub surface. Collectively, these studies highlight several challenges associated with cosmogenic ³He dating of accessory phases: the difficulty in measuring small amounts of cosmogenic ³He in the presence of large amounts of radiogenic ⁴He, the importance of production of ³He via neutron capture on ⁶Li, and the redistribution of energetic ³He and ³H between adjacent mineral phases. Despite these challenges, adopting a ¹⁰Be production rate of 4.51 at g^-1 a^-1 in quartz (Balco et al., 2008), brings three independent ³He production rate estimates into good agreement with grand means of 103 ± 3, 133 ± 6, and about 134 ± 13 at g^-1 a^-1 in zircon, apatite, and spessartine garnet respectively. Such agreement suggests that these phases are suitable for cosmogenic dating. ³He in accessory phases may enable a range of unique applications including the study of ancient sediments, paleo-altimetry, and rates of chemical weathering in soils.

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