Amidon, William H. (2010) Cosmogenic ^3He and ^(21)Ne dating in accessory mineral phases. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05192010-102315391
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 ^(10)Be, ^(26)Al, and ^(36)Cl. 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 ^(10)Be 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 ^(14)C 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 ^(10)Be 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.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Subject Keywords:||cosmogenic, helium, neon, lithium|
|Degree Grantor:||California Institute of Technology|
|Division:||Geological and Planetary Sciences|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||17 May 2010|
|Author Email:||wamidon (AT) gps.caltech.edu|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||William Amidon|
|Deposited On:||21 May 2010 17:15|
|Last Modified:||26 Dec 2012 03:25|
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