The development of the (U-Th)/He thermochronometer

Citation

Wolf, Richmond Andrew (1997) The development of the (U-Th)/He thermochronometer. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kz3w-5t57. https://resolver.caltech.edu/CaltechETD:etd-10112005-111913

Abstract

(U-Th)/He dating of apatite provides a tool for recording the low temperature (<100°C) history of the crust. A model based on stopping distances in apatite relates the fraction of alpha particles emitted from the crystal during U and Th decay to crystal size. Helium ages for different sized apatite aliquots are indistinguishable when corrected for the effects of alpha emission. Diffusion coefficients were measured by the incremental outgassing of helium from apatite. The measured range of diffusion parameters is nearly identical for apatites of different chemical composition, grain size, and grain morphology. Isothermal experiments are consistent with spherical diffusion domains which are smaller than the physical grain size.

Helium ages may reflect complex thermal histories where samples spend considerable amounts of time in the region where helium is only partially retained. Therefore, the solution to the full radiogenic helium diffusion/production equation is used to interpret helium ages instead of Dodson's (1973) closure temperature formulation. The time required to achieve a steady state between helium production and diffusion at various temperatures can be determined, as well as the range of temperatures defining the helium partial retention zone (the region where helium retentivity is most sensitive to temperature). In general, this zone resides at ~40-80°C (~2±1 km depth for typical continental geothermal gradients). This is ~35°C cooler than the analogous apatite fission track partial annealing zone.

Application of the (U-Th)/He method to natural systems has provided consistent results and useful geologic information. Helium ages from the Cajon Pass Drillhole decline from 41.1 to 0.3 Ma between 526 and 2018 m depth, and appear to be in equilibrium with the present thermal gradient. This is in contrast to the previous assertion that the region is in a thermal transient resulting from recent erosion. Helium ages from Mt. San Jacinto, California, decrease monotonically from 79 to 17 Ma with sample elevation, and suggest a modest (~7°) westward tilting of the block with no evidence of rapid exhumation during this period. Helium ages from Mt. San Jacinto and Cajon Pass are younger than other available thermochronometric techniques, consistent with predictions from laboratory diffusion data.

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