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From Radiogenic Source or Sink? --- Simple and Complex 4He Outgassing Behaviors and Diffusion Systematics Observed During Continuous Ramped Heating of Apatite
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  • Hongcheng Guo,
  • Peter Zeitler,
  • Bruce Idleman,
  • Kalin McDannell,
  • Annia Fayon,
  • Paul Fitzgerald
Hongcheng Guo
Lehigh University

Corresponding Author:[email protected]

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Peter Zeitler
Lehigh University,Dartmouth College
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Bruce Idleman
Lehigh University
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Kalin McDannell
Dartmouth College
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Annia Fayon
University of Minnesota
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Paul Fitzgerald
Syracuse University
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We performed continuous ramped heating (CRH) on apatites from various tectonic settings and found two major types of 4He outgassing behavior. Apatites with good (U–Th)/He age reproducibility show simple and unimodal incremental gas-release curves that are similar to those predicted by volume diffusion, whereas samples exhibiting greater age dispersion have complex gas-release curves that feature He ‘spikes’ and secondary gas-release peaks deferred to higher temperatures. Age dispersion from the apatites with simple outgassing behavior can be explained by variability in their relative He retentivity observed on Arrhenius arrays—with similar activation energy but different diffusivities, which may be a result of fine-scale crystal imperfections. The observed high-temperature gas component and the resulting “too-old” AHe ages, combined with an attempt at age correction based on secondary peak gas-removal, seem to indicate the existence of sink-like crystal imperfections that can trap 4He both in nature and during laboratory heating. CRH analysis at different heating rates further suggests that the second gas-release peak occurs at varying temperatures, indicating that the sink is kinetically responsive, and if characterizable, may contain additional thermal-history information. These observations suggest that (1) CRH can be deployed as a routine screening tool for (U–Th)/He dating, (2) diffusion of 4He could be complicated by imperfections beyond radiation damage, and (3) if the proposed sinks exist and retain appreciable 4He, there are opportunities to explore additional thermal histories of natural samples.