From Radiogenic Source or Sink? --- Simple and Complex 4He Outgassing
Behaviors and Diffusion Systematics Observed During Continuous Ramped
Heating of Apatite
Abstract
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.