Helium diffusion systematics in apatites: lessons from Continuous Ramped Heating analysis

There is now a growing body of literature that reports over-dispersed (U-Th)/He ages from apatites. To address this challenge, we have performed continuous ramped heating (CRH) experiments on apatites from various geologic settings to characterize grain-specific helium (⁴He) diffusion behavior. Several first-order results emerge from our CRH analyses. (1) It became clear that simple volume diffusion, even accounting for radiation damage, cannot completely describe the diffusion of ⁴He in apatite. Two major types of ⁴He degassing behavior were broadly observed. Apatites with good (U-Th)/He age reproducibility show simple and unimodal incremental degassing curves that are similar to those predicted by volume diffusion, whereas samples exhibiting greater age dispersion, often accompanied by anomalously old ages, have complex gas-release curves that feature secondary gas-release peaks deferred to higher temperatures. (2) In practice, CRH can serve as a screening tool to reduce the dispersion of apatite (U-Th)/He ages, especially for those obtained from samples that have experienced slow-cooling. (3) Even among apatites in which ⁴He does show broad volume diffusion behavior (i.e., size and radiation-damage modulated volume diffusion), CRH analysis reveals kinetic variability of ⁴He diffusion. (4) Diffusion sinks, which are capable of trapping radiogenic ⁴He during both geologic processes and laboratory heating, can explain the observed high-temperature gas release during CRH analyses. CRH results of a sample suite from an active helium partial retention zone demonstrate that the release of sink-trapped ⁴He is temperature dependent rather than being controlled by a threshold mechanism. The results from our CRH analyses carry two critical implications. First, CRH is suitable for routine implementation that enables thermochronology practitioners to focus their measurement and interpretation on apatites in which ⁴He diffusion obeys volume diffusion. Second, diffusion sinks provide opportunities to extract additional thermal-history information providing a description of grain-specific trapping dynamics. Work in this area is ongoing via ⁴He/³He diffusion experiments, through which degassing of proton-irradiated ³He in a sample provides information trapping dynamics and degassing of radiogenic ⁴He constrains the sample’s thermal history.