Behavior of helium diffusion sinks in apatite: Evidence from continuous ramped heating analysis of borehole and well-characterized samples

Abstract

Diffusion sinks have been proposed to explain the complex high-temperature 4He release often seen in Continuous Ramped Heating (CRH) analysis of apatites, which often correlates with over-dispersed and anomalously old (U-Th)/He ages. In this work, we first conducted CRH experiments with various ramping rates and cycled CRH experiments that confirm that the high-temperature 4He release from diffusion sinks is systematically temperature sensitive at laboratory time scales. We then analyzed apatite grains from the KTB borehole (German Continental Deep Drilling program) that have well-constrained thermal histories to test whether different thermal histories could alter the trapping of 4He at geologic time scales. The KTB samples show large intra-sample single-grain age dispersion, with most ages being older than predictions from a 4He radiation-damage diffusion model RDAAM. Individual grains from shallower depths show complex 4He outgassing behaviors, including the expected low-temperature volume-diffusion loss, sudden release spikes, and a higher- temperature-dependent component. Grains from the current partial retention zone and greater depths retain considerable helium and exhibit exclusively complex outgassing behavior dominated by high-temperature release in the laboratory. KTB samples show a systematic difference with depth in the proportion of low- and high-temperature gas-release gas components. Deeper samples, having long open-system thermal histories, contain a greater proportion of 4He that appears to have been trapped compared to the simpler volume-diffusion behavior exhibited by shallower samples that experienced a simpler monotonic cooling history. While greatly reduced, the high-temperature component is still present in the highest-temperature samples analyzed, well hotter than the partial retention zone. These observations confirm that in apatite trapping and release of helium from sinks is temperature dependent at geological time scales as well. This supports the suggestion that additional thermal-history information from apatites might be obtained using 4He/3He analysis, with laboratory release of 3He providing information about trap distribution and kinetics and 4He providing information about the geologic thermal history.