Systematics of helium diffusion sinks in apatite demonstrated by 4He/3He degassing experiments and modeling

Abstract

Widely reported overdispersion of apatite (U-Th)/He ages has encouraged efforts to explore He diffusion kinetics and systematics in apatite. Studies such as continuous ramped heating (CRH) of apatite have revealed complex laboratory He behavior that is often correlated with abnormally old (U-Th)/He ages. Stemming from these studies is a hypothesis that diffusion sinks within apatite grains can reversibly trap radiogenic ⁴He over both geologic time and during laboratory heating. This sink-related trapping is further hypothesized to be temperature dependent and can hence potentially provide thermal-history information. In this work, we conducted ⁴He/³He degassing experiments from proton-irradiated apatite samples using a CRH-like heating strategy. We demonstrate that the synthesized ³He also exhibits complex release behavior which (1) confirms that the proposed sink-related trapping occurs during laboratory heating, and (2) provides foundational evidence for possibly qualifying the nature and quantifying the abundance and/or distribution of diffusion sinks. Our combined ⁴He/³He observations show that an apatite sample’s thermal history controls the amount of trapping over geologic time. Building upon the dataset from one of the sink-bearing samples, we attempt to simulate the sink characteristics using a simple random-walk code that can simulate trapping. The forward modeling can reproduce the ³He release observed in the laboratory experiment if multiple sinks characterized by different kinetics are present. Using such sink kinetics and the sample’s known thermal history, we find that the prediction of laboratory ⁴He release, which reflects both geologic and laboratory volume diffusion and trapping processes, is comparable to our observations. This work confirms that a temperature-dependent trapping mechanism is required to explain the complex He diffusion behavior seen in many apatite grains, and also serves as a proof-of-concept that helium trapped in sinks can provide added thermal history information.