Cosmogenic 3He in terrestrial rocks: A review
Pierre‐Henri Blard
Abstract
This review article summarizes the state of the art of cosmogenic 3 He ( 3 He c ), with a focus on the most efficient methods and strategies for measuring this cosmogenic noble gas in the terrestrial minerals. Our ability to accurately and precisely measure cosmogenic 3 He in minerals is mainly constrained by the level of the non-cosmogenic 3 He background (i.e., magmatic, radiogenic, nucleogenic, and atmospheric 3 He), and thus by the geological characteristics of the samples. Constructing 3 He vs 4 He isochrons by analyzing several aliquots from the same surface sample constitutes a useful and overlooked method. Isochrons can be applied to mineral samples with homogeneous radiogenic 4 He concentration and variable magmatic helium concentrations. This method also allows the direct and joint determination of the cosmogenic 3 He concentration and the magmatic 3 He/ 4 He ratio, precluding the often-complicated step of vacuum crushing. I perform numerical modeling to explore the impact of the non-cosmogenic 3 He components on the final uncertainties and detection limits of 3 He c measurement. Reducing the magmatic component by selecting phenocrysts in the 100-500 m size fraction improves the precision of cosmogenic 3 He analyses. Moreover, it is important to measure U, Th, and Li concentrations in the analyzed minerals and their host rocks to ensure proper corrections for radiogenic 4 He and nucleogenic 3 He, improving both the accuracy and precision of the method. I discuss the most important aspects of analytical techniques, including the key parameters of noble gas mass spectrometry that result in accurate and precise helium isotopic measurements.