Cosmogenic 3He in terrestrial rocks: A review

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 μm 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. I also review 3 He c production rates and their spatial variability: the global database of absolute calibration sites yields a world-wide average 3 He c production rate in olivine and pyroxene of 124 ± 11 at g −1 yr −1 using the LSD scaling and the online CREp calculator ( https://crep.otelo.univ-lorraine.fr/#/ ). Cross-calibrations against 10 Be indicate that the ratio of the production rate of 3 He c in olivine/pyroxene to that of 10 Be in quartz is 33 ± 2 and increases by less than 7% between sea level and 5000 m elevation. This important observation demonstrates that 3 He in olivine/pyroxene and 10 Be in quartz are accurately scaled with existing models and can hence be considered as synchronized chronometers. However, 3 He c / 10 Be cross-calibrations based on 3 He c in accessory minerals (zircon, garnet , kyanite, apatite) yield unexpectedly high 3 He/ 10 Be production ratios of 40–60 above 3000 m elevation. As the capture of cosmogenic thermal neutrons by 6 Li is unlikely to explain this excess, other mechanisms are required, such as 3 He c inherited from previous exposure episodes, unrecognized specific reaction pathways, or the impact of snow cover. New cross-calibration data obtained by measuring 3 He c against other cosmogenic nuclides ( 14 C, 36 Cl, 21 Ne, 53 Mn) in different settings will advance our understanding of cosmogenic nuclide production rates and improve the accuracy and precision of applications relying on cosmogenic 3 He. Other improvements could extend the applicability of the 3 He geoscientific toolbox; for example, coupling 3 He with radioactive cosmogenic nuclides ( 10 Be, 14 C, 36 Cl, 53 Mn) will allow paleoaltimetry or the determination of burial ages or paleo-depths in intermediate and mafic terrains.