An inter-laboratory comparison of cosmogenic ^3He and radiogenic ^4He in the CRONUS-P pyroxene standard

This study reports an inter-laboratory comparison of the 3He and 4He concentrations measured in the pyroxene material CRONUS-P. This forms part of the CRONUS-Earth and CRONUS-EU programs, which also produced a series of natural reference materials for in situ produced ^(26)Al, ^(10)Be, ^(14)C, ^(21)Ne and ^(36)Cl. Six laboratories (GFZ Potsdam, Caltech Pasadena, CRPG Nancy, SUERC Glasgow, BGC Berkeley, Lamont New York) participated in this intercomparison experiment, analyzing between 5 and 22 aliquots each. Intra-laboratory results yield ^3He concentrations that are consistent with the reported analytical uncertainties, which suggests that ^3He is homogeneous within CRONUS-P. The inter-laboratory dataset (66 determinations from the 6 different labs) is characterized by a global weighted mean of (5.02 ± 0.12) × 10^9 at g^(−1) with an overdispersion of 5.6% (2σ). ^4He is characterized by a larger variability than ^3He, and by an inter-lab global weighted mean of (3.60 ± 0.18) × 10^(13) at g^(−1) (2σ) with an overdispersion of 10.4% (2σ). There are, however, some systematic differences between the six laboratories. More precisely, 2 laboratories obtained mean ^3He concentrations that are about 6% higher than the clustered other 4 laboratories. This systematic bias is larger than the analytical uncertainty and not related to the CRONUS-P material (see Schaefer et al., 2015). Reasons for these inter-laboratory offsets are difficult to identify but are discussed below. To improve the precision of cosmogenic ^3He dating, we suggest that future studies presenting cosmogenic 3He results also report the ^3He concentration measured in the CRONUS-P material in the lab(s) used in a given study.