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 ²⁶Al, ¹⁰Be, ¹⁴C, ²¹Ne and ³⁶Cl.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)×10⁹atg^-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)×10¹³atg^-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 etal., 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.