Determination of chlorine concentrations in whole rock: Comparison between prompt-gamma activation and isotope-dilution AMS analysis

Accurate determination of chlorine concentrations in terrestrial rocks is of importance for the interpretation of terrestrial in-situ cosmogenic ³⁶Cl. Neutron capture by ³⁵Cl, together with production from Ca and K, is one of the three major production pathways of ³⁶Cl in rocks. Here, we present an inter-comparison of chlorine determinations by two procedures. The first approach is an independent Cl determination by prompt-gamma (neutron) activation analysis (PGAA). The second method is isotope-dilution based on isotopically enriched stable chlorine carrier added during chemical sample preparation for accelerator mass spectrometry (ID-AMS). Twenty-six (26) whole rock samples have been processed for PGAA and ID-AMS analysis. This study constitutes the first published inter-comparison for concentrations below 100 μgCl/g. Our results show no significant difference in Cl concentrations between methods. This agreement indicates good retention of chloride during the procedure we employ for whole rock sample dissolution. No significant loss of stable chlorine from either the spike or the sample occurs before isotopic equilibrium is reached, prior to AgCl precipitation. Uncertainties, which are <5% for both methods, affect the uncertainty of the total ³⁶Cl production rate less than 2% for our samples.The Cl concentration measured by PGAA can be used to calculate the amount of isotopically enriched spike for AMS-ID sample preparation with the aim to optimize ³⁶Cl analysis. Furthermore, PGAA offers an advance for the interpretation of ³⁶Cl measurements. It allows measurement of concentrations of major, minor and trace elements including the elements for ³⁶Cl production (Cl, K, Ca, Ti, and Fe), as well as of neutron absorbers and neutron moderators (H, B, Sm and Gd). These measurements are performed simultaneously and with a precision necessary for calculating the relative contributions to ³⁶Cl production from the different mechanisms.