Determination of cosmogenic 36Cl in rocks by isotope dilution: innovations, validation and error propagation

Successful application of isotope dilution to 36Cl dating requires that Cl be retained during digestion. We performed extractions in a sealed acid-digestion bomb to prevent Cl losses and to reduce digestion times by more than 90%. Isotope dilution gives 36Cl/Cl values within 1σ of conventional (unspiked) values for 8 paired silicate samples, and gives Cl concentrations that are consistent with the ion specific electrode method for 14 of 17 silicate samples. Results from three spiked replicates of a carbonate sample are also consistent with the unspiked 36Cl/Cl, but we found that isotope dilution gives a more accurate estimate of native Cl concentration than the ion specific electrode method. We also prepared five limestone samples in open vessels in the presence of excess Ag+ to prevent volatilization of Cl. This method would permit processing of larger samples (the bomb's capacity is 5 g), and would be useful for samples with low concentration of Cl or low 36Cl/Cl when insoluble fluorides do not precipitate in the digestion vessel. Results from paired samples digested in both open and closed vessels suggest that open-vessel digestion is a reliable way of preparing spiked carbonate samples.