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

Measurements of cosmogenic Cl-36 in terrestrial rocks provide quantitative information about exposure ages of landforms and surface features. The isotope dilution method for preparing Cl-36 samples is now widely used because it allows Cl-36 and Cl to be measured simultaneously on a single accelerator mass spectrometry target, increases the accuracy and precision of Cl determinations, and reduces rock sample size and laboratory work. In this paper we describe a new implementation of isotope dilution to Cl-36 dating, report experimental data verifying the accuracy of this approach, and show how errors in the measured stable isotope ratio propagate to errors in exposure ages. Successful application of isotope dilution to Cl-36 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 Cl-36/Cl values within 1 sigma 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 Cl-36/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 Cl-36/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. (c) 2006 Elsevier B.V. All rights reserved.