New Analytical Developments in K-Ca Geochronology Using MC-ICP-MS

While the decay of 40K to 40Ar has been used extensively in geochronology, the other arm of this branched decay, the beta decay of 40K to 40Ca, has received much less attention, despite the system's potential application to important geologic problems. Unfortunately, measuring calcium isotopic ratios has proven to be challenging with conventional single collector TIMS instruments given the relative signal intensity of 40Ca with respect to all other calcium isotopes. Additionally, TIMS analyses take many hours (and a steady signal) to complete and the thermal source does a relatively poor job of ionizing calcium. Here we present a technique that make it possible to precisely and reproducibly measure calcium isotopic ratios in solution using a multicollector ICP-MS. The success of this technique was predicated upon overcoming two significant analytical problems: 1) the interference of argon (the carrier gas) at mass 40 and 2) the general problem of mass distribution of calcium isotopes, in which 40Ca is significantly more abundant than the other naturally - occurring isotopes. Through repeated measurement of 40Ca/42Ca in the NIST SRM 915b we achieve an external precision of ~ 0.05% at the 95% confidence interval. Internal errors on individual analyses are ~ 0.02% at the 2 σ level. We are applying our MC-ICP-MS technique for calcium isotope analysis to K-rich salts and authigenic glauconites of known age, as well as to igneous whole rock and mineral separates with variable K/Ca ratios. Measured 40Ca/42Ca ratios in these materials match well with modeled values based on their ages and K/Ca ratios.