While the decay of 40K to 40Ar has been used extensively in geochronology, the other arm of this bra

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New developments in K-Ca geochronology
Mary R. Cecil and Mihai Ducea
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 systems 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. We developed a technique
that makes 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 2s 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.