Calcium has six stable isotopes, two of which occur in nature: stable
40Ca and radioactive 41Ca with a half-life = 106
years. 97% of the element is in the form of 40Ca. 40Ca
is one of the daughter products of 40K decay, along with 40Ar.
While K-Ar dating has been used extensively in the geological sciences,
the prevalence of 40Ca in nature has impeded its use in dating.
Techniques using mass spectrometry and a double spike isotope dilution
have been used for K-Ca age dating. Russell et al. (1978) compared 40Ca
to 44Ca; Marshall and DePaolo (1982) used 40Ca and
another stable isotope, 42Ca, with a good deal of success.
The isotopic composition of calcium in rocks and minerals varies because
of the formation of 40Ca by beta decay of 40K and
because of the fractionation of Ca isotopes by natural processes. The possibility
for fractionation is enhanced by the large differences in mass of Ca isotopes.
This fractionation is important not only because it interferes with the
K-Ca dating method, but because of hopes that the variation in Ca isotopes
might convey process information.
Unlike cosmogenic isotopes that are produced in the atmosphere, 41Ca
is produced by neutron activation of 40Ca. Most of its production
is in the upper meter or so of the soil column where the cosmogenic neutron
flux is still sufficiently strong. Variations in the isotopic ratios of
calcium are due to particle-induced changes in the nucleus of the atom.
The geochemical behavior of Ca at low temperatures is mainly controlled
by calcite formation and dissolution. This would suggest that 41Ca
may be useful in studies that are focused on the carbon cycle. Calcium
may eventually be one of the most useful elements to hydrologic studies,
participating in a wide variety of hydrochemical processes. Currently,
no published studies have applied 41Ca to hydrology.
41Ca has received much attention in stellar studies because
41Ca decays to 41K, a critical indicator of solar-system
anomalies. 41K excesses found in solar-system material suggests
that stellar core material may have been added to the solar nebula Allende
as late as 2 to 20 Myr before condensation (Hutcheon et al., 1984).
Source of text: This review was assembled by Dan Snyder,
Carol Kendall and Eric Caldwell, primarily from Faure (1986), Dicken (1995)
and Nimz (1998).
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