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Periodic Table--Calcium

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).

Bowen, R. (1988). Isotopes in the Earth Sciences. Elsevier Applied Science, New York, 647 pp.
Dicken, A.P. (1995). Radiogenic Isotope Geology. Cambridge University Press, New York, 452 pp.
Faure, G. (1986). "The K-Ca method of dating", In: Principles of Isotope Geology, second edition, John Wiley and Sons, New York, pp. 275-281.
Hutcheon, I.D., Armstrong, J.T. and Wasserburg, G.J. (1984). "Excess in 41K in Allende CAZ: confirmation of a hint." In: Lunar Planet. Sci. XV, Lunar Planet. Inst., pp. 387-388.
Junge, C.E. and Werby, R.T. (1958). "The concentration of chloride, sodium, potassium, calcium, and sulfate in rain water over the United States." J. Meteorol., 15: 417.
Marshall, B. D., and DePaolo, D. J. (1982). "Precise age determinations and petrogenic studies using the K-Ca method." Geochim. Cosmochim. Acta, 46: 2537-2545.
Kutschera, W. (1990). "Accelerator mass spectrometry: A versatile tool for research." Nucl. Instr. Meth. Phys. Res., B50: 252.
Nimz, G. J., (1998). "Lithogenic and Cosmogenic Tracers in Catchment Hydrology", In: C. Kendall and J.J. McDonnell, (Eds),Isotope Tracers in Catchment Hydrology, Elsevier, Amsterdam, pp. 247-290.
Russell, W. A., Papanastassiou, D. A., and Tombrello, T. A. (1978). "Ca isotope fractionation on the Earth and other solar sstem materials." Geochim. et Cosmochim. Acta., 42, 107.
Related Links
Periodic Table
Fundamentals of Stable Isotope Geochemistry
General References
Isotope Publications
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