B. Marshall, D. DePaolo
Dec 1, 1982
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Journal
Geochimica et Cosmochimica Acta
Abstract
High precision mass spectrometric determination of calcium isotope ratios allows the 40K → 40Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. 40Ca42Ca is used to monitor enrichments in 40Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low KCa (∼0.01) that it has retained “primordial” 40Ca42Ca = 151.016 ± 0.011 (normalized to 42Ca44Ca = 0.31221), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. 40Ca42Ca ratios can be conveniently expressed relative to this value as ϵCa in units of 10−4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations (λK = 0.5543 b.y.−1, λβ−λK = 0.8952, 40K = 0.01167%). The initial 40Ca42Ca of 151.024 ± 0.016 (ϵCa = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of 40K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.