Theoretical studies of the stable isotope geochemistry of iron, chlorine, and chromium are presented, with the goal of providing a framework to aid interpretations ofnew measurements and to identify promising areas for future study. In addition, new oxygen-isotope measurements of Mesozoic and Cenozoic granitoids from the northeastern Great Basin are used to constrain the temporal evolution of magmatic sources in the region.
The stable isotope compositions of elements heavier than sulfur (atomic no. 16) are generating great geochemical interest, now that new mass-spectrometry techniquesmake it possible to measure their isotopic abundances with high precision. Theoretical calculations for three of these elements (iron, chlorine, and chromium) are made usingpublished infrared, Raman, and inelastic neutron scattering measurements of vibrational frequencies, in combination with empirical and ab initio force-field estimates of unknown frequencies. The calculations suggest that a number of natural processes can drive significant stable isotope fractionations of heavy elements, including oxidation/reduction during the precipitation or dissolution of dissolved metals (inorganically or organically),and bond-partner exchange during hydrothermal alteration, or degradation of Cl-bearing organic compounds. At equilibrium and 25°C, ^(56)Fe/^(54)Fe will be ~5‰ higher in[Fe(H_2O)_6]^(3+) than in coexisting [Fe(H_2O)_6]^(2+), ^(53)Cr/^(52)Cr will be ~6-7‰ higher in [CrO_4]^(2-) than in coexisting [Cr(H_2O)_6]^(3+) or Cr_2O_3, and aqueous Cl- will be ~2-3‰ lighter than coexisting alteration minerals like mica and amphibole.
Oxygen isotope measurements of whole-rock samples from granitoid plutons in the northeastern Great Basin suggest that two or three different types of source rocks were melted in varying proportions during the three stages of magmatism in this region in the Late Jurassic, Late Cretaceous, and mid-Cenozoic. Radiogenic-isotope measurements were previously made on the same samples. Late Cretaceous (90-70 Ma) granites have high δ^(18)O (+9.3 to + 12.1) and ^(87)Sr/^(86)Sr_i (0.711 to 0.734), and low εNd(-13 to -23) indicating that their source was dominated by evolved crustal sediments and basement. However, late Jurassic plutons in this region span a larger range of δ^(18)O values (+7.2 to + 13.2), despite having Sr and Nd isotopic compositions that are much less suggestive of an ancient crustal component (^(87)Sr/^(86)Sr_i = 0.705 to 0.711, εNd = -2.5 to -6.5) than the Late Cretaceous plutons, suggesting moderate to extensive mixing orassimilation of high-δ^(18)O sedimentary rocks into a more mafic parent melt. The 40-25 Ma Cenozoic plutons (δ^(18)O = +7.0 to + 9.7, ^(87)Sr/^(86)Sr_i = 0.707 to 0.717, εNd= -13.2 to -26.3) probably have a source dominated by continental basement. The Cenozoic plutons can be subdivided into a higher δ^(18)O (+8.6 to + 9.7) southern group and a lower δ^(18)O (+7.0 to + 8.2) northern group across a Crustal Age Boundary (CAB) at roughly 40° 40'N; this CAB coincides with a radiogenic isotope boundary defined with the same samples, as well as with the approximate southern limit of exposure of Archeanbasement. The low δ^(18)O values and depleted lead isotope compositions of the Lower Array (northern) samples indicate that Archean age basement is present beneath a largearea of the most northeasterly part of the Great Basin. A further, speculative conclusion is that δ^(18)O of the (meta)sedimentary source region may have dropped by 2-3‰ as a result of fluid-rock interaction sometime between the Jurassic and Late Cretaceous magmatic episodes.
