I. Predicting equilibrium stable isotope fractionations of iron, chlorine, and chromium. II. Oxygen-isotope investigation of Mesozoic and Cenozoic granitoids of the northeastern Great Basin, Nevada and Utah.

摘要

Theoretical studies of the stable isotope geochemistry of iron, chlorine, and chromium are presented, with the goal of providing a framework to help interpret new measurements and identify promising areas for future study. In addition, new oxygen-isotope measurements of Mesozoic and Cenozoic granites from the northeastern Great Basin are used to constrain the temporal evolution of magmatic sources in the region.; Natural variations in the stable isotope compositions of a number of elements heavier than sulfur have recently been measured. Theoretical calculations for three of these elements (iron, chlorine, and chromium) are made using published 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 stable isotope fractionations of heavy elements, including oxidation/reduction of dissolved metals, and bond-partner exchange reactions. At equilibrium and 25°C, 56Fe/54Fe will be ∼5‰ higher in [Fe(H₂O)₆]3+ than in coexisting [Fe(H ₂O)₆]2+, 53Cr/52Cr will be ∼6–7‰ higher in [CrO₄]2− than in coexisting [Cr(H₂O)₆]3+ or Cr₂O₃, and aqueous Cl− will be ∼2–3‰ lighter than alteration minerals like mica and amphibole.; Whole-rock 18O/16O measurements of granites in the northeastern Great Basin suggest that different source rocks were melted during three stages of magmatism. Radiogenic-isotope measurements were previously made on the same samples. Late Cretaceous (90–70 Ma) granites have high δ 18O (+9.3 to + 12.1), high 87Sr/86Sri (0.711–0.734), and low ϵNd (≤−13) indicating a source dominated by ancient, evolved crustal sediments. Late Jurassic plutons span a larger range of δ18O values (+7.2 to +13.2) despite having lower 87Sr/86Sri (≤0.711) and higher ϵNd (≥−6.5), suggesting that high-δ18O sediment was assimilated into mafic parent melts. The 40–25 Ma Cenozoic plutons (δ18O = +7.0 to +9.7, 87Sr/86Sr _i = 0.707 to 0.717, ϵNd = −13.2 to −26.3) appear to be dominated by continental basement-type material. Cenozoic plutons can be subdivided into a higher δ18O (>+8.6) southern group and a lower δ18O (+8.2) northern group across a boundary previously observed with radiogenic isotope measurements. Northern Cenozoic granites suggest that Archean basement underlies part of the Great Basin.