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Characterization of calcium isotopes in natural and synthetic barite |
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| Authors: | Elizabeth M Griffith Edwin A Schauble Adina Paytan |
| Institution: | a Department of Geological & Environmental Sciences, Stanford University, Building 320, Room 118, Stanford, CA 94305, USA b Institute of Marine Sciences, University of California, Santa Cruz, CA 95064, USA c Department of Earth and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095, USA d Branch of Regional Research, Water Resources Division, U.S. Geological Survey, MS 420, 345 Middlefield Rd., Menlo Park, CA 94025, USA |
| Abstract: | The mineral barite (BaSO4) accommodates calcium in its crystal lattice, providing an archive of Ca-isotopes in the highly stable sulfate mineral. Holocene marine (pelagic) barite samples from the major ocean basins are isotopically indistinguishable from each other (δ44/40Ca = −2.01 ± 0.15‰) but are different from hydrothermal and cold seep barite samples (δ44/40Ca = −4.13 to −2.72‰). Laboratory precipitated (synthetic) barite samples are more depleted in the heavy Ca-isotopes than pelagic marine barite and span a range of Ca-isotope compositions, Δ44/40Ca = −3.42 to −2.40‰. Temperature, saturation state,  , and aCa2+/aBa2+ each influence the fractionation of Ca-isotopes in synthetic barite; however, the fractionation in marine barite samples is not strongly related to any measured environmental parameter. First-principles lattice dynamical modeling predicts that at equilibrium Ca-substituted barite will have much lower 44Ca/40Ca than calcite, by −9‰ at 0 °C and −8‰ at 25 °C. Based on this model, none of the measured barite samples appear to be in isotopic equilibrium with their parent solutions, although as predicted they do record lower δ44/40Ca values than seawater and calcite. Kinetic fractionation processes therefore most likely control the extent of isotopic fractionation exhibited in barite. Potential fractionation mechanisms include factors influencing Ca2+ substitution for Ba2+ in barite (e.g. ionic strength and trace element concentration of the solution, competing complexation reactions, precipitation or growth rate, temperature, pressure, and saturation state) as well as nucleation and crystal growth rates. These factors should be considered when investigating controls on isotopic fractionation of Ca2+ and other elements in inorganic and biogenic minerals. |
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