Ca isotope study of Ordovician dolomite, limestone, and anhydrite in the WillistonBasin: Implications for subsurface dolomitization and local Ca cycling

摘要

A difference of 0.61‰ is reported between dolomite ( — 1.66‰) and its precursor, limestone ( —1.05‰), inthe Yeoman Formation (Red River equivalent) of the Williston Basin, southeastern Saskatchewan. Thesignificance of the large difference found, and the preference for light isotope enrichment in the dolomite, isevaluated with assistance from ~(87)Sr/~(86)Sr ratios and dolomitization models previously proposed for thestudied rocks. In particular, I explore the possibility that δ~(44)Ca values in dolomite reflect the δ~(44)ofthe dolomitizing fluids, without any correction for mineral-fluid fractionation. This hypothesis is based onrecent studies showing negligible isotopic fractionation between carbonates and Ca~(+2)bearing waters at verylow precipitation rates (Fantle and DePaolo, 2007; Jacobson and Holmden, 2008). If correct, calcium isotopeshold promise as a tool for discriminating among numerous hydrological models of dolomite formation. As acase in point, the δ~(44)Ca value of the Yeoman dolomite ( — 1.66‰) is too low to reflect Ca derived from thefollowing sources: (1) the original limestone (-1.05%0), (2) overlying beds of anhydrite ( — 1.28‰), or (3)evaporated seawater ( — 0.25‰). Paleozoic carbonates ranging as low as —1.7‰ appear to be the lightestsource of Ca in the basin succession, suggesting that the Mg containing dolomitizing fluid was a connatewater with a previous history of water-rock interactions with Paleozoic carbonates, possibly involvingearlier episodes of dolomitization and light Ca release into migrating basinal fluids. The relatively high seawater δ~(44)Ca values inferred from measurements of Yeoman limestone and Lake Almaanhydrite suggest that seawater in the Williston Basin (450 Ma) was 0.22-0.46%0 higher in 844Ca than thecontemporaneous ocean, based on globally distributed brachiopods (Farkas et al., 2007). This findingsuggests that δ~(44)Ca values preserved in the deposits of epeiric seas may bias δ~(reconstruction theocean secular record, if local Ca cycling effects are not taken into account. Equations describing the isotopebalance of Ca in an epeiric sea indicate that seawater δ~(44)Ca values may be higher, lower, or equal to the δ~(44)Cavalue of the ocean, depending on the sizes of local scale Ca deposition and weathering fluxes in relation toseawater Ca exchange fluxes.