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

Abstract

A difference of 0.61‰ is reported between dolomite (− 1.66‰) and its precursor, limestone (− 1.05‰), in the Yeoman Formation (Red River equivalent) of the Williston Basin, southeastern Saskatchewan. The significance of the large difference found, and the preference for light isotope enrichment in the dolomite, is evaluated with assistance from 87Sr/ 86Sr ratios and dolomitization models previously proposed for the studied rocks. In particular, I explore the possibility that δ 44Ca values in dolomite reflect the δ 44Ca values of the dolomitizing fluids, without any correction for mineral-fluid fractionation. This hypothesis is based on recent studies showing negligible isotopic fractionation between carbonates and Ca + 2 bearing waters at very low precipitation rates (Fantle and DePaolo, 2007; Jacobson and Holmden, 2008). If correct, calcium isotopes hold promise as a tool for discriminating among numerous hydrological models of dolomite formation. As a case in point, the δ 44Ca value of the Yeoman dolomite (− 1.66‰) is too low to reflect Ca derived from the following sources: (1) the original limestone (− 1.05‰), (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 lightest source of Ca in the basin succession, suggesting that the Mg containing dolomitizing fluid was a connate water with a previous history of water–rock interactions with Paleozoic carbonates, possibly involving earlier episodes of dolomitization and light Ca release into migrating basinal fluids. The relatively high seawater δ 44Ca values inferred from measurements of Yeoman limestone and Lake Alma anhydrite suggest that seawater in the Williston Basin (450 Ma) was 0.22–0.46‰ higher in δ 44Ca than the contemporaneous ocean, based on globally distributed brachiopods (Farkas et al., 2007). This finding suggests that δ 44Ca values preserved in the deposits of epeiric seas may bias the δ 44Ca reconstruction of the ocean secular record, if local Ca cycling effects are not taken into account. Equations describing the isotope balance of Ca in an epeiric sea indicate that seawater δ 44Ca values may be higher, lower, or equal to the δ 44Ca value of the ocean, depending on the sizes of local scale Ca deposition and weathering fluxes in relation to seawater Ca exchange fluxes.