Effect of dolomitization on isotopic records from Neoproterozoic carbonates in southwestern Mongolia

Abstract

Carbon isotope values from shallow-marine carbonate rocks, including those from many dolomitized successions, are the primary lens through which we interpret the ancient carbon cycle. Carbon isotopes are typically regarded as being robust to alteration during dolomitization due to the high carbon content of the rock compared to the fluid. However, chemostratigraphic studies of the Neoproterozoic Tsagaan-Olom Group in southwestern Mongolia exhibit 3–12‰ differences in carbon isotopes between stratigraphically equivalent limestone and dolomitized successions. To understand the origins of this geochemical variation, we conducted detailed geological mapping, petrographic, isotopic (δ13C, δ18O, δ44/40Ca, and δ26Mg), and fluid inclusion analyses of carbonates in the Taishir, Ol, and Shuurgat formations of the Tsagaan-Olom Group. Stratigraphic and textural constraints distinguish fabric retentive early dolomitization (Dolomitization Event 1) that occurred in the Taishir and Ol formations during and soon after deposition of the ca. 635 Ma Marinoan cap dolomite of the Ol Formation and fabric destructive dolomitization that occurred after the deposition of the younger Shuurgat Formation (Dolomitization Event 2), either prior to or during early Cambrian foreland basin formation. The dolomitizing fluids moved through porous stratigraphic units bound by impermeable shale of the Khongor and Zuun-Arts formations. Dolomitization homogenized the isotopic records of all Neoproterozoic Tsagaan-Olom Group dolomites; undolomitized successions show more extreme negative excursions from more positive background values. The salinity and isotopic values of seawater, the dolomitizing fluids, and the original platform carbonate were estimated from fluid inclusion data and a numerical model of diagenetic dolomitization. These results demonstrate that both early and late dolomitization can have a profound effect on carbon isotopic records of carbonates; however, with a multi-isotope approach the original carbon isotopic composition of platform-top carbonate can be distinguished from that of seawater and other dolomitizing fluids.