C, CA, AND MG ISOTOPE STUDY OF CO-EXISTING LIMESTONE AND DOLOMITE OF THE CRYOGENIAN TO EARLY EDIACARAN TSAGAAN-OLOM GROUP, ZAVKHAN TERRANE, MONGOLIA

Carbonate dominated strata of the Cryogenian to early Ediacaran Tsagaan-Olom Group exposed on the Zavkhan Terrane of Mongolia are well-exposed and preserved and provide an excellent opportunity to document natural variations of geochemical cycles through this critical interval of Earth History. Particularly, the Cryogenian Taishir and Ediacaran Ol formations (fms) are composed of coeval limestone and dolomitized strata providing an excellent natural laboratory to better understand dolomitization. Here we report δ¹³C_carb, δ¹⁸O_carb, δ^44/40Ca, and δ²⁶Mg values through coeval limestone and dolomitized successions in the context of detailed geological mapping, petrographic, fluid inclusion, and elemental analysis.

δ¹³C_carb values of carbonate are typically considered to be rock buffered to dolomitizing fluids. However, chemostratigraphic studies of the Taishir and Ol fms demonstrate large, <10‰, differences in δ¹³C_carb between limestone and dolomitized sections. Interestingly, the δ^44/40Ca value of carbonates of limestone section covaries with the δ¹³C_carb and the corresponding value of carbonates in dolomitized section loses its covariance. δ²⁶Mg values of dolostones show little variability through both the Taishir and Ol fms. Textural fabrics and fluid inclusion data suggest that dolomitization was driven by focused fluid flow, potentially driven by Ediacaran to Cambrian orogenic events. However, δ²⁶Mg values suggest low temperature, early diagentic dolomitization. In either case, the dolomitizing fluid drove large-scale isotopic variability, especially in δ¹³C_carb and δ^44/40Ca. We suggest that the large signal in δ¹³C_carb is related to large difference in the starting isotopic composition of the Cryogenian host rock and dolomitizing fluid as it evolved from Ediacaran seawater. During the Phanerozoic, the isotopic effect of dolomitization may be more subtle where there is less of a difference between the δ¹³C_carb values of recently deposited sediments and seawater.