A PARAGENETIC MODEL FOR ISOTOPIC VARIABILITY AND BARITE FORMATION IN BASAL EDIACARAN CAP CARBONATES OF MONGOLIA

Carbonate dominated strata of the Cryogenian Taishir Formation in Mongolia are bounded by Sturtian and Marinoan glacial deposits and are overlain by the Ediacaran Ol and Shuurgat formations. Here we develop a depositional model of the Taishir, Ol and Shuurgat formations to provide context for barite deposits, dolomitization, and carbon, oxygen, strontium, sulfur and magnesium isotope data.

These carbonates are preserved as limestone, partially dolomitized carbonate, and recrystallized dolomite. Map relationships suggest that dolomitization of the upper portion of the Taishir Formation took place during the post-Marinoan glacio-eustatic transgression and deposition of the Ol Formation. Carbonate carbon and oxygen isotope values are markedly different between the Cryogenian sections preserved as limestone and the same sections preserved as dolomite and this variability also exists in the overlying Ol Formation. The Mg isotopic composition of the dolomitized carbonates is the same as the overlying Marinoan cap carbonate that has variable carbon isotope values. This provides evidence for the dolomitization fluid/event being one and responsible for the geochemical proxy variation between the altered and unaltered carbonates.

Multiple sulfur isotope analyses (δ³⁴S and Δ³³S) on bedded barite preserved at the top of the Ol cap carbonate preserve isotopically heavy sulfur isotope values. These values are indistinguishable from barites in stratigraphically equivalent horizons in cap carbonates from West Africa and NW Canada. The existence of barites with the same sulfur isotope signature at the same horizon globally suggests that this signal is from a global, well mixed seawater sulfate reservoir. We propose that circulation cells set up during deglaciation pumped seawater through the margin, which drove both dolomitization and the mobilization of continental barium dissolved in expulsed fresh water. In this scenario, when barium-rich fresh water mixed with seawater, barite super-saturation was reached. This model can explain the patchy distribution of barites in cap carbonates and their association with dolomitization fronts in Mongolia, and can be tested with further geochemical studies.