Testing the viability of normal seawater dolomitization in lower Cambrian carbonates using clumped isotopes

While dolomitization by normal seawater has been commonly used to explain the formation of Cenozoic dolomites, ancient platform-scale dolostones are normally interpreted as being products of either evaporative brines or mixed fresh and marine fluids. However, the viability of these two models in ancient successions is continuously questioned by new studies which favour normal or slightly modified seawater as the dolomitizing fluid. This study presents an ancient example from the lower Cambrian Longwangmiao Formation (South China) which, although appearing to have formed in association with evaporite minerals and experienced meteoric subaerial exposure, was interpreted to be dolomitized by normal seawater. Three petrographic types of dolomites, formed in the penecontemporaneous to early burial diagenetic stages in the Longwangmiao Formation. These include finely, fine to medium and medium to coarse crystalline dolomites. The stable isotope compositions (δ¹³C_carb, δ¹⁸O_carb and Δ₄₇) were determined on pure dolomite samples that had been chemically isolated, and the results interpreted using a coupled numerical model. This analysis strongly suggests that the dolomite was replaced by Cambrian seawater (ca −2‰) and subsequently experienced closed-system recrystallization (water/rock ratio <0.4) in the deep burial stage. The vertical variation of the mol% Mg content of the dolomite shows multiple episodes of dolomitization with the increasing stoichiometry in each episode suggesting the control of sea-level changes. This study has revealed that the normal-seawater dolomitization could also proceed in an evaporative environment on a shallow-water platform. As in many studies of ancient carbonates, the recognition of diagenetic fluids is often hindered by complex geochemical overprinting in bulk samples.