MG ISOTOPE SYSTEMATICS DURING EARLY DIAGENETIC ARAGONITE-CALCITE TRANSITION

The seawater Mg/Ca ratio controls the mineralogical composition of calcium carbonate, i.e. precipitation of aragonite and high-Mg calcite at high Mg/Ca ratio (the aragonite sea), while low-Mg calcite formation at low Mg/Ca ratio (the calcite sea). It is proposed that the secular variation of seawater Mg/Ca ratio was primarily controlled by the spreading rate of mid-ocean ridges (MOR). To unravel the complex interactions in Earth’s history, reconstruction of seawater Mg/Ca ratio is critical. Seawater Mg/Ca ratio can be inferred from the primary mineralogy of marine carbonate, i.e., aragonite or calcite. However, this approach has been complicated by the transition of metastable aragonite and high-Mg calcite to low-Mg calcite in the early diagenesis. It is proposed that Mg isotopes (δ²⁶Mg) might be used to differentiate the mineralogy of calcium carbonate, because the isotopic fractionation for aragonite is smaller than that of calcite, and accordingly aragonite has higher δ²⁶Mg than both high-Mg calcite and low-Mg calcite. Though promising, however, it is unclear whether the isotopic composition of aragonite could be preserved during aragonite-calcite transition. Here, we measured the Mg isotopic composition of Pleistocene Key Largo (Florida) Limestone, which is at the stage of aragonite-calcite transition and consists of a mixture of calcite and aragonite. Our results show that δ²⁶Mg of aragonite-dominated samples ranges from -2.00‰ to -2.33‰, significantly higher than that of calcite-dominated samples (-3.78‰ to -4.83‰), suggesting that the δ²⁶Mg of aragonite is reset during the aragonite-calcite transition. Thus, the aragonite signal cannot be preserved in rock record, and the original mineralogy (aragonite vs. calcite) cannot be differentiated by the Mg isotopes of carbonate rocks. Furthermore, aragonite-calcite transition is also associated with Sr loss, but the Mg content remains more or less constant. This suggests that the aragonite-calcite transition could not take place in a solid-phase transition, instead, require a dissolution-reprecipitation process. We thus speculate that the dissolution and reprecipitation might be slow reactions, allowing the preservation of ultra-fine textures of calcareous fossils in the aragonite-calcite transition.