Despite prolonged research, the formation environments of dolomite remain debated. Previous studies have associated the apparent decrease in dolomite abundance in the geological record with a major transition in marine depositional environments from warm, saline shallow platforms, to deeper and cooler environments in which dolomite formation was largely inhibited. Others suggested that large volumes of pre-Cenozoic dolomites reflect dolomitization at elevated burial temperatures of these rocks, whereas Cenozoic carbonate platforms mostly never reached sufficient thermal maturity. A third, hybrid possibility is that Mg-rich dolomite precursor precipitated in shallow environments and later underwent cation ordering during burial diagenesis.
To test these models, we measured cation ordering (quantified by XRD and indicative of crystal structure), oxygen, and clumped isotopes (δ18O and TΔ47, respectively) in Triassic dolomite from the Mohila Formation in Makhtesh Ramon. This dolomite is thought to have formed in evaporative settings. It was compared to a modern analogue of dolomite from Dohat Faishakh Sabkha, Qatar. We incorporate these data into a global compilation of dolomite records. δ18O and TΔ47 data from Qatar indicate formation at Earth surface temperature and evaporated (2.5-4.8 ‰ VSMOW) seawater. In contrast, δ18O and TΔ47 of dolomites from the Mohila Fm. are consistent with cation ordering in deep burial environments flushed with normal seawater (0 to -1‰ VSMOW). Together, these data support a two-step dolomite formation process, in which carbonates were initially enriched in Mg2+ in the lagoon and later recrystallized in burial-diagenetic environments. For most dolomite in our dataset, cation ordering is positively correlated with TΔ47, suggesting that a burial recrystallization step is coupled with and possibly driven by ordering of proto-dolomite. However, dolomites from one study in the data set diverge from this trend and show a high degree of ordering despite low clumped isotope temperatures, suggesting a secondary – early dolomite formation pathway. Dolomites associated by stratigraphy with surface environments show a negative correlation between δ18O and TΔ47 values, similar to massive dolomites that lack such context. Considered together, our results suggest that burial recrystallization is a common process in the formation of dolomite records that are typically interpreted as forming under surface conditions.