Conditions and timing for the precipitation of early marine diagenetic dolomite in the Triassic Dolomia Principale (northern Italy)

ABSTRACT

Thermodynamic constraints and empirical evidence often suggest that fabric‐retentive replacement dolomites are stable carbonate minerals that preserve ancient seawater properties. This study evaluates this hypothesis by examining the Upper Triassic Dolomia Principale in Northern Italy. Using a combination of petrographic analysis, isotope geochemistry, fluid‐inclusion thermometry and U‐Pb cement dating, a paragenetic sequence of seven diagenetic phases was identified. Only one phase represents marine diagenetic (mesohaline reflux) dolomite, yet it accounts for about 90% of the bulk rock volume. In contrast, late‐stage burial dolomite cements account for 7%, and late‐stage calcite porosity‐filling cement for 3%. The mean oxygen and carbon isotopic values for the bulk Dolomia Principale closely mirror those of the marine replacement dolomite phase. Dolomite recrystallisation and reordering occurred in a marine burial setting, as shown by high average salinity (up to 130 000 ppm) and reconstructed burial temperatures of approximately 50°C, as evidenced by primary fluid inclusions. The data and regional context suggest that the extensive Dolomia Principale platform was dominated by saline, restricted platform‐top waters with only limited exchange with the Norian open ocean. Predominant mesohaline reflux dolomitisation affected all inner platform down to toe‐of‐slope facies, while the impermeable, anoxic intraplatform basinal mudstone deposits were preserved as limestone. The Dolomia Principale dolostone data, when compared to reconstructed Norian seawater values, exhibit ¹³ C depletion and, in some cases, ¹⁸ O enrichment. Similarly, ⁸⁷ Sr/ ⁸⁶ Sr ratios diverge from reconstructed seawater values. Overall, the Dolomia Principale marine replacement dolomites are more likely to preserve records of restricted platform water masses and altered marine pore fluids down to burial depths of several hundred metres, rather than Norian seawater. Although these findings may not be applicable to all dolomitised platforms in the Phanerozoic, they highlight the need for caution when interpreting marine diagenetic dolostones as archives of ancient seawater properties.