Fabric-retentive early dolomite preserves ancient seawater composition, Yates and Tansill formations, Permian Reef complex, Guadalupe Mountains, Texas and New Mexico

Abstract:

Dolomites are a major component of the carbonate sedimentary record but do not readily form in modern marine environments. Lime sediments must have experienced substantial exchange with diagenetic fluids to form dolomite, potentially confounding information about ancient seawater chemistry. In many cases, dolomite formed by the advection of many pore volumes of fluid throughout the rock’s burial history. Here, we present evidence for another dolomite formation pathway.

We report 72 carbonate clumped and oxygen isotope measurements and reconstructed pore fluid δ¹⁸O compositions of 19 dolomites and 16 calcite cements from the Yates and Tansill formations, which comprise the youngest back-reef facies of the Permian Capitan Reef in the Guadalupe Mountains, Texas and New Mexico. The dolomites have a mineral δ¹⁸O of -1.4‰ – 1.2‰ VPDB, which formed in equilibrium with water with δ¹⁸O of 0.4‰ – 8.1‰ VSMOW at temperatures between 36 and 79°C. There is no correlation between the mineral δ¹⁸O and equilibration temperature (R²=0.01) and a strong correlation between water δ¹⁸O and temperature (R²=0.84). These data indicate that the carbonate clumped isotope compositions have been reset in a burial environment nearly completely closed to later circulating fluids. Calcite spars record more open-system precipitation of meteoric-burial fluids in remaining porosity.

These results suggest that these dolomites completed stoichiometric exchange in the earliest diagenetic environment and were then buried and recrystallized in a closed-system environment with no significant alteration to mineral δ¹⁸O. After Mg incorporation, pore fluid δ¹⁸O, not rock composition, evolved during burial. The stratal geometry of dolomite in the Permian Reef has long been recognized as consistent with an early brine seepage mechanism controlled by penecontemporaneous sea level cycles, and dolomitization was largely fabric-retentive. Therefore, similar dolomites may be petrographically, stratigraphically, and geochemically identified in other successions to provide a robust archive for paleoceanographic proxies. Dolomite formed this way may be a good archive of δ¹³C, δ¹⁸O, δ³⁴S, and other proxies locally inherited from ancient seawater because the bulk chemistry of the rock is set in the early marine diagenetic environment.