Carbonate Clumped Isotopes (Δ ₄₇ ) in Cenozoic “Island Dolostones”: Dolomitization Driven by Modified Seawater at Earth‐Surface Conditions

Abstract

Dolomite [CaMg(CO ₃ ) ₂ ] is abundant throughout the geological record and constitutes a major component of many ancient carbonate platforms, yet the mechanisms responsible for its formation under Earth‐surface conditions are equivocal. Given that modern examples of low‐temperature dolomitization are rare, Cenozoic “island dolostones” provide ideal natural laboratories to evaluate these diagenetic processes. Sedimentological, petrographical, and geochemical studies indicate that “island dolostones” formed from seawater‐derived fluids, but the temperature of dolomitization and the oxygen isotopic composition of the diagenetic fluids (δ ¹⁸ O _FLUID ) remain poorly constrained. Using carbonate clumped isotope (Δ ₄₇ ) thermometry, we demonstrate that the “interior” dolostones on Grand Cayman formed at higher temperatures (TΔ ₄₇  = 26.1 ± 2.6°C), from diagenetic fluids with higher δ ¹⁸ O _FLUID values (2.1 ± 0.7‰ VSMOW), than the “peripheral” dolostones on Grand Cayman and Cayman Brac (TΔ ₄₇  = 19.1 ± 1.4°C; δ ¹⁸ O _FLUID  = 0.2 ± 0.5‰ VSMOW). Our results identify two endmember dolostone populations, consistent with dolomitization by evaporatively modified seawater in the platform interior and normal seawater along the platform margin. These observations are most readily explained by the migration of diagenetic fluids from restricted lagoonal settings toward the platform margin, where they mixed with seawater. Critically, isolated carbonate platforms host spatially heterogeneous diagenetic fluids that, when properly constrained, provide a powerful window into the geological record, thus strengthening interpretations of ancient seawater chemistry.