Paper on clumped isotopes in early dolomite
This month our paper on clumped isotope in early dolomite has been published in the Journal of Sedimentary Research. The study investigates one of the few examples of dolomitization of Lower Cretaceous shallow-water limestones from the southern Tethys carbonate platform from outcrops on the Haushi-Huqf High in central-east Oman. Mud-dominated peritidal carbonates rich in microbial mats are replaced by fine crystalline dolomite along at least 60 km in the lower 10 m of the Jurf Formation. Two, meter-thick beds in the overlying Qishn Formation are also dolomitized over lateral distances of one to two kilometers. The stratabound geometry and petrographic relations of the dolomite with all other diagenetic phases indicate that the dolomite precipitated early. The presence of only rare anhydrite relicts suggests that seawater was below gypsum saturation during most of the dolomitization, supporting the hypothesis that Cretaceous slightly evaporated water (δ18Ofluid values range from 2.8‰ to 3.5‰ SMOW) can affect pervasive dolomitization. Despite being composed of peritidal facies with features suggesting high salinity, most of the carbonate succession was not dolomitized, suggesting that the presence of microbial mats exerted a major control on the distribution of dolomite, and that salinity is only a minor control on dolomitization. Clumped isotope results indicate that the early formed dolomite re-equilibrated with fluids at 44 ± 3ºC in a shallow-burial setting. The elevated iron content (on average 6011 ppm) and 87Sr/86Sr (on average 0.70782) with respect to Cretaceous seawater suggests that the burial fluids interacted with Permian clay and feldspars of the Gharif Formation that underlies the Cretaceous carbonates. The vertical gradients of radiogenic Sr and the occurrence of small volumes of coarse crystalline dolomite in stylolites and fractures suggests that burial fluids were driven vertically and laterally by differential compaction, probably during maximum burial in the Late Cretaceous.
Sena CNH, John CM, Jourdan AL, Vandeginste V, Manning C. Dolomitization of Lower Cretaceous peritidal carbonates by modified seawater: constraints from clumped isotope paleothermometry, elemental chemistry and strontium isotopes. JOURNAL OF SEDIMENTARY RESEARCH, July 2014, v. 84, p. 552-566, published online July 15, 2014, doi:10.2110/jsr.2014.45 (Journal article)