Cédric John

The clumped isotope paleo-thermometer has become a valuable proxy for the burial history reconstruction of carbonate formations. To maximise the accuracy of these reconstructions, post-depositional alterations, such as recrystallisation and Δ47 isotope exchange reactions, must be understood. In this study, we examine the isotopic behaviour of calcites and early dolomite samples from the same stratigraphic intervals, and thus with similar burial history. This approach provides additional constraints on the kinetics of Δ47 reordering in dolomite during exhumation. Clumped isotope measurements were performed on 19 calcites and 15 early dolomites from the Permian, Jurassic, and Cretaceous periods from four locations in Oman spanning different burial regimes. The calcite and dolomite samples were collected from the rock matrix, based on the assumption that fine material was more susceptible to recrystallisation. Our results show that calcites and dolomites record different Δ47 values despite being subjected to the same thermal history. The maximum Δ47 temperature recorded in dolomites (181 ± 13 °C) corresponds to the oldest and most deeply buried Permian rock. This value is approximately 35 °C higher than those measured in the co-located and coeval calcite matrix (145 ± 14 °C). This discrepancy suggests that calcite and dolomite have different kinetic parameters. Our data confirm (1) that dolomite Δ47 values are more resistant to alteration during burial and exhumation than Δ47 calcite values, and (2) that dolomite has a higher Δ47 closing temperature than calcite during cooling. The presence of two mineral phases with distinct kinetic parameters in the same stratigraphic unit provides additional constraints on models of burial and uplift. In addition, mineralogical data coupled with Δ47 and burial depths suggest that the progressive development of dolomite cation ordering is driven by temperature elevation, as previously suggested.

Previous studies have shown that microorganisms thrive in oligotrophic fracture systems, and metabolisms include consumption and production of methane. In the Laxemar, Götemar, and Forsmark areas of Sweden, ancient microbial activity has previously been demonstrated by large δ13CVPDB variability of carbonate vein infillings within granitic host rocks. Here, we apply carbonate clumped isotope thermometry to reconstruct the temperature of precipitation of the carbonate within these veins. The carbonate clumped isotope temperatures indicate that mineralization took place between 46°C and 108°C, in line with previously published fluid inclusion data (<50°C to 113°C). The new carbonate clumped isotope data more accurately narrows temperatures at the lower end of this range as fluid inclusions are not easily applicable below 80°C, a temperature regime of high importance for paleobiological reconstructions. Our results demonstrate that large volumes of biogenic carbonate cement were formed from low-temperature microbial processes in low-salinity water, succeeding calcite of deep brine origin. The known burial and thermal history of the region combined with our carbonate clumped isotope data place the microbial activity at the end of the Variscan orogeny and later events (e.g., Jurassic). Thus, carbonate clumped isotope thermometry reduces uncertainties in deep biosphere studies by providing more accurate temperature constraints in the low-temperature regime, the biogenic processes, and the origin of the diagenetic fluids.