Coupling Δ47 and fluid inclusion thermometry on carbonate cements to precisely reconstruct the temperature, salinity and δ18O of paleo-groundwater in sedimentary basins

Abstract

Diagenetic minerals may provide information about the burial history of geological units and can have practical applications, for instance, for reconstructing the geochemical and thermal histories of sedimentary basins. Clumped isotope, or Δ47, thermometry on carbonates opens a new avenue for interpreting carbonate formation temperature and thermal history of rocks. Yet, most of current knowledge on Δ47 systematics has been acquired via theoretical or experimental studies with only limited validation by the rock-record at geological conditions/timescales. Here, we investigate calcitic and dolomitic cements representative of three genetically different cementation phases from a well-documented mineral paragenesis of a carbonate unit (Middle Jurassic, Paris basin, France). We compare Δ47 with fluid inclusions microthermometry (FIM) data that were independently obtained from the same calcite and dolomite crystal specimens. The range of homogenization temperatures (Th) found for Cal1, Cal2 and Dol1 fluid inclusions fit remarkably well (i.e. within <5°C) with the temperatures determined from the Δ47 measurements (TΔ47), for a temperature range between 60 and 100°C and salinities between 0 and 15wt% NaCl eq. This provides a consistent rock-based validation of the experimentally determined Δ47 calibration with formation temperature for both calcite and dolomite mineralogy. Such findings also confirm the applicability of Δ47 thermometry in low temperature diagenetic environments (i.e., below circa. 100°C), which provides higher precision than FIM measurements (i.e., typical uncertainties of ±6°C with three Δ47 measurements) though significantly less time-consuming. Importantly, this study underlines how the coupling of both techniques can help to evaluate the degree of preservation of the original temperature information captured by either fluid inclusions or Δ47 compositions, and interpret each proxy as confidently/accurately as possible. Moreover, because both FIM and Δ47 measurements can provide independent constraints on the geochemistry of diagenetic paleofluids (via their salinity and δ18O composition), this study also highlights the benefits of coupling both techniques to further unravel the nature of paleofuids. Finally, we propose a practical guideline as a basis for future applications of combined FIM and Δ47 thermometry.