Implications of clumped-isotope thermometry for the deposition and alteration of evaporite-carbonate sabkha cycles in the Jurassic Weald Basin, U.K

Marginal marine Jurassic evaporite-carbonate cycles in the Weald Basin, United Kingdom, are examined using clumped isotope thermometry to reconstruct the temperature of a sabkha system and determine their paleoenvironment and diagenetic histories. The objective is to provide insight into the depositional parameter and burial history of the sabkha deposits, and thus to constrain whether they have undergone secondary alteration. Previous studies of evaporite temperature distribution trends have utilized fluid inclusion micro-thermometry to derive the temperatures of precipitation for evaporites. However, we aim to couple δ¹⁸O and clumped isotope analyses to investigate meter-scale spatial variations in the temperature at which calcitic marls inter-bedded with evaporite sequences were precipitated. Furthermore, this approach will allow for the isotopic composition of the diagenetic fluid to be reconstructed. Discrete laterally-extensive marly horizons occur in anhydrite of the Lower Purbeck and our focus is placed on marly algal limestone outcropping ~300 m below surface in Brightling Mine. Preliminary data from one sample yields a datum with Δ₄₈ and δ₄₈ values of 242.6‰ and 256.2‰, respectively, and a Δ₄₈ offset from the heated gas line larger than 2‰. The Δ₄₇ and δ₄₇ values are 0.038×0.01‰ and 7.25×0.02‰, respectively. These results point out possible contamination (e.g., by sulfur) because a sample should not yield a signal at mass Δ₄₈ due to the low abundance of isotopologues of this mass. XRD analysis shows a high intensity calcite peak at 29.4 °2θ (CuKα radiation), gypsum at 11.6 °2θ, and a peak of anhydrite at 22.7 °2θ. Subsequent to cleaning the sample twice utilizing a mixture of dilute 3% H₂O₂-Calgon-Nh3 solution, a second XRD analysis was conducted and only showed a high intensity calcite peak at 29.4 °2θ. This indicates that the cleaning procedure dissolved evaporitic material and it will be repeated for future clumped isotope analyses. The reconstructed clumped isotope temperatures are independent from the δ¹⁸O composition of the fluid, and thus, the associated carbonates within evaporites can provide a more profound understanding of the temperature histories of carbonate depositional facies. Future work will also include reconstructing the temperature of precipitation and fluid composition involved in the precipitation of potential carbonate phases within another sabkha system, in the Upper Permian Zechstein Group in the Southern Permian Basin, which will allow for a comparison of precipitation temperatures within sulfate deposits.