Tracking pore-water evolution through clumped isotope analyses of a septarian concretion

Septarian concretions have been recognized in many sedimentary units spanning nearly all ages. Although they exhibit a bizarre structure, their widespread occurrence makes septarian concretions more than just simple geologic curiosities. The tapering veins, or "septaria", within these concretions are often filled with complex, relatively late-stage (post-concretion body) isopachous rim and blocky calcite mineral phases, reflecting potentially discrete episodes of successive cementation. Previous studies have used traditional carbonate carbon (δ13C) and oxygen (δ18O) isotope analyses to characterize the diagenetic fluids responsible for vein-filling mineral precipitation. Whereas these studies have provided valuable information concerning mineralization, it is impossible to resolve the individual affects of temperature and pore fluid δ18O on mineral δ18O compositions. Of course as with all diagenetic systems, both temperature and fluid oxygen isotope compositions are integral parameters to quantify in order to characterize carbonate mineral paragenesis. Here, we use the clumped isotope proxy, a paleothermometer that is independent of fluid δ18O values, in order to better constrain the formation environment of a septarian concretion of the Jurassic Ampthill Formation, United Kingdom. This concretion exhibits cements that are typical of many septarian concretions in which distinct vein-filling cementation events can be traced by color differences in carbonate phases. As a result, it is relatively easy to sample subsequent phases along the paragenetic sequence and therefore draw interpretations concerning environmental evolution. The concretion body, isopachous rim and vein-filling calcite exhibit similar clumped isotope temperatures and calculated pore-water δ18O values show a progressive depletion in the respective phases above. The isotopic data along with the crystallographic progression suggest mineral precipitation initially in modified marine fluids with later phases forming under the influence of meteoric fluids. These results highlight the utility in using the clumped isotope paleothermometer in diagenetic systems.