Laboratory calibration of the calcium carbonate clumped isotope thermometer in the 25–250°C temperature range

Many fields of Earth sciences benefit from the knowledge of mineral formation temperatures. For example, carbonates areextensively used for reconstruction of the Earth’s past climatic variations by determining ocean, lake, and soil paleotempera-tures. Furthermore, diagenetic minerals and their formation or alteration temperature may provide information about theburial history of important geological units and can have practical applications, for instance, for reconstructing the geochemi-cal and thermal histories of hydrocarbon reservoirs.Carbonate clumped isotope thermometry is a relatively new technique that can provide the formation temperature of car-bonate minerals without requiringa prioriknowledge of the isotopic composition of the initial solution. It is based on thetemperature-dependent abundance of the rare13C–18O bonds in carbonate minerals, specified as aD47value. The clumpedisotope thermometer has been calibrated experimentally from 1°Cto70°C. However, higher temperatures that are relevantto geological processes have so far not been directly calibrated in the laboratory.In order to close this calibration gap and to provide a robust basis for the application of clumped isotopes to high-temperature geological processes we precipitated CaCO3(mainly calcite) in the laboratory between 23 and 250°C. We usedtwo different precipitation techniques: first, minerals were precipitated from a CaCO3supersaturated solution at atmosphericpressure (23–91°C), and, second, from a solution resulting from the mixing of CaCl2and NaHCO3in a pressurized reactionvessel at a pressure of up to 80 bar (25–250°C).The calibration lines of both experimental approaches overlap and agree in the slopes with theoretical estimates and withother calibration experiments in which carbonates were reacted with phosphoric acid at temperatures above 70°C. Our studysuggests a universalD47-T calibration (T in K,D47in&):D47¼0:98ð 0:01Þ ð 3:407 109=T4þ2:365 107=T3 2:607 103=T2 5:880=TÞþ0:293ð 0:004ÞThis newD47-T calibration (given in the absolute reference frame), that extends the experimentally calibrated temperaturerange for clumped isotopes to 250°C, can be applied to carbonates that grew at intermediate temperatures (20–250°C)