First use of clumped-isotope thermometry to quantify temperatures of crystallization for diagenetic calcite Katharine W. Huntington Department of Earth & Space Sciences, University of Washington, Seattle WA We aim to develop clumped isotopes as a tool to quantify temperatures of sub-surface carbonate growth and re-crystallization and the d18O of fluid-rock systems in basins. To do this, we will test the method in well-characterized natural systems. Precision should be as good as ±1-2°C with >50°C calibration experiments in progress. 13C-18O clumping in CaCO3 (calcite) A single clumped isotope measurement gives independent estimates of the growth temperature and d18O of carbonate minerals, enabling the d18O of diagenetic waters from which the minerals grew to be calculated COLD Conventional d18O and petrographic/CL analysis would suggest all diagenetic calcites in this sample (below) grew from meteoric waters at Earth surface temperatures. But clumped isotope temperatures of 14-122°C for multiple phases of primary and diagenetic calcite in the same specimen demonstrate this is not the case & constrain water-rock ratios (≤0.01 wt %) in the diagenetic micro-environment. HOT Bergman et al., GSA 2010; in prep Clumped isotope results from 26 Paradox basin samples (above) indicate fault-related calcite cements precipitated at 43-97°C at equilibrium, from well mixed waters with d18O = -11.7±1.8‰ SMOW. Temperatures agree with basin models and isotopes, but are cooler than fluid inclusion temperatures. Huntington et al., for submission to J Sed Res Huntington et al., for submission to J Sed Res; to be featured in GSA 2010 keynote lecture