Clumped isotope fractionation during phosphoric acid digestion of carbonates at 70 °C

The application of clumped isotopes (Delta(47)) in carbonate minerals as a sensitive temperature proxy in paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary step of phosphoric acid digestion of the carbonate mineral to produce CO2. Published estimates for clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under debate. Differences in the sample preparation design and sample digestion temperatures are potential sources for varying acid fractionations and could be a source for discrepant Delta(47-)temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid fractionation at 70 degrees C and simultaneously account for a potential cation effect we analyzed a set of eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a stochastic isotope distribution by heating them to temperatures of 1000 degrees C. Our study reveals significant cation similar to and mineral specific differences for the Delta(47) acid fractionation of carbonate minerals digested at 70 degrees C or 100 degrees C. The Delta(47) acid fractionation at 70 degrees C for calcite is 0.197 +/- 0.002 parts per thousand, for aragonite 0.172 +/- 0.003 parts per thousand whereas dolomite has a significantly larger acid fractionation of 0.226 +/- 0.002 parts per thousand. For magnesite digested at 100 degrees C we observed a Delta(42) acid fractionation of 0.218 +/- 0.020 parts per thousand. Projected to an acid digestion at 25 degrees C, our acid fractionation for calcite of 0260 parts per thousand is statistically indistinguishable from existing studies. We further show that the Delta(47) of the calcite standards ETH-1 and ETH-2 of 0265 parts per thousand and 0.267 parts per thousand, respectively, are in the range of the determined acid fractionation projected to 25 degrees C suggesting that they have an identical and near stochastic isotope distribution. The observed differences in the Delta(47) acid fractionation between calcite and aragonite (Delta Delta(47) = -0.025 parts per thousand) and between calcite and dolomite (Delta Delta(47) = -0.029 parts per thousand) does not correlate with the phosphoric acid fractiohation of oxygen isotopes, but rather depends on the radius of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of dolomite at 70 degrees C is significantly distinct from the one of calcite, but at 90 degrees C the two are within error of each other due to the different acid fractionation temperature dependence of calcite and dolomite. Thus it is necessary to use a mineral-specific Delta(47) acid fractionation factor for dolomite to avoid differences in the final Delta(47) signal from dolomites digested at 90 degrees C and dolomites digested at lower temperatures. Similar effects may apply also to other carbonates such as magnesite and siderite. However, their mineral specific Delta(47) acid fractionation at digestion temperatures around 90 degrees C might be also similar to the one of calcite so that potential differences could be within the range of the analytical error. (C) 2016 Elsevier B.V. All rights reserved.