Kinetic oxygen isotope fractionation upon acid liberation of CO2 from artificial and natural norsethite (BaMg(CO3)2), a mineral analogue of dolomite

Rationale: The analytical method to determine the stable oxygen isotope (O-18/O-16) composition of carbonates via phosphoric acid digestion leads to temperature- and solid-dependent kinetic isotope fractionation. Values for the double carbonate norsethite (BaMg(CO3)(2)) have been unknown so far. Methods: The temperature dependence of kinetic oxygen isotope fractionation during the reaction of synthetic and natural BaMg(CO3)(2) with orthophosphoric acid (H3PO4) according to the overall reaction BaMg(CO3)(2) + 2H(3)PO(4) = Ba2+ + Mg2+ + 2HPO(4)(2-) + 2CO(2) + 2H(2)O has been examined for the first time using separate carbonate decomposition via fluorination or phosphoric acid digestion, with the resulting gases analyzed by isotope ratio monitoring mass spectrometry. Results: In the temperature range between 25 and 70 degrees C the kinetic fractionation factor between acid-generated CO2 and artificial and natural norsethite is described by (T in K): 1000ln alpha(acid)(CO2-carbonate) = A+B x 10(5)/T-2 with A = 4.15 and B = 6.47 for natural norsethite, and A = 4.77 and B = 5.94 for synthetic norsethite. The fractionation factor measured for a poorly crystallized synthetic carbonate agrees with those for the other samples at 25 degrees C, but is slightly lower at 50 and 70 degrees C. No carbon isotope fractionation was found during the unidirectional acid dissolution. Conclusions: The kinetic oxygen isotope fractionation during phosphoric acid liberation of CO2 from BaMg(CO3)(2) is quantified. Based on published results for endmember carbonates, the results at 25 degrees C for other double carbonates are estimated.

Automated summary

The kinetic oxygen isotope fractionation during the reaction of natural and synthetic norsethite (BaMg(CO3)(2)) with orthophosphoric acid was examined for the first time. The results show a significant dependence on temperature. The fractionation factor for other double carbonates at 25 degrees C can be estimated based on the published results.