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

Abstract

Rationale The analytical method to determine the stable oxygen isotope (18O/16O) 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 + 2H3PO4 = Ba2+ + Mg2+ + 2HPO42– + 2CO2 + 2H2O 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°C the kinetic fractionation factor between acid‐generated CO2 and artificial and natural norsethite is described by (T in K): 1000lnαCO2‐carbonateacidgoodbreak=Agoodbreak+Bgoodbreak×105/T2$$ 1000\ln {\alpha}_{{\mathrm{CO}}_2\hbox{-} \mathrm{carbonate}}^{\mathrm{acid}}=A+B\times {10}^5/{T}^2 $$ 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°C, but is slightly lower at 50 and 70°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°C for other double carbonates are estimated.