Rationale: The analytical method to determine the stable oxygen isotope (18 O/16 O) 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 (H3 PO4 ) according to the overall reaction BaMg(CO3 )2 + 2H3 PO4 = Ba2+ + Mg2+ + 2HPO4 2- + 2CO2 + 2H2 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°C the kinetic fractionation factor between acid-generated CO2 and artificial and natural norsethite is described by (T in K): [Formula: see text] 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.
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