Rationale: Oxygen isotopic ratios of silicates are excellent tools to reconstruct paleotemperature and isotopic composition of the precipitating fluid. However, the measurement of 17 O/16 O is difficult due to the low abundance of 17 O. The present study reports a simplified high-precision analytical technique for measuring the two oxygen isotope ratios, 17 O/16 O and 18 O/16 O, in silicates.
Methods: Silicate samples were ablated by a CO2 laser in a BrF5 environment. The released oxygen (O2 ) was then cryogenically trapped in a molecular sieve zeolite (MSZ). Associated contaminants such as BrF5 , F2 , NF3 etc. were cleaned by passing the gas through a NaCl trap followed by a cooled (-25°C) MSZ-packed U-tube trap. The purified O2 was analysed in a MAT 253 isotope ratio mass spectrometer for oxygen isotope ratios.
Results: The δδ17 O and δ18 O values of the working gas were calibrated by NBS-28 and crosschecked by inter-laboratory references UWG-2, SCO and IMAU-O2 . The average analytical precisions (using aliquots of NBS-28, UWG-2, SCO, and laboratory internal standards IIT-KGP-SQ quartz and IIT-KGP-NQ quartz) of the δ17 O, δ18 O and ∆'17 O values were 0.04‰, 0.08‰ and 4 per meg, respectively.
Conclusions: A new cryogenic cleaning technique was developed that does not require GC but efficiently removes NF3 -contaminants from oxygen gas produced by laser fluorination of silicates. The technique is simple, quick and cost-effective and provides highly precise and accurate δ17 O, δ18 O and ∆'17 O values.
© 2020 John Wiley & Sons, Ltd.