A simple cryogenic method for efficient analysis of triple oxygen isotopes in silicates

Rationale Oxygen isotopic ratios of silicates are excellent tools to reconstruct paleotemperature and isotopic composition of the precipitating fluid. However, the measurement of 17O/16O is difficult due to the low abundance of 17O. The present study reports a simplified high‐precision analytical technique for measuring the two oxygen isotope ratios, 17O/16O and 18O/16O, 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 δδ17O and δ18O 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 δ17O, δ18O and ∆′17O 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 δ17O, δ18O and ∆′17O values.