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 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 CO 2 laser in a BrF 5 environment. The released oxygen (O 2 ) was then cryogenically trapped in a molecular sieve zeolite (MSZ). Associated contaminants such as BrF 5 , F 2 , NF 3 etc. were cleaned by passing the gas through a NaCl trap followed by a cooled (−25°C) MSZ‐packed U‐tube trap. The purified O 2 was analysed in a MAT 253 isotope ratio mass spectrometer for oxygen isotope ratios. Results The δδ 17 O and δ 18 O values of the working gas were calibrated by NBS‐28 and crosschecked by inter‐laboratory references UWG‐2, SCO and IMAU‐O 2 . 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 δ 17 O, δ 18 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 NF 3 ‐contaminants from oxygen gas produced by laser fluorination of silicates. The technique is simple, quick and cost‐effective and provides highly precise and accurate δ 17 O, δ 18 O and ∆′ 17 O values.