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

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.

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.

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.

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.