An automated laser fluorination technique for high-precision analysis of three oxygen isotopes in silicates

Abstract

Rationale: The three oxygen isotopes in terrestrial/extraterrestrial silicates can provide geochemical and cosmochemical information about their origin and secondary processes that result from isotopic exchange. A laser fluorination technique has been widely used to extract oxygen from silicates for δ17O and δ18O measurements by isotope ratio mass spectrometry. Continued improvement of the techniques is still important for high-precision measurement of oxygen-isotopic ratios. Methods: We adopted an automated lasing technique to obtain reproducible fluorination of silicates using a CO2 laser-BrF5 fluorination system connected online to an isotope ratio mass spectrometer. The automated lasing technique enables us to perform high-precision analysis of the three oxygen isotopes of typical reference materials (e.g., UWG2 garnet, NBS28 quartz and San Carlos olivine) and in-house references (mid-ocean ridge basalt glass and obsidian). The technique uses a built-in application of laser control with which the laser power can be varied in a programmed manner with a defocused beam which is in a fixed position. Results: The oxygen isotope ratios of some international reference materials analyzed by the manual lasing technique were found to be isotopically lighter with wider variations in δ18O values, whereas those measured by the automated lasing technique gave better reproducibility (less than 0.2‰, 2SD). The Δ17O values, an excess of the δ17O value relative to the fractionation line, also showed high reproducibility (±0.02‰, 2SD). Conclusions: The system described herein provides high-precision δ17O and δ18O measurements of silicate materials. The use of the automated lasing technique followed by careful and controlled purification procedures is preferred to achieve satisfactory isotopic ratio results.