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

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 δ¹⁷ O and δ¹⁸ O measurements by isotope ratio mass spectrometry. Continued improvement of the techniques is still important for high-precision measurement of oxygen-isotopic ratios.

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.

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 δ¹⁸ O values, whereas those measured by the automated lasing technique gave better reproducibility (less than 0.2‰, 2SD). The Δ¹⁷ O values, an excess of the δ¹⁷ O value relative to the fractionation line, also showed high reproducibility (±0.02‰, 2SD).

The system described herein provides high-precision δ¹⁷ O and δ¹⁸ O 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.