Design,calibration and geological application of the first operational Australian laser ablation sulphur isotope microprobe

This contribution describes the setup and operating procedures of the first operational laser ablation microprobe for stable (sulphur) isotope analysis in Australia as well as some brief geological applications. A significant feature on this laser ablation microprobe is automated gas purification and analysis; operator control is only required to locate and ablate sample targets. As with other laboratories, samples were ablated in an oxygen atmosphere, producing a SO₂/O₂ gas mixture. SO₂ was separated from this mixture by either of two techniques. In the first technique, SO₂ was condensed into a liquid N₂ trap by cryogenic pumping, and O₂ was pumped away. This resulted in the collection of 60–70% of the produced SO₂. In the second technique, SO₂ was condensed into a liquid N₂ trap as the SO₂/O₂ mixture was slowly bled away. This technique collected 90–95% of the SO₂, with a small fractionation of 0.16%. Laser ablation and SO₂ collection via the second technique required a mineral dependent, additive correction of 2.85–5.75% to convert raw δ³⁴S values to δ³⁴S_CDT. These correction factors are mineral and laboratory dependent, and from our data, seem to be dependent on the quality of polish of the ablated sample. Precision (1σ) of laser ablation sulphur isotope analysis is 0.4–0.5%o for 150 μm ablation craters.

Preliminary results of studies on samples from the Broken Hill, Hellyer and active sea floor Pacmanus deposits indicate that laser ablation microprobe analysis can show subtle variations in δ³⁴S not apparent using either conventional or SHRIMP analysis. Laser ablation analysis indicates a larger range, but similar mean values, to conventional analysis on the same samples.