DESIGN, CALIBRATION AND GEOLOGICAL APPLICATION OF THE FIRST OPERATIONAL AUSTRALIAN LASER-ABLATION SULFUR ISOTOPE MICROPROBE

This contribution describes the setup and operating procedures of the first operational laser ablation microprobe for stable (sulphur) isoto pe 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 l ocate and ablate sample targets. As with other laboratories, samples w ere ablated in an oxygen atmosphere, producing a SO2/O-2 gas mixture. SO2 was separated from this mixture by either of two techniques. In th e first technique, SO2 was condensed into a liquid N-2 trap by cryogen ic pumping, and O-2 was pumped away. This resulted in the collection o f 60-70% of the produced SO2. In the second technique, SO2 was condens ed into a liquid N, trap as the SO2/O-2 mixture was slowly bled away. This technique collected 90-95% of the SO2, with a small fractionation of 0.16 parts per thousand. Laser ablation and SO2 collection via the second technique required a mineral dependent, additive correction of 2.85-5.75 parts per thousand to convert raw delta(34)S values to delt a(34)S(CDT). These correction factors are mineral and laboratory depen dent, and from our data, seem to be dependent on the quality of polish of the ablated sample. Precision (1 sigma) of laser ablation sulphur isotope analysis is 0.4-0.5 parts per thousand for 150 mu m ablation c raters. Preliminary results of studies on samples from the Broken Hill , Hellyer and active sea floor Pacmanus deposits indicate that laser a blation microprobe analysis can show subtle variations in delta(34)S n ot apparent using either conventional or SHRIMP analysis. Laser ablati on analysis indicates a larger range, but similar mean values, to conv entional analysis on the same samples.