IN-SITU OSMIUM ISOTOPE RATIO ANALYSES OF IRIDOSMINES BY LASER ABLATION-MULTIPLE COLLECTOR-INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

Abstract

In-situ isotopic ratio analyses of Os for iridosmine samples have been made by a laser ablation–multiple collector–inductively coupled plasma mass spectrometer (LA–MC–ICPMS). A frequency-quadrupled Nd-YAG UV laser (266 nm) which produces pit sizes of 10–15 μm was used to ablate iridosmine samples. The technique developed in this study enabled us to measure small deviations in isotopic and elemental ratios of solid geochemical samples with an analytical uncertainty smaller than 0.05%. The analytical precision of Os isotopic ratio measurements achieved by the present LA–MC–ICPMS technique was comparable to those achieved by negative thermal ionisation mass spectrometry (NTIMS). We have determined Os isotopic ratios for three iridosmine placers from the Urals, California and Hokkaido, Japan. The resulting 187Os/188Os ratio for iridosmines from the Urals (0.124786±0.000030, 2σ) exhibits an excellent agreement with the value reported by Allègre and Luck, 1980 (0.12479±0.00048). In the case of the iridosmine samples from the Urals and Hokkaido, the Re depletion age based on the 187Os/188Os isotopic growth curve for chondritic materials show good agreement with the metamorphic age (240 m.y. for the Urals iridosmine and 120 m.y. for the Hokkaido iridosmine). This suggests that these iridosmine samples were formed from the source material which had a chondritic Re/Os abundance ratio. In the case of the California iridosmine, the Re depletion age estimated from the 187Os/188Os isotopic growth for chondritic materials was about 700 m.y., and this is much older than the age of the volcanic activity or a metamorphic age (80–210 m.y.). This suggests that the California iridosmine analysed in this study was formed from the materials which could be depleted in Re as compared with chondritic materials. Variations in elemental ratios as well as isotopic compositions can be monitored by the present LA–MC–ICPMS technique. In this study, although no significant isotopic variations in Os isotopic ratios could be found within a single iridosmine grain, a significant variation in the W/Os abundance ratio could be found for the Hokkaido iridosmine. The data presented here demonstrate clearly that the LA–MC–ICPMS has a potential to become a strong tool for the direct detection of possible isotopic and chemical variations in solid geochemical samples.