Analyses of Pb (204-206-207-208) Isotopic Compositions in Basaltic Glasses Using Multi-ion Counter SHRIMP II

Increasingly, leading edge geochemical investigations rely on obtaining multiple types of chemical data from limited size samples, as for example melt inclusions or rare relict phases in ancient rocks. The capability of determining isotopic compositions, with high spatial resolution and minimal sample destruction, is thus essential. Here we report progress in common Pb isotopic measurements of natural glass samples with as little as 1ppm total Pb using a multi-collector ion probe.

Method: The ANU SHRIMP II is fitted with 5 ion counters (CDEMs). Pb isotopic measurements are made with 206, 207 and 208 centered in the axial collectors and 204 in the low mass collector. In contrast to most LA-ICP-MS systems, the inherent low SHRIMP backgrounds and high mass resolution (all analyses at >6000) minimise problems of isobaric interferences (e.g. from 204Hg, background common Pb, molecular species). Relative gains for the ion counters, which also account for mass bias, were established at the beginning of a 12 to 24 hour session using USGS fused glass BCR-2g and were stable at <3 per mil for all ratios. A large (25-40 nA) O- primary beam was used with a ca. 25 micron diameter analysis spot on the target. A typical analysis for low level samples comprised 1200 sec static collection to ensure >10,000 total counts of 204Pb. The external reproducibility of Loihi glass (3ppm Pb) per session is <0.3% (standard error) for the ratios 204/206, 207/206 and 208/206. A test sample of olivine hosted melt inclusions from Mauna Loa volcano, Hawaii demonstrates the feasibility of this approach for Pb isotopic measurements of melt inclusions. These natural basaltic glasses with ca. 1 ppm total Pb typically yielded an internal precision of better than 0.2% for 207/206 and 208/206 and 0.3% for 204/206. Repeat analyses of a single large inclusion measured at the beginning and end of a session agreed to +/-0.15% for 207/206 and to +/-0.4% for 204/206 and 208/206. Pb isotopic compositions for 12 melt inclusions show a limited compositional range, in overall agreement with data from Mauna Loa basalts. No exotic Pb isotopic components are yet identified for this suite.