2013 年 47 巻 3 号 p. 369-384
We present analytical protocols and the analytical performance of Pb isotope analysis of silicate minerals and rocks at the Pb = 0.02-15 ppm level using excimer laser ablation (LA) coupled with an enhanced-sensitivity multiple collector-inductively coupled plasma mass spectrometer (MC-ICP-MS). We applied Faraday cups for all the isotopes analysed, including the minor isotopes 204Pb and 204Hg. This device is, so far, the first attempt to analyse samples with low Pb concentrations by LA-MC-ICP-MS. The improved sensitivity of the mass spectrometer, along with careful correction of the Hg overlap on 204Pb, permits accurate analyses of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios. This procedure includes determination of the fractionation factor of Hg isotopes among 200Hg, 202Hg, and 204Hg. The analytical error, including precision and reproducibility, was within 1.5 per mil for 5-15 ppm Pb in glassy silicate rocks (groundmass) taken from 500 μm × 500 μm × 100 μm (0.025 mm3) craters. These results were comparable to those obtained using conventional non-spiked thermal ionization mass spectrometry after chemical separation. Major Pb isotope ratios, including 207Pb/206Pb and 208Pb/206Pb, were also analysed to within a 1 per mil error for 1-1.5 ppm Pb in groundmass and plagioclase crystals from craters 200 μm in diameter and 100 μm deep. When a larger sample size, i.e., a 500 μm × 500 μm × 200 μm crater was used, a ~10 per mil error was achievable for samples containing ~0.02 ppm Pb, which allows in situ analysis of low-Pb minerals such as clinopyroxene. Melt (glass) inclusions in olivine with a sample size of diameter ~200 μm containing 1-5 ppm Pb were also analysed with 1-10 per mil errors. The achievable accuracy and precision were significantly better for 207Pb/206Pb and 208Pb/206Pb, and better for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb than those obtained by in situ analyses using secondary ion microprobe mass spectrometry and a normal-sensitivity LA-MC-ICP-MS using one or more ion counters. The analytical performance of the present method is useful for many applications connected with geochemical studies on igneous rocks. We present examples for groundmass, plagioclase, clinopyroxene, and olivine melt inclusion analyses of ocean island basalts.
