Pb isotope analyses of silicate rocks and minerals with Faraday detectors using enhanced-sensitivity laser ablation-multiple collector-inductively coupled plasma mass spectrometry

摘要

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 ~(204)Pb and ~(204)Hg. 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 ~(204)Pb, permits accurate analyses of ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios. This procedure includes determination of the fractionation factor of Hg isotopes among ~(200)Hg, ~(202)Hg, and ~(204)Hg. 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 mm~3) craters. These results were comparable to those obtained using conventional non-spiked thermal ionization mass spectrometry after chemical separation. Major Pb isotope ratios, includ- ing ~(207)Pb/~(206)Pb and ~(208)Pb/~(206)Pb, 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 ~(207)Pb/~(206)Pb and ~(208)Pb/~(206)Pb, and better for ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb 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 con- nected with geochemical studies on igneous rocks. We present examples for groundmass, plagioclase, clinopyroxene, and olivine melt inclusion analyses of ocean island basalts.