LEAD ISOTOPE ANALYSIS OF MARINE CARBONATES AND SEAWATER BY MULTIPLE COLLECTOR ICP-MS

Abstract

A consistent method for stable lead isotope analysis of marine carbonates and seawater is presented, utilizing multiple collector ICP-MS (MC-ICP-MS). This study presents new observations of the large (0.7% amu−1), time-dependent mass bias determined by thallium normalization, including preferential light ion transmission induced by the acceleration potential and plasma interface (β=−1.3 to 0.9). These experiments show equivalent results for three empirical correction laws, and the previously proposed βPb/βTl correction does not improve isotope ratio accuracy under these conditions. External normalization to SRM-981 following the thallium correction provides one simple alternative, and a rationale is provided based on secondary bias effects. With current intensities less than 1.5×10−12 A, external isotope ratio precision better than 250 ppm for SRM-981 207Pb/206Pb and 208Pb/206Pb ratios is observed (2σ). From reconstructed lead isotopic variability in the North Atlantic, this instrumental precision results in a signal-to-noise ratio greater than 100. Matrix effects are significant with concomitant calcium in SRM-981 (−280 ppm at 257 μM [Ca]). With the appropriate corrections and minimal concomitants, MC-ICP-MS can reliably determine 207Pb/206Pb and 208Pb/206Pb ratios of marine carbonates (30 mg) and seawater (160–200 g).