Reproducibility of Pb Isotopic Compositions of Ocean Island Basalts: From Leaching to Analysis

High-precision Pb isotope MC-ICP-MS data is essential for evaluating the sources and components of ocean island basalts. We studied six compositionally distinct basalts from Hawaii and Kerguelen to assess the variability in Pb isotopic compositions due to alteration/secondary phases and the efficiency of matrix elimination by anionic exchange columns, and to evaluate how these factors influence the reproducibility of MC-ICP-MS analyses. Unleached and leached powders, with one or two passes through exchange columns, were analyzed as well as replicates on a Nu Plasma MC-ICP-MS. When possible, 3 complete duplicates were processed. Leaching leads to a weight loss of ~35% for Hawaiian samples, while for Kerguelen, weight loss varies from 30 to 60%, depending on the relative alteration (age and composition) of the samples. Unleached samples yield erroneous Pb values and higher 2sd errors. For all Hawaiian basalts, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values are consistently more radiogenic (4697, 1329 and 1980 ppm, respectively) and plot outside the accepted fields for Mauna Loa and Mauna Kea. For Kerguelen, the unleached samples are also more radiogenic, but to a minor extent (725, 1264 and 1167 ppm, respectively). For matrix removal, in Kerguelen, the Pb isotopic compositions after two-passes on columns are less radiogenic than after one-pass (by 168, 29, 164 ppm, almost within error). In Hawaii, the difference is larger (~400 ppm) and the two-pass results are intermediate between the unleached and one-pass results. This difference reflects, as expected, the cumulative effects of alteration and matrix effect on the accuracy of Pb isotopic results on basalts. Despite higher degree of alteration and older ages, the Kerguelen basalts show better reproducibility (200-250 ppm) than Hawaiian basalts (730-850 ppm). Further investigations will allow us to constrain the origin of this behavior, which accounts for some of the ``noise'' observed in recent HSDP studies. Importantly, our study shows that all steps in sample processing are critical for achieving the highest precision MC-ICP-MS Pb isotopic compositions of oceanic island basalts.