SIMULTANEOUS PB-ISOTOPE AND TRACE-ELEMENT CHARACTERIZATION OF GALENA BY QUADRUPOLE-BASED LA-ICP-MS
Whereas Pb-isotopes in galena are typically measured by TIMS or (LA)-MC-ICP-MS in order to obtain high-precision measurements on individual sample aliquots, Q-LA-ICP-MS could potentially achieve usable precision (e.g, 0.1%) on Pb-isotopes using pooled weighted averages for larger (e.g., n >20) populations. Ablation conditions for galena were optimized to produce a near-steady-state mass transfer to the Q-ICP-MS plasma: 0.35 J/cm2 fluence, 2.5 Hz pulse rate, 60 µm diameter craters and 60 second ablation. The large Pb ion beams produced under these conditions allow short quadrupole dwell times to capture time-dependent fluctuations while at the same time maintaining good counting statistics. All data are externally standardized to a fragment of Broken Hill galena using Pb-isotope reference values of Stacey et al (1969) and trace-element concentrations measured in-house relative to USGS MASS-1. This approach provides near-perfect matrix-matching. A 40s background is sufficient to return to baselines and the high Pb/Hg (typically >1E6 cps) precludes any interference from 204Hg. Several important trace-elements (Ag, Cd, Sn, Sb, and Tl) can also be simultaneously analyzed.
Propagated errors on individual spots is typically between 0.3% to 0.6%. The Isoplot ‘Weighted Average’ routine (weighted by assigned 2σ error, and reported at 95% confidence) reduces these errors to <0.10% for 207Pb/206Pb, ~0.10% for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/206Pb and 0.12% for the 208Pb/204Pb ratios. Although these errors are still significantly larger than TIMS or MC-ICP-MS they are of sufficient precision for rapid reconnaissance studies. The validity of the method was tested by analyzing galena from a wide variety of well-known VMS and vein deposits in Canada, Germany, Ireland, and the USA.