In situ Pb-isotope analysis of pyrite by laser ablation (multi-collector and quadrupole) ICPMS

Abstract

Pb-isotope ratios, measured in the mineral pyrite, provide a valuable petrogenetic tool with widespread applicability. In order to interpret complex structural and mineralogical textures, however, a method of in-situ analysis is essential. While laser ablation ICPMS is ideally suited to this task, the low melting point of sulfide, the highly variable and often high Pb contents, and the potential presence of relatively radiogenic inclusions introduce a number of analytical problems unique to pyrite Pb-isotope analysis. Here we address these issues using results obtained on two very different analytical systems based around multi-collector and quadrupole ICPMS instruments respectively. We suggest that controlled ablation of pyrite is only achieved at low laser fluence and that, under these conditions, standardisation using silicate reference materials is inappropriate and natural pyrite standards are to be preferred. The inherent variability in Pb (and sometimes U) concentrations in pyrite requires careful selection of detector systems for optimal analysis and in this regard both quadrupole and multi-collector ICPMS instruments can play important and complimentary roles. Multi-collector instruments provide higher precision analyses but detector configurations can prohibit simultaneous measurement of U, Th and Pb. Furthermore, the micrometer-scale variability in Pb concentrations can cause problems for both Faraday cup and ion counting detection systems. In contrast, quadrupole ICPMS systems allow simultaneous measurement of U, Th and Pb, and have more flexible detection systems with many orders of magnitude dynamic range but are unable to produce high precision data. Results are presented for two different analytical systems and demonstrate a very strong dependence of data quality upon signal size. In addition they allow some estimation of the limiting precision obtainable by these methods. Finally, a geological example is provided from the giant Sukhoi Log sedimentary Au deposit of Russia.