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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 insitu
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.

Item Type:	Article
Authors/Creators:	Woodhead, JD and Hergt, J and Large, RR and Meffre, Sebastien and Danyushevsky, Leonid and Gilbert, SE
Keywords:	sulfide, pyrite, Pb-isotope, In-situ analysis, laser ablation, ICPMS
Journal or Publication Title:	Chemical Geology
ISSN:	0009-2541
DOI / ID Number:	10.1016/j.chemgeo.2009.02.003
Additional Information:	The definitive version is available at http://www.sciencedirect.com
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