Precise and accurate 186Os/188Os and 187Os/188Os measurements by Multi-Collector Plasma
Ionisation Mass Spectrometry, part II: laser ablation and its application to single-grain Pt–Os and Re–Os geochronology

Nowell, G.M.; Pearson, D.G.; Parman, S.W.; Luguet, A.; Hanski, E.

Abstract

The abundance of platinum group alloy (PGA) grains available from some alluvial deposits combined with their great chemical resistance to conventional acid attack generates the need for an alternative method of obtaining routine, rapid yet precise Os isotopic data. Laser ablation multi-collector ICPMS (LA-MC-ICPMS) is ideally suited for this purpose and has been applied, in a relatively limited extent, to Os-rich samples by previous workers (e.g., Hirata et al., 1998; Walker et al., 2005). The wide variety of PGA minerals yield a broad spectrum of Pt, Re and Os contents which translate into a wide spread in parent-daughter ratios that is attractive for geochronology using the Re-Os and Pt-Os decay schemes. In fact, we show that single grains, consisting of multiple, syngenetic PGE minerals can contain sufficient internal Re, Pt and Os elemental variability that they present an opportunity to obtain isochron ages from two different isotope decay systems. We present a rapid (40 second acquisition time), precise and accurate LA-MC-ICPMS methodology suitable for applying Pt-Os and Re-Os geochronology approaches to single PGA grains, for use in dating chromitite deposits and identifying and dating multiple sources in alluvial PGA deposits. Because of isobaric interferences from Pt, PGA analyses by LA-MC-ICPMS are best corrected for instrumental mass fractionation using the 189Os/188Os ratio. We demonstrate that, within our analytical uncertainties, mass bias effects for Os isotope ratios during LA-MC-ICPMS are similar in nature and magnitude to mass bias variation produced during normal solution mode analysis. Robust elemental interference corrections can be applied for Re and W on the Os mass spectrum and for Os on the Pt mass spectrum, to constrain Pt/Os ratio measurement simultaneously with Os isotope ratio measurement. Accurate and precise geochronology by La-MC-ICPMS can be limited by poorly quantified fractionation of the Manuscript Click here to download Manuscript: Manuscript04-04-07.doc parent/daughter elemental ratio during ablation. Although further work is required to understand the nature and exact extent of inter-element fractionation of the Pt/Os and Re/Os ratios during laser ablation analysis of Pt- and Re-rich PGA grains, our best estimate is that fractionation of the Pt/Os and Re/Os ratios during laser ablation is less than 5-10%. Using this approach we present multi-grain isochrons and single grain internal isochrons for the Re-Os and Pt-Os isotope systems in PGA grains from Lapland. In all cases, the Pt-Os isotope system gives significantly more precise isochrons than the Re-Os system. We interpret this discordance between the two systems as being most likely due to Re disturbance from the PGA grains. We show that a suite of Pt-rich PGA grains from the Central Lapland greenstone belt yields a spectrum of internal Pt-Os isochron ages that coincides well with pulses of formation of chromitite-bearing ophiolitic material within this magmatic terrane and we take this to indicate the record of chromitite formation in different magmatic units. The single-grain Pt-Os and Re-Os geochronology technique shows great promise in the dating of chromitite deposits in ophiolites and in the discrimination of multiple source regions within alluvial PGA deposits.

Citation

Ionisation Mass Spectrometry, part II: laser ablation and its application to single-grain Pt–Os and Re–Os geochronology. Chemical Geology, 248(3-4), 394-426. https://doi.org/10.1016/j.chemgeo.2007.12.004

• Journal Article Type: Article
• Publication Date: 2008
• Journal: Chemical Geology
• Print ISSN: 0009-2541
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 248
• Issue: 3-4
• Pages: 394-426
• DOI: https://doi.org/10.1016/j.chemgeo.2007.12.004