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Durham University

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G.M. Nowell, D.G. Pearson, S.W. Parman, A. Luguet & E. Hanski (2008). 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. Chemical Geology 248(3-4): 394-426.

Author(s) from Durham

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.