Problems in obtaining precise and accurate Sr isot

Authors: P Z Vroon B van der Wagt J M Koornneef G R Davies

Publish Date: 2007/12/14

Volume: 390, Issue: 2, Pages: 465-476

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Abstract

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry LAMCICPMS in the period 1995–2006 This technique has been shown to have great potential but the accuracy and precision are limited by 1 large instrumental mass discrimination 2 laserinduced isotopic and elemental fractionations and 3 molecular interferences The most important isobaric interferences are Kr and Rb whereas Ca dimer/argides and doubly charged rare earth elements REE are limited to sample materials which contain substantial amounts of these elements With modern laser 193 nm and MCICPMS equipment minerals with 500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution spot size of approximately 100 μm The LA MCICPMS analysis of 87Sr/86Sr of both carbonate material and plagioclase is successful in all reported studies although the higher 84Sr/86Sr ratios do suggest in some cases an influence of Ca dimer and/or argides High Rb/Sr 001 materials have been successfully analysed by carefully measuring the 85Rb/87Rb in standard material and by applying the standardsample bracketing method for accurate Rb corrections However published LAMCICPMS data on clinopyroxene apatite and sphene records differences when compared with 87Sr/86Sr measured by thermal ionisation mass spectrometry TIMS and solution MCICPMS This suggests that further studies are required to ensure that the most optimal correction methods are applied for all isobaric interferencesThe radioactive beta β− decay of 87Rb to 87Sr is an important isotope system that has been widely applied for geochronological purposes More importantly the isotopic system is extensively used to constrain the rates and fluxes involved in a wide range of geological processes operating from within the hydrosphere of the Earth to the deep mantle In addition recently Sr isotopes have become widely applied as provenance tracers in many different scientific disciplines eg biology nutrition medical forensic and art history 1 2 3 4 5 6 7 Sr isotope ratios have always been difficult to analyse to high precision better than 20 ppm because there are large differences in the abundances of the isotopes 84Sr 86Sr 87Sr and 88Sr In many materials the low abundance of Rb and their relatively young age leads to small isotopic variations eg 005 in mantle rocks and biogenic carbonates This range is considerably smaller than the combined UThPb isotope system for example Recent technical improvements in thermal ionisation mass spectrometry TIMS design most notably in terms of Faraday collectors and amplifier electronics now allow analytical precisions better than 00005 5 ppm 1 SE Despite this significant improvement in precision the classical TIMS technique requires timeconsuming liquid chromatographic techniques to remove matrix and interfering elements 87Rb inhibiting any possibility for an “online” in situ analytical techniqueThe arrival of the first commercial multicollector inductively coupled plasma mass spectrometry MCICPMS instruments in the first half of the 1990s coupled with laser ablation LA facilities promised an enormous new potential for in situ analytical techniques in petrology marine sciences and many other applications eg see 8 9 The study of smallscale variations of Sr isotopes in geological materials has provided very important constraints on the rates of volcanic processes and the fluxes that operate in diverse geological environments eg see 2 10 However these studies are extremely time consuming typically averaging more than 10 hours per sample because they require careful microdrilling techniques lowblank liquid chromatographic separation techniques and TIMS analyses The MCICPMS coupled with a laser ablation system should be able to perform a Sr isotope analyses within minutes while importantly maintaining the spatial resolution and avoiding the extensive wet chemistry and warm up times on the TIMS instrumentDespite the great potential of LAMCICPMS analyses for Sr isotopes and its availability for more than a decade the technique has not become routine Eight out of eleven papers published in the period 1995–2006 that reported in situ analyses of Sr isotopes by LAMCICPMS are mainly focussed on technique development 11 12 13 14 15 16 17 18 19 20 21 There is clearly a huge potential and a demand for the application of in situ Sr isotope analysis Therefore the goals of this paper are to 1 describe the different approaches that have been followed to date 2 summarize the problems encountered 3 discuss the solutions attempted to solve the encountered problems and 4 discuss studies which are needed in the near future to improve the LAMCICPMS technique so that “routine” Sr isotope analysis becomes possible

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