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High precision osmium stable isotope measurements by double spike MC-ICP-MS and N-TIMS

Nanne, J.A.M.; Millet, M.-A.; Burton, K.W.; Dale, C.W.; Nowell, G.M.; Williams, H.M.

High precision osmium stable isotope measurements by double spike MC-ICP-MS and N-TIMS Thumbnail


Authors

J.A.M. Nanne

M.-A. Millet

K.W. Burton

C.W. Dale

H.M. Williams



Abstract

Osmium stable isotopes provide a new, potentially powerful tool with which to investigate a diverse range of geological processes including planetary formation, ore-genesis and weathering. In this paper, we present a new technique for high precision measurement of osmium (Os) stable isotope ratios by both Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) and Negative ion Thermal Ionisation Mass Spectrometry (N-TIMS). We use a 188Os–190Os double spike, composed of 61% 188Os and 39% 190Os, to correct for mass dependent fractionation resulting from sample preparation and isotope measurement, with the ideal spike to sample ratio being 55 : 45. Isotope ratios are expressed as δ190Os which is the per mil deviation in the measured 190Os/188Os ratio relative to isotope reference material DROsS. Repeated analyses of double spiked DROsS for both MC-ICP-MS (n = 80 cycles) and N-TIMS (n = 280 cycles) show that an internal precision of 0.01–0.02‰ on δ190Os (2 se) can be attained, with a long-term reproducibility of 0.016‰ and 0.029‰ (2 sd; n = 91 and 83, respectively). The better reproducibility on MC-ICP-MS than on N-TIMS is, predominantly, due to measurement at higher beam intensities (11–18 V with consumption of ∼200 ng natural Os vs. 2–18 V with consumption of 2.3–45 ng natural Os, respectively). In addition to stable isotope compositions, our method allows for simultaneous measurement of 187Os/188Os and 186Os/188Os ratios with a precision of <40 ppm (2 se; 80 cycles for MC-ICP-MS and 280 cycles for N-TIMS) and an external reproducibility of 123–268 ppm and 234–361 ppm (2 sd; n = 91 for MC-ICP-MS and n = 83 for N-TIMS), respectively. We demonstrate that a similar precision and reproducibility can be obtained for other pure Os solutions as well as for geological materials. Furthermore, with a range of of analytical tests we evaluate and demonstrate the robustness of our method with regards to residual matrix effects and interference correction, signal intensity and on-peak zero on MC-ICP-MS, and the effect of oxygen corrections and isobaric interference on N-TIMS. Finally, we report the first Os stable isotope compositions for geological reference materials, including mantle peridotites and chromitites, and one ordinary chondrite.

Journal Article Type Article
Acceptance Date Mar 3, 2017
Online Publication Date Mar 6, 2017
Publication Date Apr 1, 2017
Deposit Date Mar 21, 2017
Publicly Available Date Mar 3, 2018
Journal Journal of Analytical Atomic Spectrometry
Print ISSN 0267-9477
Electronic ISSN 1364-5544
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
Volume 32
Issue 4
Pages 749-765
DOI https://doi.org/10.1039/c6ja00406g
Public URL https://durham-repository.worktribe.com/output/1360910