HR: 09:30h
AN: V41E-07    [Abstracts]
TI: High-Precision Double-Spike Sr Isotope Measurements: Applications to Geochemistry and Cosmochemistry
AU: * Parkinson, I J
EM: I.J.Parkinson@open.ac.uk
AF: The Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Burton, K W
EM: K.W.Burton@open.ac.uk
AF: The Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Burton, K W
EM: K.W.Burton@open.ac.uk
AF: Université de Toulouse, LMTG, Toulouse, F-31400, France
AU: Mokadem, F
EM: F.Mokadem@open.ac.uk
AF: The Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Charlier, B L
EM: B.L.A.Charlier@open.ac.uk
AF: The Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AB: Strontium isotope systematics consist of the stable ⁸⁸Sr/⁸⁶Sr ratio, the radiogenic ⁸⁷Sr/⁸⁶Sr ratio and the ⁸⁴Sr/⁸⁶Sr ratio which is stable in terrestrial materials and may vary due to nuclear- synthetic processes in some meteorites [1]. Growing interest in natural mass dependent ⁸⁸Sr/⁸⁶Sr variations indicate that terrestrial samples vary by ~500 ppm and have been measured to a precision of 50-70 ppm by MC-ICP-MS [2, 3]. However, the precision is large relative to the likely variation in many terrestrial systems. Furthermore, ⁸⁴Sr/⁸⁶Sr ratios are poorly determined due to spectral interferences and therefore MC-ICP-MS techniques are not appropriate for high-precision analyses of all the Sr isotope ratios. Recent advances in TIMS allow ⁸⁷Sr/⁸⁶Sr and ⁸⁴Sr/⁸⁶Sr ratios to be measured to an external precision of ~5 and 60 ppm respectively with instrumental fractionation being corrected by internal normalisation to a constant ⁸⁸Sr/⁸⁶Sr ratio. Following on from the pioneering work of [4] we use an ⁸⁴Sr- ⁸⁷Sr double-spike coupled with TIMS analyses. Optimal spiking, extended ion collection times and a 25-fold improvement in the determination of the spike composition compared to [4] allow us to measure ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr ratios to better than 10 ppm external precision. The double-spike determination consists of an un-spiked and spiked measurement, with spiking prior to chemical separation because column separation produces a consistent and resolvable light fractionation effect of 21 to 93 ppm. The un-spiked run allows us to assess any non-mass dependent fractionation effects. Double-spike deconvolution is performed in ⁸⁷Sr denominator space. Our preliminary high-precision data allows us to resolve small (25 ppm) mass-dependent shifts in seawater ⁸⁸Sr/⁸⁶Sr, which may reflect short-timescale Sr fluxes in the oceans. References [1] D.A. Papanastassiou, G.J. Wasserburg, Geophys. Res. Lett. 5 (1978) 361-376. [2] J. Fietzke, A. Eisenhauer, Geochem. Geophys. Geosyst., 7, (2006) Q08009, doi:10.1029/2006GC001243. [3] G.M. Nowell, et al., Geochim. Cosmochim. Acta. 71 (2007) A725. [4] P.J. Patchett, Nature, 283 (1980b) 438-441.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting