HR: 11:50h
AN: V51K-07    [PDF]
TI: Assessing Kinetic Mass Fractionation of Si Isotopes Measured by High Resolution MC-ICP-MS.
AU: * Reynolds, B C
EM: reynolds@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH Zentrum, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AU: Williams, H M
EM: williams@erdwethz.ch
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH Zentrum, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AU: Frank, M
EM: frank@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH Zentrum, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AU: Halliday, A N
EM: halliday@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH Zentrum, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AB: The introduction of silicon into a MC-ICP-MS via a desolvating nebuliser system is a recently developed method to measure Si isotope abundances with high precision and accuracy (Cardinal et al., 2003). Silicon is the most abundant rock-forming element. This technique allows for the investigation of Si isotope fractionation between different mineral phases in terrestrial and extra-terrestrial materials. Understanding the distribution of Si isotopes between different terrestrial reservoirs and thus the Si isotope terrestrial mass-balance is fundamental for the development of Si isotopes as a tracer of biogeochemical cycles. Furthermore, like oxygen and magnesium, Si has three stable isotopes, such that mass-dependent and -independent processes can be assessed. However, a terrestrial mass fractionation line has yet to be characterized for Si isotopes. The measurement of Si isotopes by MC-ICP-MS is limited by large isobaric interferences, particularly on mass 30, which can only be separated at a mass-resolution of at least $\sim$1500. On the high resolution Nu1700 MC-ICP-MS these interferences can all be separated, and we are thus able to measure all three isotopes $^{28}$Si-$^{29}$Si-$^{30}$Si isotopes directly. A range of standards and natural samples have been measured which define a single, purely kinetic mass fractionation line (e.g. a slope of 0.5092 for measured ln($^{30}$Si/$^{28}$Si) versus ln($^{29}$Si/$^{28}$Si)). However, not only do different cone geometries cause different mass-fractionation factors but the standard wide-angle cones also cause the measured isotope abundances to differ from the purely kinetic mass fractionation line. Although some of this effect may be caused by 'Si-H' interferences, the intensities of these species alone cannot account for the differences observed between cone geometries. The internal mass fractionation lines that we measure do not agree with the certified values for Si isotope reference materials based on spiked fluorination gas-source mass-spectrometry measurements. For the international NIST NBS-28 isotopic reference material we obtain a $\delta ^{29}$Si isotope value that is over 4 $\permil$ lighter for the given $^{30}$Si/$^{28}$Si ratio. Unfortunately, we also observe differences in the relative isotope composition between NBS28, IRMM017 and IRMM018 from those previously measured, suggesting significant heterogeneity of the standards. Clearly, there is need for inter-laboratory collaboration to define suitable isotopic reference materials. Cardinal et al., 2003, Journal of Analytical Atomic Spectrometry 18(3): 213-218
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting