HR: 0800h
AN: V51B-0565    [Abstracts]
TI: In-Situ Silicon Isotope Analysis of Archean Cherts by Laser Ablation MC-ICPMS
AU: * Vroon, P Z
EM: pieter.vroon@falw.vu.nl
AF: Dept. of Petrology, Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081 HV, Netherlands
AU: van den Boorn, S H
EM: boorn@geo.uu.nl
AF: Dept. of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3584 CD, Netherlands
AU: van der Wagt, B J
EM: wagb@falw.vu.nl
AF: Dept. of Petrology, Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081 HV, Netherlands
AU: van Bergen, M J
EM: vbergen@geo.uu.nl
AF: Dept. of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3584 CD, Netherlands
AB: We present in-situ stable silicon isotope results for Archean Cherts from the Pilbara region, Western Australia. Analyses were performed using a Geolas Laser Ablation (LA) system equipped with a 193nm Excimer laser and a ThermoFinnigan Neptune MC-ICPMS. The MC-ICPMS was used in medium resolution mode (RP=4000) to resolve molecular isobaric interferences (e.g., ¹²C¹⁶O⁺, ¹⁴N₂⁺, ¹⁴N¹⁶O⁺). We used an ablation pit size of 49 by 300μm with a 7Hz repetition rate and 5 J.cm^-2. Tuning conditions and cup settings were similar as those described by Van den Boorn et al. (2006; 2007) for solution work. To assess precision and accuracy of the LA technique, chert samples were analyzed that were previously characterized for silicon isotopes by micro-drilling and subsequent liquid chromatographic purification. A chemically homogenous chert sample that is well characterized for silicon isotopes was used as a standard. This in-house standard has a δ³⁰Si of 0.50 ± 20 (2sd, n=4) relative to NIST RM8546 (=NBS28). Our precision with the LA technique of 0.2‰ (2sd, n=11), based on repeated measurements of the standard, is slightly better than the long-term precision of 0.3‰ for solution work (Van den Boorn et al. 2006). Micro-drill and laser data are in excellent agreement (less than 0.4‰), which is well within the variations recorded in individual mm-cm sized chert laminae. By producing 3cm scans across chert bands, inhomogeneities of up to 0.5‰ can be resolved within a single band. Matrix effects might be significant in LA work. For example, borate silicate glass was up to 2.15‰ heavier than values obtained by solution work. This suggests that ablation induces isotopic fractionation and/or that matrix elements cause a shift in mass bias for silicon in the plasma. Because Archean cherts generally contain more than 95% SiO₂, offsets due to matrix effects will be small. However, the use of a standard with a composition close to samples is recommended. References: Van den Boorn et al. (2006) J. Anal. At. Spectrom., 21, 734–742. Van den Boorn et al. (2007). Geology, 35: 939-942.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1051 Sedimentary geochemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting