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Free keywords:
Si isotopes; UV-femtosecond laser; MC-ICP-MS
DDC:
550 - Earth sciences
Abstract:
A new method for the in situ measurement of stable Si isotope ratio using UV-femtosecond laser ablation connected to a multiplecollector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been established. The use of medium mass resolution mode (with a resolving powerm/Δm=8000) permits to resolve spectral interferences on 28Si, 29Si and 30Si allowing for determination of the 29Si/28Si and 30Si/28Si ratios with high accuracy and precision on wide, interference-free plateaus. A three-isotope plot demonstrated that interferences, if existing, are negligible for Si isotope ratios. The laser spot size is 35 μm and measurements are made using square rasters of 150 μmside length. Different types of elemental Si, Si-oxide and silicate matrices have been analysed by laser ablation using the international standard NBS28 as the bracketing standard. Thus δ29Si and δ30Si have been determined for the silicon isotope standard IRMM-017 (δ30Si±2 S.D.=−1.26±0.24‰; n=89) and BigBatch (δ30Si=−10.55±0.42‰; n=15), as well as San Carlos Olivine (δ30Si=−0.81±0.19‰; n=14), Caltech Rose Quartz (δ30Si=0.10±0.13‰; n=14) sponge needles (Stylocordyla borealis; δ30Si=−2.19±0.32‰; n=14) and JER-diopside glass (δ30Si=0±0.09‰; n=14) samples. The overall repeatability achievable is 0.15‰ (2 S.D.) on δ29Si and 0.24‰ (2 S.D.) on δ30Si. The silicon isotope standard IRMM-018 has also been measured and was confirmed to be heterogeneous. The two isotope ratios follow an equilibrium mass-dependent fractionation law which can be represented as δ30Si=1.93× δ29Si. Published extractions methods have been used to wet-chemically purify Si from the JER-diopside glass. The Si extracted was presented as Si-gel and ablated like a solid. The results show excellent agreement with in situ measurements of this glass that confirms that this technique can be employed to a wide variety of matrices, including Si purified from solution. The new technique presents a viable alternative to solution MC-ICP-MS for bulk measurements and the most precise technique so far for in situ measurement of Si isotope ratios.
