In situ Rb-Sr dating by collision cell MC-ICP-MS
Abstract
The 87Rb-87Sr system is fundamental to geochronology. Yet, as a beta decay, the inevitable isobaric overlap of parent and daughter nuclides critically limits the in situ analysis of high Rb/Sr phases. The development of 'Proteus', a one-of-a-kind tribrid MC-ICPMS, coupled with an extensive literature of ion gas reactions, provides a solution to overcome this analytical challenge. Proteus utilises the unique combination of quadrupole mass pre-filter and collision cell to limit the mass range of ion species entering the cell. To provide chemical resolution of Rb+ from Sr+we used SF6 in He as a reaction/collision gas. This gas exhibits an efficient exothermic reaction with Sr+ forming SrF+ and, since F is monoisotopic, a simple mass spectrum results. Crucially, SF6 displays no reactivity with Rb+, therefore providing complete resolution of the 87Rb-87Sr overlap.
Here we demonstrate that the combination of pre-cell mass filter and reaction with SF6 can yield accurate 87Sr/86Sr for samples with 87Rb/86Sr >80. We also report accurate in situ 87Sr/86Sr ratios despite using poorly matrix-matched reference materials such as NIST SRM® 610, highlighting the robustness of the technique. Furthermore, the first application of in situ Rb-Sr dating using collision cell MC-ICPMS for the Dartmoor and Shap granites is presented: Rb-Sr isochrons were created from spot analyses on K-feldspar and plagioclase feldspar only. Both samples yielded accurate ages and 87Sr/86Sri, within uncertainty of previous TIMS analysis. All our analyses effectively demonstrated the superiority of Proteus for in situ Rb-Sr dating compared to TQ-ICPMS. This proof of concept using collision cell MC-ICPMS expands the variety of geological samples available for in situ Rb-Sr dating and validates the potential of this technique as a rapid and versatile dating method within geochronlogy.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V14B..03B
- Keywords:
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- 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRY;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 1094 Instruments and techniques;
- GEOCHEMISTRY