IN-SITU Rb-Sr DATING USING PROTEUS: COLLISIONCELL MC-ICPMS

摘要

The beta decay of ~（87）Rb to ~（87）Sr was one of the earliest examples of a radioactive system beingharnessed as a tool for geochronology. However, a characteristic and complicating feature ofisotopic analysis of β-decay systems such as Rb-Sr is the presence of an isobaric overlap of theparent and daughter nuclides. Previously this meant that time-consuming chemical separation ofthe two elements was required before analysis, and in-situ analysis was extremely difficult. Thedevelopment of collision cell inductively coupled plasma mass spectrometer (ICPMS) technologyand advances in the understanding of ion gas reactions has now permitted the in-situ analysis ofRb-rich major mineral phases (rock forming minerals) such as biotite, k-feldspar. A variety ofreaction gases such as N2O, SF6 and CH3F gases have been utilised for ion gas reactions with Sr+within a collision cell to form products with a greater mass [1]. Crucially Rb+ does not display anyreactivity towards these gases and does not form a polyatomic ion. The contrast in behaviour of Srand Rb with these reaction gases means that chemical resolution of the two elements can beachieved directly during analysis. This method has been successfully attempted using inductivelycoupled plasma – triple quadrupole mass spectrometer (ICP-QQQ) technology [2, 3].Here we demonstrate the inherit advantages of using the prototype collision cell multi-collector –inductively coupled plasma mass spectrometer (MC-ICPMS) ‘Proteus’ compared to ICP-QQQ.Our results (Fig. 1) show that using either SF6 or CH3F reaction gases, Sr+ can be chemicallyresolved from Rb+ through ion gas reactions, preserving up to 80 % of the original Sr+ analytesignal as a polyatomic product that is produced in the collision cell. In addition to the chemicalresolution of Sr, the in-situ Rb-Sr analysis conducted here utilises the recently developedtechnology of nanopowder tablets, which are used in this work as a standard that is matrixmatchedto the mineral being analysed. The use of Proteus with these reaction gases and theapplication of nanopowder tablets provides sufficient evidence to proceed in using this techniqueto date a multitude of geological samples using in-situ laser ablation coupled with the firstapplication of collision cell MC-ICPMS for Rb-Sr dating.