In Situ Rb-Sr Dating By Collision Cell, Multicollection Inductively-Coupled Plasma Mass-Spectrometry With Pre-Cell Mass-Filter, (Cc-Mc-Icpms/Ms)

We document the utility for in situ Rb-Sr dating of a one-of-a-kind tribrid mass spectrometer, 'Proteus', coupled to a UV laser ablation system. Proteus combines quadrupole mass-filter, collision cell and sector magnet with a multicollection inductively-coupled plasma mass spectrometer (CC-MC-ICPMS/MS). Compared to commercial, single collector, tribrid inductively-coupled plasma mass spectrometers (CC-ICPMS/MS) Proteus has enhanced ion transmission and offers simultaneous collection of all Sr isotopes using an array of Faraday cups. These features yield improved precision in measured Sr-87/Sr-86 ratios, for a given mass of Sr analysed, approximately a factor of 25 in comparison to the Thermo Scientific (TM) iCAP TQ (TM) operated under similar conditions. Using SF6 as a reaction gas on Proteus, measurements of Rb-doped NIST SRM (standard reference material) 987 solutions, with Rb/Sr ratios from 0.01-100, yield Sr-87/Sr-86 that are indistinguishable from un-doped NIST SRM 987, demonstrating quantitative 'chemical resolution' of Rb from Sr. We highlight the importance of mass-filtering before the collision cell for laser ablation Sr-87/Sr-86 analysis, using an in-house feldspar standard and a range of glass reference materials. By transmitting only those ions with mass-to-charge ratios 82-92 u/e into the collision cell, we achieve accurate Sr-87/Sr-86 measurements without any corrections for atomic or polyatomic isobaric interferences. Without the pre-cell mass-filtering, measured in situ(87)Sr/Sr-86 ratios are inaccurate. Combining in situ measurements of Rb/Sr and radiogenic Sr isotope ratios we obtain mineral isochrons. We utilise a sample from the well-dated Dartmoor granite (285 +/- 1 Ma) as a calibrant for our in situ ages and, using the same conditions, produce accurate Rb-Sr isochron ages for samples of the Fish Canyon tuff (28 +/- 2 Ma) and Shap granite pluton (397 +/- 1 Ma). Analysing the same Dartmoor granite sample using identical laser conditions and number of spot analyses using the Thermo Scientific (TM) iCAP TQ (TM) yielded an isochron slope 5x less precise than Proteus. We use an uncertainty model to illustrate the advantage of using Proteus over single collector CC-ICPMS/MS for in situ Rb-Sr dating. The results of this model show that the improvement is most marked for samples that have low Rb/Sr (<10) or are young (<100 Ma). We also report the first example of an in situ, internal Rb-Sr isochron from a single potassium-feldspar grain. Using a sample from the Shap granite, we obtained accurate age and initial Sr-87/Sr-86 with 95% confidence intervals of +/- 1.5% and +/- 0.03% respectively. Such capabilities offer new opportunities in geochronological studies.