Speed dating with ion specificity: in situ Rb-Sr ages using novel mass-spectrometry

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An ideal geochronological technique should be able to produce a date from a single, easy analysis. Although in situ dating of zircons has come close to this goal, it is limited to this accessory phase, which typically needs to be separated from larger volumes of sample. Even more appealing would be the ability to date samples from a petrographic section, preferably on major mineral phases. This possibility exists for some high Rb-Sr phases using the 87Rb- 86Sr system, but this currently requires time consuming micro-drilling, separation chemistry and subsequent analysis. The problem with the 87Rb- 87Sr pair, and other beta decay systems commonly used in geological dating, is that the parent inevitably forms an isobaric interference with the daughter, such that phases with high parent daughter ratios, which have the potential to give the most precise dates, have the most interfered isotope ratios. These elemental inferences cannot be resolved by even the largest geometry mass spectrometers. A novel approach is to use a collision cell to achieve chemical specificity rather than mass resolution. Ion-gas reactions in the collision cell can reduce isobaric interferences by many orders of magnitude. Such a scheme exists for the Rb-Sr system and we wish to explore the potential of this approach with our unique collision cell, plasma multi-collector mass-spectrometer (Proteus). The student will work in both refining the reaction schemes to remove Rb from Sr (in collaboration with our CASE partner, Thermo Fisher Scientific), and harnessing these results to develop protocols on Proteus to measure isotope ratios from laser ablated material. The resulting methodology will be applied to the dating of inclusions in ultra-deep diamonds, the ages of which are a source of long standing debate. Laser ablation is an efficient means to sample these valuable samples of the deep mantle and the prospect of coupling this with isotopic analysis is a valuable goal.