In situ Cr isotope measurements and early solar system history

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Project BackgroundThe project will develop methods to make in situ, laser ablation measurements of Cr isotope to improve our understanding of the earliest history of the solar system. Bulk meteorites show variations in 'mass independent' 54Cr, implying inhomogeous mixing of a 54Cr-rich carrier [Trinquier et al., 2007]. Finding the carrier is important to understand what causes this variability in the composition of planetary forming materials. Studies have found oxide grains that apparently carry anomalous 54Cr, but these required large corrections for 54Fe interferences, which could not be resolved and had to be assumed to be 'normal' [Dauphas et al., 2010; Qin et al., 2011]. Moreover, the grains were the residues of meteorite dissolutions and not found in situ. Ultimately we wish to find in the matrices of primitive meteorites pre-solar grains carrying a 54Cr anomaly.Project Aims and MethodsThis work will exploit the capabilities of our unique, Proteus collision cell multi-collector inductively coupled plasma mass-spectrometer. This project will look for Cr isotope anomalies in situ, and using the capabilities of Proteus, remove 54Fe interferences (by oxidising Fe to FeO in the collision cell). This is a challenging topic, which will require extensive analytical experimentation. As an initial step in establishing a protocol, the project will try a simpler set up to measure the Cr isotopic composition of Mn-rich sulphides (niningerites) in highly reduced, enstatite chondrites [Ehlers and El Gorsey, 1988]. This holds the promise of providing precise ages, of the chondrules which host the sulphides, using the 53Mn-53Cr chronometer (t1/2 = 3.7Ma). There are few precise ages for chondrules in enstatite chondrites and so this would yield valuable new information. In any in situ study, it is important to benchmark standards against bulk techniques. So the project will also involve analysing Cr isotopes by thermal ionisation mass-spectrometry. In all the work will provide cutting edge training in a wide range of analytical skills, focussed on topical cosmochemical problems.