Development of a Method to Measure Seven Metal Stable Isotope Ratios in Small Carbonaceous Chondrite Samples

Asteroid return missions have brought back small amounts of pristine carbonaceous chondrite-like material to Earth to further our understanding of the origin and evolution of objects formed early in solar system history. To obtain the maximum amount of information from the smallest possible amounts of sample, optimisation of methods that allow determination of multiple chemical and physical parameters are required. Here we present a method that allows for sequential determination of primary mineralogy via micro computed X-ray tomography (micro-CT) and isotope ratios (δ-values) of seven different elements (Mg, Fe, V, Zn, Sr, Cd and Tl) on carbonaceous chondrite samples as small as 24.1 mg. These elements have the advantage that they cover a broad range in half-mass condensation temperatures, which allows simultaneous investigation of processes occurring throughout an asteroidal lifetime. We test our methodology via analyses of the USGS reference materials AGV-2 and BHVO-1 as well as three chips of CM carbonaceous chondrite ALH 83100 with masses of ~ 24–47 mg. We use sequential ion exchange chromatographic separation to produce pure mono-elemental solutions for each of the seven target elements. Subsequent isotope ratio measurements using a Neptune multi-collector inductively coupled plasma-mass spectrometer and a thermal ionisation mass spectrometer produced accurate data for AGV-2 and BHVO-1 with precisions comparable to those obtained in the recent literature. Isotope composition data obtained for the three ALH 83100 fragments are consistent with previous results reported for CM carbonaceous chondrites. We also combine our micro-CT data with the isotope data obtained for the three chips of ALH 83100. These comparisons suggest that different components of carbonaceous chondrites may preserve isotope variability in Fe, Sr, Zn, Cd and Tl isotopes as a function of their modal mineralogy.