Oxygen isotope measurements of cometary filamentary enstatite

Each sample was transferred to a sputter-cleaned Au foil mount using a computer-controlled Omniprobe micro-manipulator in an FEI Quanta 3D FIB. Within a 10 μm radius of each sample, we placed three grains from a crushed sample of the Nor-ton County aubrite. This geometry allowed for simultaneous measurements of the sample and internal standards. We acquired 12×12 μm, 256×256 pixel ion raster images using the Wash U Cameca NanoSIMS 50. Each sample was pre-sputtered with a 78 pA Cs+ beam for 300 s to remove any adsorbed water. Measurements were collected using a 2 pA Cs+ primary beam focused to approximately 100 nm. We simultaneously collected 16O−, 17O−, and 18O− on separate electron multipliers. For all four samples, the mass-resolving power for 17O− was ~5500, sufficient to resolve the interference from16OH−. We collected ∼100 frames (2.5 hours) for each sample, after which the enstatite whiskers (samples A and B) were entirely consumed. However, material from the enstatite ribbons (samples C and D) remained after the measurements. Isotopic data from the NanoSIMS analyzed were analyzed using a custom Matlab script derived from the Look@NanoSIMS package. We aligned the images from each cycle and defined regions of interest (ROIs) to avoid grain and raster edges and reduce topography-induced instrumental fractionation effects. The aligned count data from each ROI was then summed to calculate the isotopic ratios.