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 a10 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 UCameca 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.

本研究采用计算机控制的Omniprobe微操纵器（Omniprobe），将每个样品转移至经溅射清洁的金箔样品台，实验操作在FEI Quanta 3D聚焦离子束（FIB）系统内完成。在每个样品的10 μm半径范围内，我们放置了三颗取自诺顿县顽火辉石无球粒陨石（aubrite）粉碎样品的颗粒，该几何布局可实现样品与内标物质的同步测量。我们采用华盛顿大学配套的卡麦伽（Cameca）NanoSIMS 50型纳米离子探针，采集了视场为12×12 μm、像素分辨率为256×256的离子光栅扫描图像。对每个样品均以78 pA的Cs+束预溅射300秒，以去除表面吸附的水分；测量采用聚焦至约100 nm的2 pA Cs+一次束流完成。我们通过独立的电子倍增器同时采集了16O−、17O−和18O−信号。针对全部四个样品，17O−的质量分辨率约为5500，足以区分16OH−带来的信号干扰。每个样品采集了约100帧图像（耗时2.5小时），此后样品A与B的顽火辉石晶须便被完全溅射消耗；但测量完成后，样品C与D的顽火辉石条带仍残留有样品物质。本研究采用基于Look@NanoSIMS软件包定制开发的Matlab脚本，对纳米离子探针获取的同位素数据进行分析处理：我们对每个扫描周期的图像进行配准，并划定感兴趣区域（ROI），以避开颗粒与扫描边缘，同时降低由表面形貌引发的仪器分馏效应；随后对每个感兴趣区域的配准计数数据进行求和，以此计算同位素比值。