High-Precision Oxygen Isotope Analysis via Ion Microprobe IMS-1280: Analytical Developments & Applications

1. Analytical developments on oxygen three isotope analyses using a new generation Ion Microprobe IMS-1280. N. T. Kita, T. Ushikubo, B. Fu, M. J. Spicuzza, and J. W. Valley (LPSC, March 2007) Oxygen isotopes in meteorites show a wide range of mass dependent and mass independent isotopic fractionations and provide a unique tool for studying the early evolution of the solar system. Studies of oxygen isotopic variations at the level of a few ‰ in the µm to sub-mm scale in meteorite samples are very limited because of difficulties in analyzing low natural abundance 17O and 18O isotopes (18O/16O~2.005210-3 and 17O/16O~3.8310-4) with sufficient precisions. We developed analytical methods to achieve sub‰ precisions on both17O/16O and 18O/16O ratios by using a new generation ion microprobe CAMECA IMS-1280 at the University of Wisconsin-Madison. KEYS to High Precision and Accuracy (1) Higher Secondary ion intensity Focused Cs+ primary beam to obtain higher primary ion intensity High secondary ion transmission >70% (2) Multi-collection Faraday Cup (FC) Cancel primary and secondary beam instability (3) Stability of Electron gun Tune electron beam with homogeneous density (4) Automated analyses for reproducibility Centering of secondary beam Identical presputtering time (5) Sample flatness Tilted or rough surface creates distorted ion extraction field and causes instrumental mass fractionation. (6) Calibration of Standards Reproducibility of oxygen three isotope analyses for terrestrial olivine standard using three FCs. The analytical conditions include (1) Primary focused Cs+ ions (12µm, 4nA), (2) Electron gun for charge compensation, (3) Manual Z-focus and automated X-Y centering of secondary beam, (4) Transfer optics of 200 magnification, (5) NMR probe for magnetic field control (stability ≤ 5ppm over 10 hours), (6) Mass Resolving Power of ~5000 for 17O, (7) Secondary 16O intensity 3.5109 cps. A single analysis takes ~7min consisting of presputtering (~100s), automatic beam centering (~60s) and integration of oxygen isotopes (~200s). Kita et al. (2007a) LPSC Abstract #1981

2. CL image 17 33,34 36 32 15 22 16 18 31 20 19 Application to Cosmochemistry: Oxygen three isotope analyses of chondrules. Micro-distribution of oxygen isotope compositions within a single chondrule provide strong constraints on their precursor materials, the physical conditions of their formation and information of their environments. FeO-poor olivine rich chondrule (CH44) from Semarkona meteorite contains unusual forsterite grains showing both blue and red cathodoluminescence (CL) enclosed in Al, Ca-rich glass. Blue CL olivine at the rim is depleted in FeO (Fo>99.5) and enriched in refractory elements such as Al and Ca, while red CL olivine at core is slightly FeO-richer (Fo<99.5). Oxygen three isotope compositions of blue forsterite and glass are enriched in 16O relative to red forsterite. The result indicates that CH44 formed by the mixing of 16O-rich and refractory element-rich solid precursor and 16O-poor normal relict forsterite grains from the previous generation of chondrule. Similar oxygen isotope compositions between olivine rim and glass indicates absence of oxygen isotope exchange between 16O-poor gas reservoir. TF indicates terrestrial fractionation line (17O=0.52x18O). Primitive meteoritic samples plot along mixing lines between 16O-rich refractory inclusions (17O, 18O ~-50‰) and chondritic components near TF line. Numbers in the figure indicate SIMS analysis spots shown on the back scattered electron (BSE) image. Orange, blue and green colors represent red and blue CL olivines and Al, Ca-rich glass, respectively. Kita et al. (2007b) LPSC Abstract #1791