![]() ![]() The fractionation of oxygen isotopes between magnetite and quartz (SiO 2) is the most sensitive oxygen isotope geothermometer (Valley, 2001). Oxygen isotope ratios ( δ 18O) in magnetite can provide important information on fluid conditions and temperatures during sedimentation, diagenesis, metamorphism, magmatism, and the genesis of iron ore deposits and banded iron formations (BIFs). Magnetite (Fe 3O 4) is a common mineral in sedimentary, metamorphic, and igneous rocks. Importantly, crystal orientation effects have not been identified at levels of ± 0.3‰ for δ 18O in silicates or other minerals analyzed by WiscSIMS though many minerals remain to be examined. The grain-to-grain precision in measured δ 18O for hematite improves from ± 2.1‰ to ± 1.0‰ (2SD) at + 10 kV/− 10 kV and + 3 kV/− 10 kV analysis respectively, while precision in single grains is ± 0.3‰ (2SD) for both. Instrumental bias in δ 18O also varies with crystal orientation for hematite at similar levels as is seen for magnetite. The grain-to-grain precision in measured δ 18O for magnetite improves from ± 2.9‰ to ± 0.8‰ (2SD) at + 10 kV/− 10 kV and + 3 kV/− 10 kV analysis respectively, while precision in single grains is ± 0.4‰ for both. The best results were obtained in experiment (4) at primary/secondary accelerating voltages of + 3 kV/− 10 kV respectively with an incident Cs + beam angle of 14°. Four analytical experiments were conducted in attempts to improve the grain-to-grain precision in measured δ 18O for magnetite: (1) applying an energy offset of 50 eV, (2) using a Köhler illuminated beam (shallow-pit mode), (3) reducing the total impact energy, and (4) varying the primary and secondary accelerating voltages. Routine δ 18O analysis at WiscSIMS utilizes a Gaussian focused Cs + primary beam (deep-pit mode) at primary and secondary voltages of + 10 kV and−10 kV respectively (total impact energy 20 keV). High values of δ 18O are measured when the Cs + beam is parallel to, from to, preferred channeling and focusing directions for magnetite. The crystal orientation for each magnetite grain is plotted relative to the incident angle of the SIMS primary Cs + beam. Electron backscatter diffraction shows that individual grains of magnetite are single crystals and that crystal orientation varies randomly from grain-to-grain. In contrast, the average precision is five to ten times worse, ± 2–3‰ (2SD), from grain-to-grain of magnetite due to variation in instrumental bias with crystal orientation. Multiple analyses of δ 18O have an average precision of ± 0.4‰ (2SD) in single grains of magnetite, close to ± 0.3‰, that obtained for multiple grains of UWQ-1, a homogeneous quartz standard. Furthermore, the spinel-based monoclinic model is more accurate than the monoclinic nonspinel model.In situ high precision analysis of oxygen isotope ratios ( δ 18 O) by secondary ion mass spectrometry (SIMS) reveals that instrumental bias in δ 18O for magnetite varies due to crystal orientation effects. ![]() Our work indicates that the traditional cubic spinel model is a more accurate model of γ-Al 2O 3 than the other models considered. ![]() The single-crystal SAED spot pattern reflected symmetry consistent with both the cubic spinel and tetragonal nonspinel models, yet, the Al cation distribution better matched the cubic spinel model based on the relative intensities of diffraction spots. The lattice interplanar distances derived from the polycrystalline SAED pattern most closely matched the cubic spinel γ-Al 2O 3 model. Single crystal and textured polycrystalline γ-Al 2O 3 thin films were synthesized by oxidation of NiAl(110) in air at 850 ☌ for 1 and 2 hours, respectively, and used to evaluate the accuracy of two spinel-based and two nonspinel models by comparison of selected-area electron diffraction (SAED). ![]()
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