Crystal orientation effects in δ18O for magnetite and hematite by SIMS

Abstract

In situ high precision analysis of oxygen isotope ratios ( δ 18O) by secondary ion mass spectrometry (SIMS) reveals that instrumental bias in δ 18O for magnetite varies due to crystal orientation effects. 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. 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. Electron backscatter diffraction shows that individual grains of magnetite are single crystals and that crystal orientation varies randomly from grain-to-grain. The crystal orientation for each magnetite grain is plotted relative to the incident angle of the SIMS primary Cs + beam. High values of δ 18O are measured when the Cs + beam is parallel to < uv0>, from [110] to [100], preferred channeling and focusing directions for magnetite. 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). 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. 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°. 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. 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 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. 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.