RATIONALE: The precise determination of Δ'17 O values in terrestrial material is becoming increasingly important to understand the mass-dependent fractionation processes that cause variations in oxygen isotope ratios. San Carlos olivine is widely used as the reference material for oxygen isotope measurements of terrestrial and extraterrestrial materials. We report new Δ'17 O values for San Carlos olivine that were independently determined in two different laboratories (Geoscience Center [GZG], University of Göttingen) and Institute for Study of the Earth's Interior [ISEI], Okayama University, Misasa) in direct comparison with VSMOW2 and SLAP2 water standards. METHODS: The δ 17 O and δ 18 O values of VSMOW2, SLAP2, GISP, and San Carlos olivine were determined relative to reference gas. In both laboratories, water and San Carlos olivine samples were prepared by BrF 5 fluorination. In both laboratories, the O 2 released from water and olivine samples was passed through the same purification system and measured using the same mass spectrometer relative to the same reference gas. RESULTS: In both laboratories, the δ 17 O 0.528 value (calculated from the VSMOW2-SLAP2 scaled δ values) of San Carlos olivine from GZG was À36 ± 9 ppm and, from ISEI, a value of À40 ± 7 ppm (1σ standard deviation) was determined. These values are~50 ppm higher than previously reported from the same laboratories. Possible causes for the observed discrepancies are discussed. CONCLUSIONS:The results of this study in comparison with previous data from the same laboratories demonstrated that for high accuracy determination of Δ'17 O values: (i) calibration of the reference gas relative to O 2 released from primary standards (VSMOW2, SLAP2) in the same laboratory is highly recommended, (ii) non-linearity of the mass spectrometer may not only affect δ 17O values, and (iii) the VSMOW2-SLAP2 scaling should also be applied to analyses of rocks and minerals. Studies that are concerned with small differences in Δ'

Molten I-type cosmic spherules formed by heating, oxidation and melting of extraterrestrial Fe,Ni metal alloys. The entire oxygen in these spherules sources from the atmosphere. Therefore, I-type cosmic spherules are suitable tracers for the isotopic composition of the upper atmosphere at altitudes between 80 and 115 km. Here we present data on I-type cosmic spherules collected in Antarctica. Their composition is compared with the composition of tropospheric O2. Our data suggest that the Earth's atmospheric O2 is isotopically homogenous up to the thermosphere. This makes fossil I-type micrometeorites ideal proxies for ancient atmospheric CO2 levels.

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