The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach

Arshad ALI1,2, Iffat JABEEN2, Neil R. BANERJEE2, Gordon R. OSINSKI2,3, Ian NICKLIN4, David GREGORY5, and Patrick HERRMANN6
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13072]
1Earth Sciences Research Centre (ESRC), Sultan Qaboos University, Al-Khoudh, Muscat 123, Sultanate of Oman
2Department of Earth Sciences/Centre for Planetary Science and Exploration (CPSX), University of Western Ontario, London,Ontario, Canada
3Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
4Department of Natural History, Royal Ontario Museum, Toronto, Ontario N5R 4P5, Canada
5230 First Ave., Suite 108, St. Thomas, Ontario, Canada
6Pallasite.ca, Toronto, Ontario, Canada
Published by arrangement with John Wiley & Sons

Oxygen isotope measurements of olivine in main group (MG) pallasites by traditional laser fluorination method are associated with some uncertainties including terrestrial weathering, incomplete olivine reaction, and sample state. We improved our laser fluorination approach by pretreating olivine grains with acid to remove terrestrial weathering products and by modifying the sample holder for an efficient and complete laser reaction. Our experiments on Brahin olivine demonstrate that acid‐washing successfully removes the terrestrial weathering with <0.1‰ variation in δ18O value and, at the same time, improving the ∆17O value significantly. We also achieved a complete olivine fluorination by employing a custom‐designed sample holder with “V”‐shaped profile having rounded bottom because incomplete/partial reaction of olivine gives comparatively lighter δ18O values. Using these new techniques, we present precise triple oxygen isotope data (N = 72) of 25 olivine samples separated from main group pallasites. The data are, on average, ~0.5‰ heavier in δ18O relative to the values published in the literature for the same samples. Critically, the ∆17O values of MG pallasites and to some extent their Fo‐contents suggest that there are at least two populations of olivine. Based on our improved data set, we propose that MG pallasites potentially have high‐∆17O‐ and low‐∆17O‐bearing subgroups that are statistically distinct. The subgroups present average ∆17O values of −0.166 ± 0.003 (2SE;= 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high‐∆17O‐bearing subgroup samples trend toward lower Fo‐contents compared to the other subgroup. Taken together, our data provide evidence that argues against a single parent body origin for MG pallasites.

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