The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach. (14th March 2018) - Ali, Arshad
Abstract: 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 δ 18 O value and, at the same time, improving the ∆ 17 O 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 δ 18 O 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 δ 18 O relative to the values published in the literature for the same samples. Critically, the ∆ 17 O 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‐∆ 17 O‐ and low‐∆ 17 O‐bearing subgroups that are statistically distinct. The subgroups present average ∆ 17 O values of −0.166 ± 0.003 (2SE; N = 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high‐∆ 17 O‐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.