Abstract-We report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty-Nakhla-Chassigny (SNC) meteorites using enhanced laserassisted fluorination technique. This study shows that SNCs have remarkably identical D 17 O and a narrow range in d18 O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px-ol and mask-ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass-dependent fractionation line for bulk materials with a slope of 0.526 AE 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 AE 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 AE 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with D 17 O = 0.318 AE 0.016& (SD) for bulk materials and 0.316 AE 0.009& (SD) for separated fractions. The conformity in slopes and D 17 O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.

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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; N = 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.