Oxygen isotope variation in primitive achondrites: The influence of primordial, asteroidal, and terrestrial processess
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A detailed oxygen isotope study of the acapulcoites, lodranites, winonaites, brachinites and various related achondrites has been undertaken to investigate the nature of their precursor materials. High levels of terrestrial alteration displayed by many of these samples have been mitigated by leaching in ethanolamine thioglycollate (EATG) solution. Due to their high metal and sulphide content, acapulcoite, lodranite and winonaite samples show much greater isotopic shifts during weathering than brachinites. As observed in previous studies, Antarctic weathered finds are displaced to lighter oxygen isotope compositions and non-Antarctic finds to heavier values.Leached primitive achondrite residues continue to show high levels of oxygen isotope heterogeneity. This variation is reflected in the 2σ error on group mean Δ17O values, which decrease in the following order: acapulcoite–lodranite clan > brachinites > winonaites. On an oxygen three-isotope diagram, the acapulcoite––lodranite clan define a limited trend with a slope of 0.61 ± 0.08 and an intercept of −1.43 ± 0.27 (R2 = 0.78). A broad positive correlation between Δ17O and olivine fayalite contents displayed by both acapulcoite and lodranite samples may be the result of early aqueous alteration and subsequent dehydration. Winonaites experienced a greater degree of differentiation than the acapulcoite–lodranite clan and define a distinct mass fractionation line, with a slope of 0.53 ± 0.01 and an intercept of −0.53 ± 0.04 (R2 = 1). A number of samples currently classified as acapulcoites (NWA 725, NWA 1052 and Dho 1222) have oxygen isotope compositions indicating that they are winonaites. The relatively high level of oxygen isotope heterogeneity displayed by the brachinites supports their designation as primitive achondrites. A number of ungrouped olivine-rich achondrites (Divnoe, NWA 4042, NWA 4363, NWA 4518, NWA 5400, Zag (b)) as well as the unique plagioclase-rich achondrites GRA 06128 and GRA 06129 have similar oxygen isotope compositions to the brachinites. It remains unclear whether the brachinites and related olivine-rich achondrites are from a single or multiple parent bodies. The primitive achondrites and related samples represent material from at most only 18 parent bodies, compared to an estimated 65 for the iron meteorites. This suggests that asteroidal mantle material is underrepresented in the meteorite record. Early fragmentation of differentiated asteroids, followed by preferential destruction of their silicate-rich mantles, offers a possible explanation for this discrepancy.On an oxygen three-isotope diagram, primitive chondrule-bearing winonaites (Dho 1222, NWA 725, NWA 1052, NWA 1463, Mt. Morris (Wisconsin)) plot close to the Young & Russell (Y&R) slope 1 line, with more evolved samples extending away from it towards the CCAM line. A similar relationship is shown by the CR chondrites. The acapulcoite–lodranite clan plots between the slope 1 and CCAM lines. However, the precursor material to the clan may have had a composition close to the slope 1 line prior to parent body processing. These relationships support the view that primordial oxygen isotope variation in the early solar system is best represented by the slope 1 (Y&R) line.
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