Heterogeneity of Bulk Oxygen Isotopic Compositions in Anhydrous Interplanetary Dust Particles

Anhydrous interplanetary dust particles are among the least altered ancient solar system materials. While hydrous IDPs are 16O-poor, likely due to the interaction with isotopically heavy H2O [1], oxygen isotopes of anhydrous IDPs seem to be highly variable, although data is scarce [2]. Bulk oxygen isotopic compositions of 70 nm thin ultramicrotomed sections of three anhydrous IDPs were measured by NanoSIMS. δ17OSMOW ranges from −24.2 ± 5.4 ‰ (1σ) to 7.8 ± 3.4 ‰ and δ18OSMOW from −25.3 ± 3.6 ‰ to 9.5 ± 3.1 ‰, with the heavier isotopic compositions attributed to the particles that show signs of mild thermal alteration. Isotopic heterogeneity is best explained by contribution of grains sampling different O isotope reservoirs in the protoplanetary disk, created by self-shielding and photodissociation of CO [3]. This process resulted in 16O-rich CO gas and 16O-poor H2O in the outer disk. H2O froze as ice-mantles onto dust grains and evaporated from grains transported to the inner disk, enriching it in 17O and 18O [4]. Therefore, anhydrous IDPs contain a mixture of materials from different times and spaces in the protoplanetary disk, as has been described for material from comet 81P/Wild2 revealed by the STARDUST mission [5].

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