Oxygen ablation during atmospheric entry: Its influence on the isotopic composition of micrometeorites

Micrometeorites (MMs) offer glimpses of the diverse nature of parent bodies that accreted during the first few million years after the formation of the proto-Sun. The present work explores this by evaluating the ablation of oxygen from MMs during atmospheric entry, and the resulting effect on the oxygen isotopic composition. A Chemical ABlation MODel (CABMOD) combined with the measured oxygen isotope composition of MMs, shows that at temperatures below 2000 K a relatively small percentage (∼0%–5%) of oxygen ablates; the temperature is nevertheless sufficient to induce diffusion among the different silicate phases of MMs. The large δ 18O composition found within different MM types with low oxygen ablation indicates that exchange with atmospheric oxygen is insignificant during entry. Therefore, to explain the large δ 18O values existing in heated MMs, where oxygen ablation is less than a few percent, we propose that these particles are from distinct C-type asteroids that have undergone nebular gas exchange and/or aqueously altered in their parent bodies. This is supported by the evidence from unmelted MMs that have not exchanged oxygen during atmospheric entry or undergone ablation, but have large δ 18O values. However, the oxygen isotope composition of different types of cosmic spherules does not appear to vary systematically with temperature and could be due to the heterogeneity of their precursors. This investigation overall provides insights into the oxygen ablation of the particles during atmospheric entry, oxygen isotopic alteration, and the reservoirs of the diverse extraterrestrial objects that prevailed in the early solar system.