The oxygen isotope compositions of large numbers of small cosmic spherules: Implications for their sources and the isotopic composition of the upper atmosphere

Cosmic spherules are micrometeorites that melt at high altitude as they enter Earth’s atmosphere and their oxygen isotope compositions are partially or completely inherited from the upper atmosphere, depending on the heating experienced and the nature of their precursor materials. In this study, the three oxygen isotope compositions of 137 cosmic spherules are determined using 277 in-situ analyses by ion probe. Particles of each different type of cosmic spherule (scoriaceous, porphyritic, cryptocrystalline, barred, glass, calcium aluminium and titanium (CAT), G-type and I-type) in the diameter range ~52–480mm were analysed. The results confirm that the three oxygen isotope compositions of melted micrometeorites reflect a combination of their precursor composition, exchange with the atmosphere and mass fractionation owing to evaporation during entry heating. The data appear to reveal an increase in average δ18O values of silicate dominated (S-type) spherules in the series scoriaceous<...