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<porphyritic<barred<glass<CAT spherules (~20, 22, 25, 26 and 50‰)  that is consistent with the evolution of oxygen isotopes by mass fractionation owing to increased average entry heating. The trend of δ^17,18O is broadly parallel to the terrestrial fractionation line and thus suggests mass fractionation dominates changes in isotopic composition, with atmospheric exchange a less significant effect. The D¹⁷O values of spherules, therefore, are mostly preserved and suggest that ~80% of particles are related to the carbonaceous chondrites (CC) and are probably samples of C-type asteroids. The genetic relationships between different S-types can also be determined with scoriaceous, barred and cryptocrystalline spherules mostly having low D¹⁷O values (≤0‰) suggesting they are mainly derived from CC-like sources, whilst porphyritic mostly have positive D¹⁷O (>0‰) suggesting they are largely from ordinary chondrite (OC)-like sources related to S(IV)-type asteroids. Glassy and CAT-spherules have D¹⁷O values indicating they formed by intense entry heating of both CC and OC-like materials. I-type cosmic spherules have a narrow range of δ¹⁷O (~20–25‰) and δ¹⁸O (~38–48‰) values, with D¹⁷O (~0‰) suggesting their oxygen is obtained entirely from the Earth's atmosphere, albeit with significant mass fractionation owing to evaporation during entry heating. The observed range of δ¹⁸O with the size is suggested here to reflect entry angle with high values representing enhanced heating at high angle. Finally, G-type cosmic spherules have unexpected isotopic compositions suggesting little mass-fractionation from a CC-like source and are suggested to have sulphide-silicate precursors with relatively low melting temperatures. The results of this study provide a vital assessment of the wider population of extraterrestrial dust arriving at the Earth.