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

1N. G. Rudraswami,2Matthew J. Genge,3Yves Marrocchi,3Johan Villeneuve,4S. Taylor
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006414]
1National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa, India
2Department of Earth Science and Engineering, Imperial College London, London, UK
3CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre‐les‐Nancy, France
4Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, USA
Published by arrangement with John Wiley & Sons

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 amount of 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. Our results indicate a possible correlation between an increasing average δ18O compositions of silicate dominated (S‐type) spherules along the series scoriaceous<porphyritic<barred<cryptocrystalline<glass17O values of spherules, therefore, are mostly preserved and suggest that ~80% of particles are 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 ∆17O values (≤0‰) mainly derived from CC‐like sources, whilst porphyritic spherules mostly have positive ∆17O (>0‰) are largely derived from ordinary chondrite (OC)‐like sources related to S (IV)‐type asteroids. Glass and CAT‐spherules have variable ∆17O 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 ∆17O (~0‰) suggesting their oxygen is obtained entirely from the Earth’s atmosphere, albeit with significant mass fractionation owing to evaporative heating. Finally, G‐type cosmic spherules have unexpected isotopic compositions demostrate little mass‐fractionation from a CC‐like source. The results of this study provide a vital assessment of the wider population of extraterrestrial dust arriving at the Earth.

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