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

Citation

Rudraswami, N. G. (2020), The oxygen isotope compositions of large numbers of small cosmic spherules: Implications for their sources and the isotopic composition of the upper atmosphere, Dryad, Dataset, https://doi.org/10.5061/dryad.zw3r2285s

Abstract

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 δ17,18O 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.

Methods

Electron probe micro analyzer (EPMA, Cameca SX5 at National Institute of Oceanography, Goa) was used to acquire the major and minor elemental composition. Most cosmic spherules are in the size range of less than a few hundred µm thus the instrument can perform a detailed study of specific areas in particles. Chemical analyses were performed using electron microprobe on selected cosmic spherules phases with accelerating voltage ~15 kV, ~ beam current ~12 nA, ~1–2 µm beam diameter for spot analyses, and ~5 µm beam diameter for bulk analyses.

Funding

Ministry of Earth Sciences