DOI: 10.5061/dryad.zw3r2285s
title: The oxygen isotope compositions of large numbers of small cosmic spherules: Implications for their sources and the isotopic composition of the upper atmosphere

description: 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&lt;porphyritic&lt;barred&lt;glass&lt;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 (&gt;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.
description: 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.