10.5061/DRYAD.ZW3R2285S
Rudraswami, N. G.
0000-0002-3375-9860
National Institute of Oceanography
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
2020
cosmic spherule
Ministry of Earth Sciences
https://ror.org/013cf5k59
2020-07-16T00:00:00Z
2020-07-16T00:00:00Z
en
606277 bytes
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CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
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.
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.