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Identification of isotopically primitive
interplanetary dust particles: a NanoSIMS isotopic imaging
study
Floss C., Stadermann F. J., Bradley J. P., Dai Z. R.,
Bajt S., Graham G. and Lea A. S. (2006) Geochim.
Cosmochim. Acta, 70, 2371-2399.
doi:10.1016/j.gca.2006.01.023
ABSTRACT
We have carried out a comprehensive survey of the
isotopic compositions (H, B, C, N, O, S) of a suite of
interplanetary dust particles (IDPs), including both cluster
and individual particles. Isotopic imaging with the NanoSIMS
shows the presence of numerous discrete hotspots that are
strongly enriched in 15N, up to ~1300 permil. A
number of the IDPs also contain larger regions with more
modest enrichments in 15N, leading to average
bulk N isotopic compositions that are
15N-enriched in these IDPs. Although C isotopic
compositions are normal in most of the IDPs, two
15N-rich hotspots have correlated 13C
anomalies. CN-/C- ratios suggest that
most of the 15N-rich hotspots are associated with
relatively N-poor carbonaceous matter, although specific
carriers have not been determined. H isotopic distributions
are similar to those of N: D anomalies are present both as
distinct D-rich hotspots and as larger regions with more
modest enrichments. Nevertheless, H and N isotopic anomalies
are not directly correlated, consistent with results from
previous studies.
Oxygen isotopic imaging shows the presence of abundant
presolar silicate grains in some of the IDPs. The O isotopic
compositions of the grains are similar to those of presolar
oxide and silicate grains from primitive meteorites. Most of
the silicate grains in the IDPs have isotopic ratios
consistent with meteoritic Group 1 oxide grains, indicating
origins in oxygen-rich red giant and asymptotic giant branch
stars, but several presolar silicates exhibit the
17O and 18O enrichments of Group 4
oxide grains, whose origin is less well understood.
Based on their N isotopic compositions, the IDPs studied
here can be divided into two groups. One group is
characterized as being "isotopically primitive" and consists
of those IDPs that have anomalous bulk N isotopic
compositions. These particles typically also contain
numerous 15N-rich hotspots, occasional C isotopic
anomalies, and abundant presolar silicate grains. In
contrast, the other "isotopically normal" IDPs have normal
bulk N isotopic compositions and, although some contain
15N-rich hotspots, none exhibit C isotopic
anomalies and none contain presolar silicate or oxide
grains. Thus, isotopically interesting IDPs can be
identified and selected on the basis of their bulk N
isotopic compositions for further study. However, this
distinction does not appear to extend to H isotopic
compositions. Although both H and N anomalies are frequently
attributed to the survival of molecular cloud material in
IDPs and, thus, should be more common in IDPs with anomalous
bulk N compositions, D anomalies are as common in normal
IDPs as they are in those characterized as isotopically
primitive, based on their N isotopes.
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