Microscale Diversity of H, C, and N Isotopes in Asteroid Ryugu
Larry R. Nittler1, Jens Barosch1, Bradley T. De Gregorio2, Rhonda M. Stroud2, Hikaru Yabuta3, and The Hayabusa2initial-analysis IOM team, Hisayoshi Yurimoto4, Tomoki Nakamura5, Takaaki Noguchi6, Ryuji Okazaki7, Hiroshi
Naraoka7, Kanako Sakamoto8, Shogo Tachibana8,9, Sei-ichiro Watanabe10 and Yuichi Tsuda8
1Carnegie Institution of Washington, Washington DC, USA, 2US Naval Research Laboratory, Washington DC, USA,
3Hiroshima Univ., 4Hokkaido Univ., 5Tohoku Univ., 6Kyoto Univ., 7Kyushu Univ., 8JAXA, 9Univ. of Tokyo, 10Nagoya
Univ.
Understanding the nature and origin of organic matter in asteroid 162173 Ryugu is one of the key science goalsof the Hayabusa2 sample-return mission. Macromolecular organic matter (MOM) makes up to a few % of primitive
meteorites and displays a wide range of H, C, and N isotopic ratios on whole-rock to sub-µm scales [1]. Both its origin(for example, whether in the protosolar molecular cloud or in the protoplanetary disk) and the degree to which it ismodified on asteroidal parent bodies is debated, but it may represent the dominant form of C delivered to early Earth.
NanoSIMS-based isotopic imaging surveys of both bulk CI, CR, CM, and CO chondrites and purified MOM residues
from them [2-6] have revealed that, while bulk samples are typically modestly enriched in D and 15N, relative to theEarth, a small fraction of the material shows much more extreme enrichments and/or depletions in these isotopes.
Large 13C enrichments and depletions are also seen in chondrites, both in presolar circumstellar SiC and graphite
grains [7], and in rare cases in MOM particles [8].
We have begun conducting similar NanoSIMS surveys of Ryugu samples. Four grains from Chamber Aaggregates (A0108) were embedded in S and sectioned with an ultramicrotome equipped with a diamond knife.
Relatively thick (250-nm) slices were deposited onto Si wafers for NanoSIMS analysis while adjacent thin sections
were kept for coordinated analysis by transmission electron microscopy (TEM) and synchrotron X-ray microscopy(STXM). After the S was sublimated, we applied a thin Au coat to the Si wafers and analyzed them with a NanoSIMS
50L ion microprobe in multicollection imaging mode. Microtome sections were first analyzed for C and N isotopes(as 12C2, 12C13C, 12C14N, and 12C15N, plus 16O, 28Si, MgO and secondary electrons) with a 0.4 pA, <200-nm Cs+ beam.
The sections were then re-analyzed at the same magnification with a 1-pA beam and collection of 1H, D, and 12C
secondary ions and secondary electrons. Five of the sections were subsequently re-measured for C and N to improvethe counting statistics. Total counting times varied but were typically of order ~0.1 s/0.01 µm2 for each run.
The measurements reveal broadly similar results to similar data obtained on primitive carbonaceous chondrites.
The four particles show modest bulk enrichments of D and 15N, with D/H and 15N/14N ratios in general agreement withthose of CI chondrites. The NanoSIMS images show that much of the C is present as particles typically a few hundred
nm in size, but ranging up to 2 µm. As in chondrites, a fraction of these particles show more extreme enrichments ordepletions (“hotspots” and “coldspots”) which span ranges similar to those recently seen in the least-altered CM
chondrites Asuka 12169, Asuka 12236, and Paris [3]. A much smaller fraction of C-rich grains shows 13C anomalies,
associated both with presolar grains and organic particles. The bulk and microscale isotopic distributions vary to someextent among the four Ryugu grains, indicating heterogeneity at the 100-µm scale. Planned TEM and STXM
measurements of slices from the same grains will help unravel the chemical nature of the organic particles and theirrelationship to inorganic phases in the asteroid.
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R. et al. 2018. Geochimica et Cosmochimica Acta 226:107. [7] Zinner E., 1.4 - Presolar grains, in: A.M. Davis(Ed.), Meteorites and Cosmochemical Processes (Vol. 1), Treatise on Geochemistry (Second Edition, eds: H. D.

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