Nitrogen isotopes in Ryugu return samples revealed by the stepwise combustion analysis,in comparison with CI falls and Antarctic finds
Ko Hashizume1, Akizumi Ishida2, Ayano Chiba1, Ryuji Okazaki3, Toru Yada4, Fumio Kitajima3,
Hisayoshi Yurimoto5, Tomoki Nakamura2, Hiroshi Naraoka3, Takaaki Noguchi6, Hikaru Yabuta7, Shogo Tachibana8,
Sei-ichiro Watanabe9, Kanako Sakamoto4 and Yuichi Tsuda4
1Faculty of Science, Ibaraki University, 2Department of Earth Science, Tohoku University, 3Department of Earth and Planetary Sciences,Kyushu University, 4Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 5Department of Earth andPlanetary Sciences, Hokkaido University, 6Division of Earth and Planetary Sciences, Kyoto University, 7Department of Earth and PlanetarySystems Science, Hiroshima University, 8UTokyo Organization for Planetary and Space Science, The University of Tokyo, 9Department ofEarth and Environmental Sciences, Nagoya University

Nitrogen isotope ratio is a powerful tool in cosmochemistry to decipher theorigin and evolution of planetary materials. The nitrogen isotope ratio 15N/14N,
usually expressed by the delta notation, δ15N ≡ [(15N/14N)/(15N/14N)AIR - 1] x1000 (‰), exhibits large variations among different meteorites (Fig. 1). The
isotope ratio of nitrogen, being a typical volatile element, is particularly useful
to constrain the formation and evolution processes of planetary atmosphere forEarth and other rocky planets. Viable Earth formation models assume thatCI/CM-like volatile-rich carbonaceous chondrites may have contributed as oneof the important volatile sources for Earth, if not entirely on the source for therocky body of this planet. CI chondrites, the most volatile-rich chondritic groupknown so far, exhibit δ15N values typically around +40 ‰. The higher δ15N
values for the CI chondrites than for the present Earth atmosphere may impose
important constraints on the formation and evolution history of the Earthatmosphere.
The texture and mineralogical composition of Ryugu samples are similar toCI chondrites. Three fall meteorites categorized as CI chondrites, Alais, Ivunaand Orgueil, have been studied so far for nitrogen isotopes. However, recent
search on the vast stock of Antarctic meteorites has revealed that there exist
several Antarctic meteorites which exhibit similar texture and mineralogicalcomposition with the fall CI chondrites. Though similar, these potential CI
Antarctic meteorites still exhibit marked difference from the CI falls, particularly
from the volatile-element points of view: The majority of phyllosilicates are
dehydrated in these Antarctic CI meteorites, while the phyllosilicates of CI falls
mostly remain hydrated [4]. Fig. 2 compares bulk nitrogen isotope compositionof Ryugu samples with those of CI chondrites, falls (Alais, Ivuna and Orgueil)
and an Antarctic find (Y-980115). Interestingly, the δ15N values are correlatedwith the nitrogen concentrations: lower the concentration, lower the δ15N value.
Note that this correlation is not compatible with the one we normally expect tooccur by Rayleigh-distillation type devolatilization process, nor by addition of aterrestrial contamination at different degrees. Apparently, an unknown isotope
fractionation (or mixing) effect in the nitrogen isotope composition has emerged
among the CI-chondrite family.
The stepwise analyses may provide isotope “fingerprints” to compare theidentities of planetary materials. By this method, we may distinguish possible
endmembers co-existing in the samples. Fig. 3 demonstrates the usefulness of
this method, for example, by the profiles obtained from a Ryugu bulk sample,compared with the one for CI chondrite Orgueil. These profiles provide
information, not only the bulk δ15N value and bulk N and C concentrations, but
include points such as, (a) presence of a nitrogen component easily combustible
at low (>200oC) temperatures with δ15N < +10 ‰, (b) a component released

Fig 1. Bulk nitrogen isotope composition amongdifferent meteorites. Compilation by Hashizume(2015) [1]. Important isotope data beyond thisrange include protosolar composition (-380 ‰;Marty et al, 2011 [2]) or cometary range (+600 +1000‰; Marty et al, 2016 [3])Fig 2. Bulk nitrogen isotope compositionof Ryugu samples and CI chondrites.
Ryugu data labelled A0105-07 and C010607 are reported in [5] (Ibaraki data);
A0105-05 and C0106-06 in [6] (Nancydata); A0106 in [7] (JAMSTEC data). The
other solid marks are literature bulk data
for CI chondrites from [8-13]. Open marks(Ivuna and Y-980115) are sums of
unpublished stepwise combustion datarecently acquired at Ibaraki Univ.

spike-wise at ~500oC with δ15N « 0 ‰, which is inferred to be hosted by presolar
nano-diamonds, (c) one released at 600-800oC exhibits a plateau δ15N value of+17‰, and (d) finally the one released at 1100oC and above with a δ15N value as
low as -20‰. Apparently, nitrogen in Ryugu consist of several components withdistinct origins. The combustion profile of Orgueil, though shows partially similar
isotope signature with Ryugu, for example the one presumably from presolar
microdiamonds, exhibits generally δ15N higher than Ryugu by several tens of ‰,and higher N concentrations. The differences are particularly prominent at low
temperature range below 600oC, which is generally considered to be the range
where N from the organic matter is released. Seemingly the organic matter with
δ15N values as high as +60‰ is missing in Ryugu, which could correspond to the“missing component” inferred by comparing the bulk composition for samples
belonging to the CI chondrite family (Fig. 2). It is worth to note that, though themissing component could be inferred to be an organic component from thecombustion temperature, it seems to be decoupled with the bulk of carbon, andcould be a component with a peculiar chemical composition with much higher N/Cratios than the bulk composition.
In this presentation, we will compare the stepwise combustion profiles
obtained for Ryugu, with those of Ivuna and Y-980115 recently obtained using thesame facility equipped at Ibaraki University. Though part of the Ryugu data
(A0105-07 and C0106-07) exhibit similar bulk nitrogen concentrations and δ15N
values with Y-980115, we will demonstrate that the combustion profiles appear
quite different, possibly suggesting that the N isotope evolution mechanism could
be different between these two kinds of CI-like samples.

Fig 3. Comparison of stepwise combustion
profiles for Ryugu sample C0106-07 (bttm)
obtained at Ibaraki Univ, and literature data
[11] for Orgueil (top). The concentrations
are plotted in arbitrary unit, although theareas of the bars between different
temperature ranges, and also between thetwo samples for the same element correctly
represent the relative concentrations.

References[1] Hashizume K. 2015. In “Encyclopedia of Astrobiology” doi:10.1007/978-3-642-27833-4_1065-3. [2] Marty B. et al.
Science 332:1533. [3] Marty B. et al. 2016. Earth and Planetary Science Letters 441:91. [4] King AJ. et al. 2019. Geochemistry79:125531. [5] Okazaki R. et al. 2022. Abstract #1348. 53rd LPSC.[6] Byrne DJ. et al. 2022. Abstract #2096. 53rd LPSC. [7]Naraoka H. et al. 2022. Abstract #1781. 53rd LPSC. [8] Kerridge JF. 1985. Geochimica et Cosmochimica Acta 49:1707. [9]Robert F. & Epstein S. 1982. Geochimica et Cosmochimica Acta 46:81. [10] Yang J. & Epstein S. 1983. Geochimica etCosmochimica Acta 47:2199. [11] Grady MM. et al. 2002. Meteoritics & Planetary Science 37:713. [12] Pearson VK. et al.
2006. Meteoritics & Planetary Science 41:1899. [13] Chan QHS. et al. 2016. Earth, Planets and Space 68:7.