Can SIMS measurements constrain the D/H ratio of water on Ryugu?
Larry R. Nittler1,2, Jens Barosch,1 and Rhonda M. Stroud3,2
The Hayabusa2-initial-analysis Organic Macromolecule and Core Teams
1Earth and Planets Laboratory, Carnegie Institution of Washington, USA.
2
School of Earth and Space Exploration, Arizona State University, USA
3
US Naval Research Laboratory, Washington DC, USA
We have previously reported the use of the Cameca NanoSIMS ion microprobe to characterize H, C, and N isotopes
on a sub-micrometer scale in particles returned by the Hayabusa2 spacecraft from C-asteroid Ryugu [1, 2]. These data
show that macromolecular organic-rich particles in Ryugu samples are remarkably isotopically heterogeneous on µm
scales with compositional distributions similar to those of carbonaceous chondrites (CCs), including CIs, the closest
meteorite analog to Ryugu [3]. Recently, Piani et al. [4] have used large-geometry SIMS (Cameca ims-1280) data toconclude that Ryugu water has δD/H = +74 ± 117 ‰. Since the NanoSIMS has much higher spatial resolution than
the ims-1280, we have re-examined our data to investigate whether they can provide additional information onRyugu’s water composition. Our results agree well with those of [4], but we argue below that it is not possible withcurrent ion probe data sets to infer the H2O D/H composition on Ryugu with any degree of confidence.
Prior work on CCs and Ryugu samples [4-6] has assumed that ~10-µm-scale SIMS H-isotope measurements
represent two-component mixtures between water of hydration (with C/H=0) and organic matter (with higher C/H),
based on observed correlations between measured C/H and D/H ratios. Extrapolation of fitted linear trends to the data
to C/H=0 then yields the inferred D/H of the intrinsic H2O. Shown as grey circles in Fig. 1d are the C/H ratios and δDvalues we have previously found in organic-rich particles, typically a few 100 nm in diameter, based on 20 fragments
of Ryugu particles A0108-13 (12 fragments) and C0109-2 (8 fragments) pressed into Au foils. The instrumental massfractionation and C/H calibration were corrected based on a terrestrial organic standard. The large scatter in these dataprecludes linear extrapolation. We thus chose to bin the imaging data by defining regions-of-interest (ROIs) in each
image whose C/H ratios spanned specific intervals (e.g., Figs. 1a-c). The C/H and D/H ratios are generally correlated
within an image (e.g., Fig. 1c) but the variable presence of anomalously D-rich or D-poor grains (hot/coldspots) often
leads to non-linear trends. Nevertheless, when the data are combined for all 20 particles, clear linear trends areobserved below C/H~1 for both the Chamber A and Chamber C grains (green and orange symbols in Fig. 1d-e).
Outliers are due to the presence of D hotspots or coldspots. The solid green and orange lines indicate linear fits to thedata below C/H=1 (excluding outliers). The fit lines intersect the Y-axis at δD=142 ‰ and 82 ‰, respectively, inexcellent agreement with the results of [4] (red error bar), acquired with a different instrument on different samples.
The linear trends seen in Fig. 1d,e support the idea that, when averaged over relatively large areas, the data represent
two-component mixing. At the high C/H end, the endmember is clearly the average of the ubiquitous C-rich particles
present throughout the Ryugu samples, i.e., the grey symbols in Fig. 1d. We note that insoluble organic matter (IOM)
extracted from CI chondrites [magenta star; 7] lies close to but slightly above the Ryugu trend lines. This supports
previous measurements indicating that Ryugu IOM has lower D/H than IOM from CIs [8] but could also reflect apossible mis-calibration of the C/H ratios from measured C/H secondary ion signals in the NanoSIMS. The
identification of the low C/H endmember is more ambiguous. Transmission electron microscopy and synchrotronbased transmission x-ray microscopy studies have shown that in Ryugu samples the phyllosilicates are intimately
mixed with a “diffuse” organic material [9-11]. This diffuse C (“dC”) is spectroscopically distinct from the larger
particulate organic grains, for example showing a CO3 feature (not associated with carbonate minerals) and often
showing a higher abundance of aliphatic compounds [11]. The intimate association of the dC with the clay minerals
means that even at the highest NanoSIMS spatial resolution, one cannot distinguish H coming from the dC and thatcoming from the water. Thus, the trend in fact represents 3-component mixing where the low C/H endmember is itself
a mixture of H2O and dC. Since the two forms of organics (dC and particles) are spectrally distinct, they also couldδD/H

have distinct C/H and D/H ratios on average. For example, the black lines on Fig 1e indicate mixing between waterwith δD= -200 or -500 ‰ (as inferred for CI chondrites by [12]) and supposed diffuse C components arbitrarily
assumed to have C/H~0.6 (based on assumed higher aliphatic content) and δD~2000 or 3000‰, respectively. Since
these lines intersect with the low-C/H end of the Ryugu trends, the data are fully consistent with Ryugu water having
a D/H much lower than inferred by [4], if the dC indeed has higher D/H and/or lower C/H on average than the larger
organic grains and the mixing ratio between dC and phyllosilicates is roughly constant on the scale of themeasurements. Without a way to independently determine the composition of the dC, we conclude that SIMS
measurements like those used here and by [4-6] cannot unambiguously resolve the D/H of water in Ryugu samples.
Moreover, dC has also been seen to be present in CI, CM, and CR chondrites [13], suggesting that previous SIMS
D/H measurements of these should be revisited. Finally, we note that under O- primary ion bombardment, thesecondary ion yield of H+ is higher from hydrated silicates than from organics [14]. High-resolution NanoSIMS
measurements of Ryugu
2000abcsamples with the Hyperion RF1500HCplasma O source, compared
1000
with Cs-based measurementsof the same materials, may
500
allow for better discrimination
C/H
0-0.180.86-1.00
0.18-0.34 1.0-1.2
of the water endmember from
0.34-0.52 1.2-1.4
C/H
5 µm
0.52-0.7 1.4-1.6-500
0.7-0.86
0.0
0.5
1.0
1.5
2.0 the dC and provide a better
estimate of the D/H of Ryugu
4000
deC-rich Particles
water.
Binned C/H A
3000

δD/H (‰)

Binned C/H CCI IOM

Diffuse
C?

2000
1000
00
0.0

0.5

1.0

1.5

C/H2.02.5-1000
3.0
0.0

0.2

0.4

0.6

0.8

C/H

1.0

1.2

1.4

Figure 1. a) Color-coded
NanoSIMS map of H and C in aRyugu particle b) ROIs definedby pixels within selected C/Hratio bins. c) δD versus C/H for
ROIs from panel b. d,e) δD versus
C/H for 20 analyzed Ryugu
fragments. Vertical red error bar
is y-intercept reported by [4].

References[1] Barosch J. et al. 2022. Abstract #2050. 53d LPSC. [2] Nittler L. R. et al. 2022. Goldschmidt Conference. [3]Yokoyama T et al. 2022. Science abn7850. [4] Piani L. et al. 2022. Abstract #6058. 85th MetSoc Meeting. [5] PianiL. et al. 2021. Earth & Planetary Science Letters 567: 117008. [6] Piani L. et al. 2018. Nature Astronomy 2:317-323.
[7] Alexander C. M. O’D. et al. 2007. Geochimica et Cosmochimica Acta 71: 4380–4403. [8] Remusat L. et al. 2022.
Abstract #6197. 85th MetSoc Meeting. [9] Yabuta H. et al. 2022. Abstract #2241. 53d LPSC. [10] De Gregorio B.
T., et al. 2022. Abstract #1634. 53rd LPSC. [11] Stroud R. M., et al. 2022. Abstract #2052. 53d LPSC. [12] Alexander
C. M. O’D. et al. 2012. Science 337: 721–723. [13] Le Guillou C. et al. 2014. Geochimica et Cosmochimica Acta
131: 368–392. [14] Deloule E. and Robert F. 1995. Geochimica et Cosmochimica Acta 59: 4695–4706.
The Hayabusa2-initial-analysis Organic Macromolecule and Core Teams: H. Yabuta, G. D. Cody, C. Engrand,
Y. Kebukawa, B. De Gregorio, L. Bonal, L. Remusat, R. Stroud, E. Quirico, L. R. Nittler, M. Hashiguchi, M. Komatsu,
E. Dartois, J. Mathurin, J. Duprat, T. Okumura, Y. Takahashi, Y. Takeichi, D. Kilcoyne, S. Yamashita, A. Dazzi, A.
Deniset-Besseau, S. Sandford, Z. Martins, Y. Tamenori, T. Ohigashi, H. Suga, D. Wakabayashi, M. Verdier-Paoletti,
S. Mostefaoui, G. Montagnac, J. Barosch, K. Kamide, M. Shigenaka, L. Bejach, T. Noguchi, H. Yurimoto, T.
Nakamura, R. Okazaki, H. Naraoka, K. Sakamoto, S. Tachibana, S. Watanabe, and Y. Tsuda