Microstructural and microchemical characteristics of dolomite in Ryugu regolith particles
Falko Langenhorst1,2, Kilian Pollok1, Tomoki Nakamura3, Tomoyo Morita3, Kana Amano3, Daisuke Nakashima3
and the Hayabusa2 initial analysis core team1
Institute of Geoscience, Friedrich Schiller University Jena, Germany
2
Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, USA
3
Tohoku University, Sendai, Japan.
Introduction. Previous studies of Ryugu material have reported that the regolith particles record extensive aqueous alteration
as the result of water activity in the original parent body [1,2]. The aqueous alteration finally resulted in the almost complete
overprint of the primary mineralogical composition and the precipitation of secondary minerals, i.e. phyllosilicates, carbonates,phosphates, sulfides, and oxides. The isotopic 18O/16O and 13C/12C signatures of carbonates were recently studied in great detailto obtain information on the conditions of aqueous alteration [3]. Here, we focus on the characterization of the defectmicrostructures and nanoscale element distributions in dolomite of Ryugu samples to obtain further insights into the formation
history and mechanisms of carbonates.
Samples and methods. In the context of the Hayabusa2 initial analysis phase, we have mainly studied polished sections
prepared from larger particles recovered in the C chamber, i.e. C0033-01 and C0033-04, and C0055-01 [detailed in 1]. Elevenslices of carbonate-bearing regions were cut from the thin sections by focused ion beam (FIB) preparation on a scanningelectron microscope (SEM). Subsequent analytical transmission electron microscopy (TEM) were performed according to theprocedure described by [4].
Results and discussion. TEM observations reveal numerous curled dislocations as the most dominant lattice defect in almost
all dolomite grains. This shape of dislocation lines indicate that dislocations are not in glide configuration and thus may havebeen incorporated during imperfect growth from the aqueous fluid. In the vicinity of fractures, sample C0055-01 contains alsostraight glissile dislocations, which were previously assigned to mild shock [5]. Deformation twins, which are generally
regarded as typical for shock-metamorphosed carbonates, are however absent in C0055-01. An explanation for their absence
could be that deformation twinning in dolomite requires enhanced temperatures [6], which do not prevail during low shock
metamorphism [6].
Mapping of elemental distributions in scanning mode using energy-dispersive X-ray emission spectroscopy show complex andvariable zoning of dolomite grains. Many grains show sharp, non-concentric, up to 1 µm thin rims with a significant
enrichment in manganese (up to 8 wt% MnO). This observation points, at least, to a second precipitation event, when the fluid
became apparently enriched in manganese. A preferential mobilization of manganese might be explained by a (localized)change in the reduction potential. In addition, manganese-enriched dolomites with continuously decreasing manganeseconcentration from core to rim exist and monitor a gradual change in the fluid chemistry controlled by carbonate growth.
Most interesting is the observation of nanoscale modulations in the rims of dolomite. Such microstructures have also been
observed in sedimentary calcian dolomites and are interpreted as growth microstructures [7]. The modulations appear as
almost parallel lamellae with regular alternating light and dark contrast, resembling microstructures known for spinodal
decomposition. A periodical variation of the fluid composition at the mineral-solution interface during progressing growth is aplausible explanation for this microchemical peculiarity [8].
Acknowledgements. We thank JAXA for providing access to the Ryugu particles. FIB-TEM facilities at FSU Jena were
funded by the Gottfried-Wilhelm-Leibniz award to FL (LA 830/14-1).
References[1] Nakamura T. et al. 2023 Science 379:eabn8671. [2] Nakamura E. et al. 2022 Proc. Jpn. Acad. Ser. B:227-282. [3] Fujiya W.
et al. 2023 Nature Geoscience 16:675-682. [4] Langenhorst F., et al. 2014 Earth Planets and Space 66:118. [5] Kikuiri M. et al.
2022 85th Annual Meeting of The Meteoritical Society:No. 2695. [6] Barber D.J. and Wenk H.R. 1979 Physics Chemistry
Minerals 5:141-165. [7] Reeder R.J. 1992 In: P. Buseck (editor), Reviews in Mineralogy 27:381-424. [8] Ruiz-Agudo E. andPutnis C.V. 2012 Mineralogical Magazine 76:227-253.