# Low temperature brine formation by serpentinization on asteroid (162173) Ryugu

Guy Libourel, Marc Portail, Vincent Guigoz, Virginie Brändli, Cyrille Collin, Nathalie Vigier, Tomoki Nakamura, Hikaru Yabuta, Shogo Tachibana, Cecile Engrand

► To cite this version:

Guy Libourel, Marc Portail, Vincent Guigoz, Virginie Brändli, Cyrille Collin, et al.. Low temperature brine formation by serpentinization on asteroid (162173) Ryugu. Earth and Planetary Science Letters, 2026, 679, pp.119885. 10.1016/j.epsl.2026.119885. hal-05520467

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Low temperature brine formation by serpentinization on asteroid
(162173) Ryugu

Guy Libourel¹, Marc Portail², Vincent Guigoz², Virginie Brändli², Cyrille Collin³, Nathalie Vigier⁴,
Tomoki Nakamura⁵, Hikaru Yabuta⁶, Shogo Tachibana⁷, Cecile Engrand⁸.

¹Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France.

²Université Côte d'Azur, CNRS, CRHEA, Valbonne, France.

³PSL Research University, MINES Paris-PSL, CEMEF, Valbonne, France.

⁴CNRS, Sorbonne Université, LOV, IMEV, Villefranche sur Mer, France.

⁵Department of Earth Science, Tohoku University, Sendai, Japan.

⁶Earth and Planetary Systems Science Program, Hiroshima University, Higashi-Hiroshima, Japan.

⁷UTokyo Organization for Planetary and Space Science, University of Tokyo, Tokyo, Japan.

⁸IJCLab, UMR 9012, Université Paris-Saclay, CNRS, Orsay, France.

Corresponding author: Guy Libourel, libou@oca.eu

To be submitted to *Earth and Planetary Science Letters*

Abstract: 190 words

Text: 6193 words

References: 69

Figure: 7 items

+ Supplementary informations
43 Abstract

Secondary mineral prevalence in Ryugu samples, similar to primitive carbonaceous-Ivuna type (CI) chondrites, suggests that aqueous alteration was a key factor in its formation. However, this general consensus masks our limited understanding of the specific mechanisms and environmental conditions involved in water-rock interactions on primitive asteroids. High-resolution cathodoluminescence (CL) analysis of the ubiquitous dolomite crystals in Ryugu samples reveals concentric epitaxial overgrowths with varying levels of Mn²⁺-activated luminescence. CL panchromatic images and spectral deconvolution provide compelling evidence for the evolution of aqueous fluids toward highly saturated brines. Given the close association of dolomite with widespread intergrowths of serpentine and saponite in the matrix, we propose that brine formation occurs as a byproduct of serpentinization. Unlike large-scale evaporation or freezing, this process can locally cause the hydrothermal fluid to dry out, significantly increasing its salinity over time. This leads to the sporadic precipitation of an evaporite mineral sequence, with dolomite forming at an early stage. This serpentinization-driven brine formation model offers a convincing alternative to a purely prograde alteration history for Ryugu. It may also provide a better explanation for the alteration processes of Bennu and other CI chondrite parent bodies.

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64 1. Introduction

The Hayabusa2 mission, conducted by the Japan Aerospace Exploration Agency, successfully retrieved samples from the near-Earth asteroid 162173 Ryugu. Spectroscopic analysis classified Ryugu as a C-complex asteroid, a rubble-pile formed from fragments of a larger parent body. Returned sample analyses revealed that most Ryugu samples are breccias consisting of small clasts with different degrees of aqueous alteration (Nakamura et al., 2022, 2023). This alteration resulted in the formation of secondary minerals, such as pyrrhotite, magnetite, dolomite, magnesite, and apatite in a matrix composed mainly of phyllosilicates (saponite and serpentine). Primary minerals like anhydrous silicates are relatively rare, though small mineral fragments of olivine and low-Ca pyroxene were identified, as well as tiny polymineralic fragments, similar to refractory inclusions and chondrules in carbonaceous chondrites (Liu et al., 2022). Overall, the mineralogy and petrology of Ryugu bears resemblance to CI (Ivuna-type) chondritic meteorites as do both whole-rock chemical and isotopic compositions (Yokoyama et al., 2022; Nakamura et al., 2022; Ito et al., 2022; Nakamura et al., 2023; Noguchi et al., 2023a; Tsuchiyama et al., 2024).

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Returned sample analyses revealed that most Ryugu samples are breccias consisting of small clasts with different degrees of aqueous alteration (Nakamura et al., 2022, 2023). This alteration resulted in the formation of secondary minerals, such as pyrrhotite, magnetite, dolomite, magnesite, and apatite in a matrix composed mainly of phyllosilicates (saponite and serpentine).

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Primary minerals like anhydrous silicates are relatively rare, though small mineral fragments of olivine and low-Ca pyroxene were identified, as well as tiny polymineralic fragments, similar to refractory inclusions and chondrules in carbonaceous chondrites (Liu et al., 2022). Overall, the mineralogy and petrology of Ryugu bears resemblance to CI (Ivuna-type) chondritic meteorites as do both whole-rock chemical and isotopic compositions (Yokoyama et al., 2022; Nakamura et al., 2022; Ito et al., 2022; Nakamura et al., 2023; Noguchi et al., 2023a; Tsuchiyama et al., 2024).

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Among these secondary minerals, ubiquitous carbonates are particularly noteworthy, as they provide insights into the hydrothermal conditions prevalent within Ryugu's parent body and offer a broader comparison to carbonaceous chondrite parent bodies. Recent studies focused on their mineralogical, chemical and isotopic composition of carbonates (Nakamura et al., 2022; Nagashima et al. 2022; Yokoyama et al., 2023; Fujiya et al., 2023; Kita et al., 2024). It has been shown that carbonate minerals in Ryugu, dominated by dolomite and iron-bearing magnesite (hereafter magnesite) with rarer occurrence of calcite formed under different alteration conditions. Calcite forming first and dolomite forming later. Sodium carbonates have also been found (Matsumoto et al., 2024), together with chlorides and sulfates which indicate that alkaline, salt-rich brine once flowed through Ryugu parent body. Analysis of oxygen isotope ratios indicates that the aqueous fluids in Ryugu's parent body were heterogeneous, spatially,

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provide insights into the hydrothermal conditions prevalent within Ryugu's parent body and offer a broader comparison to carbonaceous chondrite parent bodies. Recent studies focused on their mineralogical, chemical and isotopic composition of carbonates (Nakamura et al., 2022; Nagashima et al. 2022; Yokoyama et al., 2023; Fujiya et al., 2023; Kita et al., 2024). It has been shown that carbonate minerals in Ryugu, dominated by dolomite and iron-bearing magnesite (hereafter magnesite) with rarer occurrence of calcite formed under different alteration conditions. Calcite forming first and dolomite forming later. Sodium carbonates have also been found (Matsumoto et al., 2024), together with chlorides and sulfates which indicate that alkaline, salt-rich brine once flowed through Ryugu parent body. Analysis of oxygen isotope ratios indicates that the aqueous fluids in Ryugu's parent body were heterogeneous, spatially,

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offer a broader comparison to carbonaceous chondrite parent bodies. Recent studies focused on their mineralogical, chemical and isotopic composition of carbonates (Nakamura et al., 2022; Nagashima et al. 2022; Yokoyama et al., 2023; Fujiya et al., 2023; Kita et al., 2024). It has been shown that carbonate minerals in Ryugu, dominated by dolomite and iron-bearing magnesite (hereafter magnesite) with rarer occurrence of calcite formed under different alteration conditions. Calcite forming first and dolomite forming later. Sodium carbonates have also been found (Matsumoto et al., 2024), together with chlorides and sulfates which indicate that alkaline, salt-rich brine once flowed through Ryugu parent body. Analysis of oxygen isotope ratios indicates that the aqueous fluids in Ryugu's parent body were heterogeneous, spatially,

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Nagashima et al. 2022; Yokoyama et al., 2023; Fujiya et al., 2023; Kita et al., 2024). It has been shown that carbonate minerals in Ryugu, dominated by dolomite and iron-bearing magnesite (hereafter magnesite) with rarer occurrence of calcite formed under different alteration conditions. Calcite forming first and dolomite forming later. Sodium carbonates have also been found (Matsumoto et al., 2024), together with chlorides and sulfates which indicate that alkaline, salt-rich brine once flowed through Ryugu parent body. Analysis of oxygen isotope ratios indicates that the aqueous fluids in Ryugu's parent body were heterogeneous, spatially,

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shown that carbonate minerals in Ryugu, dominated by dolomite and iron-bearing magnesite (hereafter magnesite) with rarer occurrence of calcite formed under different alteration conditions. Calcite forming first and dolomite forming later. Sodium carbonates have also been found (Matsumoto et al., 2024), together with chlorides and sulfates which indicate that alkaline, salt-rich brine once flowed through Ryugu parent body. Analysis of oxygen isotope ratios indicates that the aqueous fluids in Ryugu's parent body were heterogeneous, spatially,

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found (Matsumoto et al., 2024), together with chlorides and sulfates which indicate that alkaline, salt-rich brine once flowed through Ryugu parent body. Analysis of oxygen isotope ratios indicates that the aqueous fluids in Ryugu's parent body were heterogeneous, spatially,

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and/or temporally. Aqueous alteration is inferred to occur below ≤ 100 °C (Nakamura et al., 2022) with dolomite-magnetite equilibrium temperature in the range of 37 ± 10°C to 92 ± 21°C (Nagashima et al., 2022; Yokoyama et al., 2023; Fujiya et al., 2023; Kita et al., 2024). δ¹⁸O of carbonates decreased in the order calcite, dolomite, and magnesite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration, along a prograde evolution.

Evaporation or freezing are the leading mechanisms proposed to explain the precipitation of salts on the parent bodies of asteroids Ryugu (Matsumoto et al., 2024; Tsuchiyama et al., 2024) and Bennu more recently (McCoy et al., 2025, Zega et al., 2025). These processes are crucial because they concentrate dissolved ions in brine until the solution becomes supersaturated, forcing a salt sequence to crystallize. By focusing on dolomite—the most prevalent carbonate among the secondary phases in Ryugu samples—we show that high resolution cathodoluminescence (CL) helps to shed a new light on hydrothermal activity in Ryugu. As an alternative to the evaporation or freezing of water-rich fluids, we propose that brine and salt formation occurs as a byproduct of serpentinization. Implications on Bennu and other carbonaceous chondrite parent bodies are then discussed.

## 2. Materials and methods

### 2.1 Samples

We prepared the Ryugu samples studied here in polished section during the initial analyses period, at the end of which they were sent back for storage at the Ryugu curation facility at ISAS. The polished sections were further obtained through the Ryugu announcement of opportunity 4 (AO4). We analyzed 18 polished particles from Ryugu's A and C sampling sites (see supplementary materials): A0064-FO013-17 (2), C0002-FC016 (6), C0040-FC025 (4), C0046-FO004-010 (6), whose mineralogy predominantly composed of inter-layered smectite- and serpentine-group minerals. Detailed mineralogy and modal proportions of particles have been obtained by processing EDX spectroscopy chemical maps with the XMapTools software (https://xmaptools.ch/). Dolomite is present in almost all particles, magnesite and calcite are rarer in the studied set of samples.

### 2.2 X-ray spectroscopy chemical maps

The general overview of Ryugu particles with coupled backscattered electron (BSE) images and energy-dispersive X-ray (EDX) spectroscopy chemical maps was acquired during initial analyses at ISMO with a SEM/FIB Zeiss 1540 equipped with an ultra-high resolution Gemini column, and a QUANTAX EDX system. Measurements were done at 15kV. During the CL session, Images and EDX maps were acquired at CEMEF Mines Paris-PSL-Nice (France) with a MEB Zeiss Supra 40. Measurements were operated at 15KeV and 2300pA beam current with a BRUKER Quantax XFlash 5030 detector (30mm²) at 60Kcps. BRUKER Microanalysor QUANTAX was associated with the software ESPRIT (semi-quantitative analyses without standard by P/B-ZAF method).

### 2.3 Electron microprobe analysis

Electron microprobe (EPMA) data on some carbonates were acquired at the CAMPARIS facility at Jussieu (Sorbonne University, Paris), at 15 kV and 10 nA before the CL study. Counting times varied from 10s (major elements) to 20s (minor elements). Some beam damage
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## 2.4 High-resolution cathodoluminescence

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The high-resolution cathodoluminescence facility consists of a MonoCL4 GATAN system equipped with both a high-sensitivity array detector (UV enhanced Pixis100 camera) and a high-sensitivity photomultiplier mounted on a Field Emission Gun Scanning Electron Microscope (FEG-SEM; JEOL JSM7000F at CRHEA, Valbonne, France) and suitable both for hyperspectral analyses and panchromatic imaging. The electron beam current used in the study ranged typically from 1 to 4 nA. Panchromatic images were acquired at 3 keV, for a spatial resolution at the nanometer scale, with a scanning rate of 700 µs/pixel. Filtered panchromatic images were made using bandpass filter from Thorlabs centered on 400 ±20 nm and 620 ±10 nm. For spectral and hyperspectral analysis, the luminescence coming from the excited sample is directed towards a Czerny-Turner type 300 mm focal length monochromator equipped with a 150 grooves/mm grating blazed at 500 nm; the entrance slit has been fixed to 300 µm, which gives a spectral resolution better than 3 nm. Dispersed light is collected as a whole using the CCD detector having a sensitivity range from 250 to 1050 nm. Excited photons under a 10 keV beam were integrated for 20 sec. All measurements and observations were done at room temperature (See supplementary information).

# 3. Results

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## 3.1 Mineralogy and petrology

We analyzed 18 polished particles from Ryugu's A and C chambers, corresponding to the samples collected from two different collecting sites. Their mineralogy, predominantly composed of phyllosilicate of the inter-layered smectite-group and serpentine-group minerals, has been already described (e.g., Nakamura et al., 2022, 2023; Tachibana et al., 2022; Tsuchiyama et al., 2024). Dolomite is present in almost all particles, but its modal abundance exhibits considerable variability, ranging from trace amounts to as high as 20% (Fig. 1). Calcite is rare (See Supplementary information Fig. S1). Our samples consist of small Ryugu fragments in polished section (typically < 200µm), in which dolomite crystals exhibit a wide range of sizes, with some reaching 20-30µm. They are dispersed throughout the phyllosilicate matrix, either as individual crystals or as small clusters associated with magnetite with or without Fe-sulfide (Nakamura et al. 2022). Dolomite crystals display subhedral to euhedral morphology, with well-defined crystal faces. In some cases, intergrowth between dolomites and phyllosilicates is noticeable. Dolomites are close to their ideal 1:1 Ca:Mg stoichiometry (e.g., Ca/Mg =1.02 ±0.04, n=49, Fujiya et al., 2023; Ca/Mg =1.07 ±0.06, n=18, Kita et al., 2024; Ca/Mg =1.08 ±0.09, n=18, Supplementary information Table S1) with a slight CaCO₃ endmember enrichment. Mn and Fe contents, at the percent level, exhibit enrichment towards the dolomite edge (Nakato et al., 2022; Fujiya et al., 2023). Sulfide, magnetite and Ca-phosphate are frequently found in inclusions in dolomite (Fig. 1a). By comparison, magnesite may host up to 15-20 mol% of siderite component (FeCO₃) and about 10 mol% of rhodochrosite component (MnCO₃). Calcite has almost stoichiometric composition with only trace amounts of Mn (Nakato et al., 2022; Fujiya et al., 2023; see Supplementary information Fig. S3). This suggests that the manganese content in carbonates is as follows: MnMagnesite ≥ MnDolomite > MnCalcite.
## 3.2 Cathodoluminescence

Panchromatic cathodoluminescence images of dolomites reveal intricate internal zoning at the micron scale (Fig. 2 and 3; Supplementary information Fig. S1-S2), which is often imperceptible with traditional back-scattered electron scanning electron microscopy (BSE-SEM) techniques (Libourel and Portail, 2018; Guigoz et al., 2024). The luminescence intensity varies, generally increasing from the core to the edge of the dolomite grains. However, most grains exhibit oscillatory zoning, characterized by alternating bands of high and low CL intensity. Low CL intensity cores, which display a predominantly prismatic to rhombohedral morphology, are overlayed by several high CL intensity fibrous orthogonal overgrowths, giving each crystal distinctive crenel-shaped edges. This alternating pattern suggests that the dolomite formed through multiple concentric epitaxial overgrowths of fibrous crystals around regularly-shaped equant layers (Fig. 2). Notice that in contrast to the CL panchromatic images of dolomites, the calcite CL images are more homogeneous (See Supplementary information Fig. S4); the luminescence of magnesites being null due to their higher iron content.

Dolomites from the Ryugu samples exhibit two primary spectral luminescence centers (Fig. 3 and Supplementary information Fig. S3; see Methods): a violet-blue emission band (300-425 nm) and a more intense orange-red emission band (550-750 nm). These luminescence centers are commonly attributed to intrinsic structural defects within the dolomite lattice and to extrinsic centers resulting from Mn ion substitutions, respectively (Sommer 1972a, 1972b; Machel, 2000 and Supplementary information Fig. S3). Notably, the peak of the Mn CL emission band differentiates between dolomite (655 nm), calcite (620 nm), and aragonite (550 nm). In contrast, iron-rich magnesite is non-luminescent due to the presence of iron, while magnesite exhibits a Mn CL emission peak at 655 nm (Sommer 1972a, 1972b). Point analyses of individual dolomite grains within Ryugu samples revealed a systematic broadening of the emission band from the core to the edge, accompanied by the emergence of a shoulder at shorter wavelengths (500-600 nm) (Fig. 3). Hyperspectral analyses across multiple dolomite grains confirmed this trend, demonstrating variations in the Mn CL 655 nm emission band that correlate with the oscillatory zoning observed in panchromatic images. Importantly, CL spectra of calcite exhibit an extremely weak emission band around 620 nm, consistent with their very low Mn concentrations (See Supplementary information Fig. S4, Fujiya et al., 2024).

# 4. Discussion

## 4.1 Brine-driven dolomite formation

The rhombohedral crystal lattice of an ideally ordered and stoichiometric dolomite consists of alternating Ca²⁺ and Mg²⁺ cation layers separated by CO₃²⁻ carbonate anion layers. Although both Ca and Mg are divalent, they differ so much in size (Ca²⁺- 1.0 Å vs Mg²⁺- 0.72 Å) that there is very limited substitution of one by the other. The majority of terrestrial dolomites contain iron impurities and trace amounts of Mn (up to several hundreds of ppm) as isomorphous replacement in the Mg sites according to Goldschmidt rule of cations substitution (Mn²⁺-0.67Å). Using CL and X-ray induced luminescence in the visible range of the spectrum for Mn²⁺ activated carbonate minerals, Sommer (1972a) indicated that dolomite shows an emission behavior that is "mixed" between those of calcite and magnesite, with Mn²⁺ populating both Ca and Mg octahedral sites. El Ali et al. (1993) confirmed this finding and shows that CL
236 in dolomite is characterized by two overlapping Gaussian peaks at 578 nm and 655 nm, 237 corresponding to Mn²⁺ substitution for Ca²⁺ and Mg²⁺, respectively.

238 To quantify the Mn distribution between Mg and Ca sites, dolomite spectra were deconvoluted 239 using a least-squares fitting algorithm that minimized the difference between the experimental 240 data and the sum of five Gaussian curves. When converted to energy, the two emission peaks 241 at 1.90 eV and 2.18 eV enable us to constrain the Mn²⁺ ion partitioning between the two 242 crystallographic sites as follows: 243

244 $$K_D = \frac{Mn_{Mg \text{ site}}^{2+}}{Mn_{Ca \text{ site}}^{2+}}$$

245 where Mn²⁺ in the Mg site (E₁) and Mn²⁺ in the Ca site (E₂) stand for the area of the 246 corresponding emissions in the CL spectra (Fig. 3 and Supplementary information Fig. S5). 247

248 Numerous studies (Sommer 1972a, b; Lumsden and Lloyd 1984; Walker et al., 1989; El Ali et 249 al. 1989; Gillhaus et al., 2000) have demonstrated that the stoichiometry, K_D, and luminescence 250 of natural terrestrial dolomites, as determined by electron paramagnetic resonance (EPR) and/or 251 cathodoluminescence spectroscopy, can be used to distinguish between hydrothermal, 252 evaporitic and non-evaporitic origins (Lumsden and Lloyd 1984; El Ali et al. 1989). Dolomites 253 formed in non-evaporitic or hydrothermal environments have a wide range of calcium content 254 (50-56 at%; 1 < Ca/Mg < 1.27) and exhibit a low manganese distribution ratio (K_D) around 5.0 255 or lower. In contrast, dolomites from evaporitic sediments have a narrower calcium content 256 range (49.5-51 at%; 0.98 < Ca/Mg < 1.04) but a much wider variation in K_D, with values ranging 257 from ~5.0 to 70–80. This high variability in manganese distribution suggests a formation under 258 non-equilibrium conditions for evaporitic dolomites (Lumsden et al., 1984, 1988). 259

260 The K_D Mn distribution diagram (Fig. 4) indicates a clear affinity of Ryugu dolomites with 261 those from terrestrial evaporitic environments. This is consistent with their Ca/Mg 262 stoichiometry and the characteristics of their Mn²⁺-activated luminescence (Lumsden and Lloyd 263 1984; El Ali et al. 1989). While more research is needed to better link K_D-values to geological 264 formation conditions like temperature and salinity (El Ali et al., 1989; Toffolo et al., 2019), the 265 current crystal-chemistry data for Ryugu dolomite supports the existing hypothesis of low- 266 temperature alteration by saline water and brine formation on the Ryugu parent body, and 267 possibly formation under non-equilibrium conditions. Beyond Ca-Mg carbonates, it has been 268 shown that Ryugu samples contain Mg, Na phosphate (Nakamura et al., 2022, Pilorget et al. 269 270 271 272 273 274 275

276 That dolomite formation is brine-driven also aligns with growing evidence suggesting that 277 dolomite on Earth is more prevalent in modern evaporitic saline environments than in open- 278 marine sediments (Warren 2021; Gregg et al. 2015). In the past decades, dolomite has been 279 frequently found in modern saline lakes (Li et al., 2020), coastal evaporitic sabkhas (Shalev et 280 al., 2020), and saline soils (Kohut et al., 1995), while by contrast, almost no dolomite 281 precipitation has been reported in the ocean (Gregg et al., 2015). The elevated concentrations 282
of various ions, including Na⁺, Ca²⁺, Mg²⁺, Cl⁻, and SO₄²⁻, in these evaporative saline systems, compared to surface seawater (Deng et al., 2010), strongly suggest a potential link between dolomite precipitation, high salinity, high sulfate concentration, high pH, and elevated Mg/Ca molar ratios in the associated solution (see also Li et al., 2020).

However, the concentric epitaxial overgrowths and small-scale CL oscillatory zoning in Ryugu's dolomites (Fig. 2 and 3) point to a complex precipitation history from aqueous solution. The peculiar morphology of these dolomite grains— featuring a fibrous overgrowth oriented perpendicularly to the surface of a regularly shaped equant core —is a key indicator of the formation conditions (Fig. 5). The duplication of this pattern suggests that environmental conditions varied significantly during the aqueous alteration of the Ryugu parent body. Fibrous dolomite overgrowths have high surface area-to-volume ratios, which results in a high surface-to-bulk energy ratio. This morphology is characteristic of crystallization under conditions of high supersaturation or a high Peclet number (Pe = LR/D), where L is the characteristic length, R is the growth rate, and D is the diffusion coefficient. Under these conditions, the growth process becomes subject to the Mullins-Sekerka morphological instability, which favors the growth of high surface area structures, such as dendrites or fibrous crystal structures during crystal growth (i.e., as we observed it at dolomite edge). In contrast, the regularly shaped equant crystals (i.e., dolomite core) likely precipitated under conditions of low supersaturation or a low Peclet number, which prevented this morphological instability (See also Tsuchiyama et al. 2024). In short, by controlling the mode of solute transport, variations in the Peclet number directly influence local variations in supersaturation. Assuming that cathodoluminescence (Mn²⁺) variations (Fig. 5) are directly related to the conditions of precipitation, the oscillatory zoning in the dolomite of Ryugu samples indicates very likely cyclic variations in the degree of supersaturation of the brine. The systematic increases in oscillations frequency toward the dolomite's edges (Fig. 2 and supplementary information) suggest higher states of supersaturation as dolomite precipitation progressed.

This echoes recent experimental work on dolomite precipitation in near ambient conditions (Kim et al., 2023), suggesting progress toward resolving the ‘dolomite problem’ (Warren, 2021; Roberts, 2024). The dolomite enigma stems from the difficulty of synthesizing dolomite under standard Earth surface conditions, a contradiction that has puzzled geologists for centuries (Roberts 2024). Kim et al. (2023) reported that dolomite initially precipitates with a disordered surface layer, which hinders crystal growth. The strong hexahydrate shell around the magnesium ions (i.e., Mg²⁺·6H₂O) in solution results indeed in a kinetic barrier that inhibits wide-scale growth of dolomite at ambient conditions (see Kim et al., 2023; Hobbs et al. 2024 for review). However, Kim and co-workers have shown that under repeated cycles of oversaturated and undersaturated conditions, this disordered layer can dissolve and be replaced with a more ordered structure, significantly accelerating the growth process at near ambient conditions. By analogy, it is likely that the successive episodes of dolomite growth in Ryugu samples (Fig. 2 and 5) are related to fluctuations in fluid saturation, possibly due to changes in pH or salinity.

Other studies (Fang & Xu, 2022; Hobbs et al. 2024) have also shown that dissolved silica catalyzes the direct precipitation of dolomite from evaporite brines. Silica reduces the activation energy for Mg-water complex dehydration, promoting dolomite nucleation and growth at ambient conditions (Fang & Xu, 2022). Moreover, silica can facilitate the co-precipitation of dolomite and Mg-rich smectite clays (e.g., Al-rich saponite, Hobbs et al. 2024), with the clays serving as a silica sink during serpentinization. The close association between precipitating
331 clays and dolomites in evaporitic settings, mimics what is observed in Ryugu parent body
332 (Nakamura et al. 2022, 2023; Tsuchiyama et al. 2024), and this may be more than coincidental.

### 4.2 Brine formation by serpentinization

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335 Dolomite on Earth is widely recognized as an evaporite mineral and most dolomites owe their
336 origin to hypersaline brines (Friedman, 1980; Warren, 2021; Hobbs et al., 2024). Conceptually,
337 heating drives the loss of water as it transitions into vapor, while the residual brine concentrates
338 to elevated salinities at which stage a suite of evaporite salts or salt deposit (including dolomite)
339 saturate and precipitate.
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341 Solar-driven evaporation, as well as non-solar-induced heating or freezing, have been proposed
342 as mechanisms for the formation of brines. On Earth and in asteroids, these brines precipitate
343 to form a variety of minerals, such as carbonates, sulfates, and halides (Warren 2021;
344 Matsumoto et al., 2024; McCoy, Russell et al., 2025). However, several points cast doubt on
345 the feasibility of such processes on asteroids. Firstly, the complete sequence of salts found on
346 Ryugu (and similarly on Bennu), which includes highly soluble minerals like sodium
347 carbonates and halite, would have been destroyed if they had been exposed to the harsh
348 conditions of space for extended periods. Their pristine state suggests they were formed and
349 remained contained within the parent body's interior. In this context, it's difficult to understand
350 how evaporation cycles could have occurred, as the interior of an asteroid has a much higher
351 thermal inertia compared to the loose material of the subsurface, making the necessary
352 temperature fluctuations far less likely. No significant traces of evaporation and outgassing
353 were observed in the samples either. Another related issue is the source of the heating. If
354 radioactive decay of short-lived isotopes like ²⁶Al provided enough heat to melt ice and form
355 liquid water within the asteroid's core, the core would cool over millions of years. This implies
356 a protracted timing for the evaporation of the liquid brines and the precipitation of the evaporitic
357 sequence, which could be at odds with the somewhat similar mineralogy and isotopic signatures
358 we observed. Freezing also seems an unlikely option, as the temperatures needed for carbonate
359 formation are believed to be well above the freezing point (Nakamura et al. 2023; Yokoyama
360 et al., 2023; Kita et al. 2024; see below). Finally, both evaporation and freezing are large-scale
361 processes. They fail to adequately explain the local variations in the mineralogy, petrology, and
362 chemistry observed in the Ryugu samples, suggesting that a different, more localized process
363 may have been at play.
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365 As an alternative to both evaporation or freezing, we suggest that brine and salt formation is
366 related to serpentinization of amorphous and crystalline silicate precursors (Fig. 6). Assuming
367 that water is present in the parent body, this process of crystallizing Mg-rich hydrous minerals
368 likely contributed to the drying out of the aqueous solution, as serpentine minerals contain
369 around 12-15 wt% H₂O, potentially through reactions of the type:
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$$371 \quad 2\text{Mg}_2\text{SiO}_4\text{(forsterite or amorphous-like)} + 4\text{H}_2\text{O(aq)} \rightarrow \text{Mg}_3\text{Si}_2\text{O}_5(\text{OH})_{4(\text{serpentine})} + \text{Mg(OH)}_2\text{(brucite)} \quad (1)$$

$$372 \quad \text{Mg(OH)}_2\text{(brucite)} + \text{CO}_{2(\text{aq})} \rightarrow \text{MgCO}_{3(\text{in dolomite})} + 2\text{H}_2\text{O(aq)} \quad (2)$$

$$374 \quad (\text{Mg,Fe})_2\text{SiO}_4\text{(forsterite or amorphous-like)} + \text{H}_2\text{O(aq)} \rightarrow \\ \quad (\text{Mg,Fe})_3\text{Si}_2\text{O}_5(\text{OH})_{4(\text{serpentine})} + (\text{Mg,Fe})(\text{OH})_2\text{(brucite)} + \text{Fe}_3\text{O}_4\text{(magnetite)} + \text{H}_2\text{(vapor)} \quad (3)$$

377
378 Serpentinization encompasses a series of hydration reactions starting from crystalline (Frost
379 and Beard, 2007; Malvoisin et al., 2012; Lamadrid et al., 2017, 2021; Schwander et al., 2023)
380 or amorphous magnesium-rich silicates (Nakamura-Messenger et al., 2011; Igami et al. 2021)
to produce serpentine and a suite of minor phases such as clays, iron oxides and hydroxides, sulfides and carbonates that occurs during water-rock interactions at relatively low temperatures and pressures (Klein and McCollom, 2013; Le Guillou et al. 2015; among others). Crucially, this process acts as a water sink: as new hydrated silicate phases are formed, the total amount of liquid water decreases (Fig. 5; Debure et al., 2019). This "drying out" effect dramatically impacts the fluid chemistry, leading to a significant increase in the brine's alkalinity and salinity. In the specific case of the Ryugu parent body, the presence of almost equal amounts of saponite (with 15–25 wt% H₂O) suggests this clay mineral played a similar role in consuming aqueous solution. Although we will continue to use the term serpentinization to generally describe the alteration, this water consumption—whether by serpentine or saponite—is the core process that drives fluids toward a highly concentrated state, closely linked to the initial mineralogy and salt solubilities. This alternative process is supported by several arguments.

At first, a simple mass balance calculation confirms that the serpentinization of magnesium-rich silicates (reaction 1), driven by a low-salinity aqueous fluid (1 wt% NaCl equivalent), leads to the formation of highly concentrated brines. This process can even reach salt (NaCl) saturation at advanced stages of the reaction (e.g., ≥ 70% advancement, see Supplementary information). In addition to supporting the serpentinization process, these results are consistent with observations from Ryugu samples. The samples show a high abundance of hydrated phases and a rarer occurrence of salts and other secondary phases like carbonates and phosphates (Nakamura et al. 2022, 2023; Matsumoto et al., 2024). While beyond the scope of this study, the primary phases before serpentinization in Ryugu were very likely dominated by amorphous materials, some of which resembling magnesium-rich GEMS-like materials (Nakamura et al. 2023, Leroux et al., 2023; Aleon-Toppani et al., 2024). Drawing on multiple experimental hydration studies, the formation rate of hydrated phases from amorphous silicates (Le Guillou et al. 2015; Nakamura-Messenger et al., 2011; Igami et al., 2021) is considerably higher than from crystalline silicates (Lamadrid et al., 2021), a key finding that may help to constrain the alteration timescales in the Ryugu parent body and chondrites in general.

The intimate association between phyllosilicates and other secondary phases in Ryugu samples provides further convincing evidence for serpentinization as the underlying process (reactions 1-3). Advanced TEM studies have indeed revealed a direct crystallographic relationship between phyllosilicates and other secondary minerals, such as elays, carbonates, magnetite, and sulfides (Leroux et al., 2023; Tsuchyiama et al. 2024; Mouloud et al., 2024; Aleon-Toppani et al., 2024). This close intergrowth is a key indicator of their co-formation through a shared aqueous alteration process. The presence of a mixture of serpentine and saponite, frequently decorated with micro- to nanoscale iron sulfides, magnetite, and carbonates, is particularly notable in this respect (Tsuchyiama et al. 2024; Mouloud et al., 2024; Aleon-Toppani et al., 2024). This suggests that the products of the serpentinization reaction are consistent with the inferred sequence of alteration in Ryugu samples (Tsuchiyama et al., 2024; Aleon-Toppani et al., 2024). This process begins with the sequential formation of different generations of phyllosilicate phases, including magnesium silicate hydrate, serpentine, and saponite, which are followed or accompanied by the precipitation of minor phases such as sulfides, carbonates, and magnetite; salts with « evaporite » minerals forming later (Matsumoto et al., 2024).

CL panchromatic images of dolomites provide further evidence for the evolution of aqueous fluids toward highly saturated brines. The dolomite morphology from equant to fibrous toward the crystal edges (Fig. 5) can be directly linked to Peclet number variations, a key indicator of fluctuating fluid saturation. A high Peclet number (Pe > 1) signifies that fluid convection dominates, leading to the rapid delivery of high-supersaturation zones to crystal surfaces and
accelerating crystallization. Conversely, a low Peclet number (Pe < 1) indicates that diffusion is the primary transport mechanism, promoting slower, more uniform crystal growth. It's therefore plausible that the irregular morphological oscillations observed in the dolomites are linked to the growth and relative abundance of different generations of phyllosilicates. These phases, including magnesium silicate hydrate (MSH, Tsuchiyama et al., 2024), serpentine, and saponite, have distinct water contents (MSH: <15 wt% H₂O; serpentine: 12-15 wt% H₂O; saponite: 15-25 wt% H₂O) which likely influenced the local chemical environment and fluid flow dynamics. This relationship between dolomite growth, hydrated phase formation and fluid dynamics is one of the crucial aspects of the serpentinization process.

Furthermore, the serpentinization reaction is known to cause a significant volume increase (ΔV<sub>reaction1</sub> > 0) in the solid rock. This expansion is the primary source of the internal stresses and pressures generated within the matrix of a CI-like parent body. While the estimated overpressure, calculated using the bulk modulus of serpentinite (≈ 68 GPa) and a typical 12.85% volume increase (P ≈ 8.7 GPa), is unrealistically high, the calculation illustrates the immense stress potential. If we assume the Ryugu parent body's initial material was highly porous—allowing pore space to partially buffer this volume change—a residual overpressure is still certain to exist. This pressure is crucial, as it triggers internal fracturing that creates necessary conduits for water and other reactive fluids to penetrate deeper into the asteroid's matrix. By analogy on Earth, this creates a self-propagating cycle of alteration and physical breakdown: the increased fluid flow accelerates the serpentinization reaction, which in turn generates more volume expansion and pressure, leading to more fractures at different scales. This feedback loop could be a key mechanism responsible for hydrating large volumes of asteroidal material, (and possibly for carbonate veins formation?), a process observed in bodies like Ryugu and Bennu and in CI chondrites, which distinguishes it from large-scale evaporation or freezing.

## 4.3 Serpentinization and carbonation on Ryugu: insights from fluid chemistry and isotopic evidence

Serpentinization on Earth is also associated with increasing alkalinity which promotes low temperature carbonatation in the hydrothermal fluid favoring the precipitation of several carbonate generations crosscutting the serpentine mesh textures. This strongly suggests that serpentinization pre-dates carbonatation (Picazo et al., 2020). Redox potential, silica activity and salinity of the reacting fluid are thus critical factor controlling the formation and the nature of the secondary phases (including clays) and the rate of the serpentinization process (Frost and Beard, 2007; Lamadrid et al., 2021).

In the course of this alteration, abiotic organic matter, such as carboxyl groups, can also play a significant role in complexing and dehydrating Mg ions present in the brine, and this process may participate to promote dolomite nucleation and growth (Roberts et al., 2013). This is because serpentinization leads to the drying out of the system and the subsequent formation of macromolecular carbon (Andreani et al., 2023). In Ryugu samples, diffuse organic matter containing diverse carbonyl and carboxyl functional groups has been shown to be closely associated with phyllosilicate minerals (Viennet et al., 2023; Stroud et al. 2024; Mouloud et al. 2024; Aleon-Toppani et al., 2024; De Gregorio et al. 2024). This organic matter is also characterized by molecular carbonate (CO₃) absorption features, resembling those observed in clay-bound organic matter within CI meteorites. (Nakamura et al., 2022; Yabuta et al., 2023; De Gregorio et al., 2024; Stroud et al., 2024). Analyses of fluid inclusions within a Ryugu pyrrhotite crystal using X-Ray Computed Tomography (XRCT) and Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) comfirm this view of a complex brine composition
481 comprising water, CO₂, sulfur species, organic material, and various elements (H, C, O, S, F,
482 Cl, Ni, Na, Mg, Al, Cr, K, Ca), with the detection of larger organic fragments (C²⁻, C₂H⁻, C³⁻,
483 CO⁻, CN⁻) indicating the presence of more complex organic molecules (Dolocan et al., 2024).
484

485 The identification of lizardite as the primary serpentine polymorph in Ryugu's matrix (Mouloud
486 et al., 2023) suggests that alteration may have occurred at low temperatures. Assuming low
temperature serpentinization/carbonatation processes (Nakamura et al., 2022; Nagashima et al.
487 2022; Yokoyama et al., 2023; Fujiya et al., 2023; Kita et al., 2024), the conditions for dolomite
488 precipitation on Ryugu parent body can thus be estimated. Magnesium isotopic analysis of
489 Ryugu carbonates indicates a sequential mineral formation process: Mg-rich phyllosilicates
500 initially precipitated, followed by dolomite (Yoshimura et al., 2024). Based on the inferred
501 alteration sequence, the Mg²⁺ ions are initially integrated into phyllosilicates, followed by
502 magnesite. This process significantly depletes the fluid's original Mg²⁺ content (by
503 approximately 80%), leading to an increased Ca/Mg ratio and favoring the precipitation of
504 dolomite. This depletion led to the evolution of the remaining fluid into a highly alkaline Na-
505 Cl brine with salting-in properties and enrichment in Ca²⁺ (Nakamura et al. 2023; Yoshimura
506 et al., 2023, 2024; Matsumoto et al. 2024). Following Pokrovski (1998), where MgCO₃
507 saturation governs Mg incorporation into carbonates, a magnesite-dolomite-calcite sequence is
508 expected. This sequence might be partially reflected in Mn distribution, given the similar ionic
509 radii and hydration shells of Mg²⁺ and Mn²⁺. However, the complexity of Ryugu samples, being
510 breccias with clasts exhibiting varying degrees of aqueous alteration, may suggest a more
511 intricate carbonate formation process than a simple sequential model, because calcite occurs
512 predominantly in the less altered clasts, while magnesite-dolomite occur in the altered clasts
513 (Nakamura et al., 2023).
514

505 Observed small-scale CL zoning and overgrowths in dolomites (Fig. 2, 3 and 5) can thus be
506 explained by subtle variations of several of the following parameters. Given the lack of talc or
507 brucite minerals associated with the matrix serpentine-saponite on Ryugu, it is expected that
508 the brine will exhibit low to intermediate silica activities, estimated to be between –3 < log
509 aSiO₂ (aq) < –2 (Frost and Beard, 2007; Fang and Xu, 2022; Hobbs et al. 2024). The same range
510 of aSiO₂ (aq) could potentially lead to saponite formation, which may in turn result in further
511 desiccation of the residual aqueous fluid (Hobbs et al. 2024). Variations in oxidation-reduction
512 potential (Eh) are also expected, possibly driven at a small scale by the interplay of the
513 serpentinization reactions (e.g., reactions 1-3). They are consistent with previous data on iron
514 oxidation state of Ryugu samples (Roskosz et al., 2024). Maintaining the stability of Mn
515 carbonate, organic matter, and pyrrhotite, while preventing MnS alabandite formation, requires
516 moderately reducing range (-0.3 < Eh < -0.2; Maynard 2003; Guigoz et al. 2024). Even more
517 reduced conditions may be possible if H₂-bearing aqueous fluids are expected (see reaction 3;
518 Eh < -0.45; Schwander et al., 2023), while higher Eh values will be needed for dolomite-
519 magnetite precipitation (Maynard 2003; Guigoz et al. 2024; Nakamura et al. 2022; Yokoyama
520 et al., 2023; Kita et al., 2024). Additionally, alkalinity conditions are estimated to be between
521 7.5 < pH <10, which are conducive to dolomite and saponite precipitation and the absence of
522 siderite (Matsumoto et al. 2024). These fluctuations may explain the uneven distribution of
523 dolomite within the Ryugu matrix, highlighting the importance of matrix permeability and its
524 evolution during alteration, a phenomenon that would be particularly significant if changes in
525 stress and pressure related to the serpentinization reaction were considered (see above).
526

527 The proposed model of brine formation by low temperature serpentinization (Fig. 6) can be
528 further tested by examining the oxygen-isotope distribution among the aqueously altered
529 minerals in Ryugu samples. It posits that a single aqueous fluid evolving in composition is
530
531 responsible for both serpentinization (lizardite) and carbonate (magnesite, dolomite, calcite)
532 formation within Ryugu parent body. A specific mineral sequence is predicted: serpentine
533 forming initially, followed by magnesite/dolomite. Based on various studies, the aqueous
534 alteration temperatures can be assumed to be 100°C or below (Nakamura et al., 2022;
535 Nagashima et al., 2022; Fujiya et al., 2023; Yokoyama et al., 2023; Kita et al., 2024). We can
536 first assume that bulk δ¹⁸O isotope of Ryugu samples is identical to that of serpentine (green
537 bar, Fig. 7), since this is the most dominant hydrous phases (≈ 85 vol%). Appropriate
538 equilibrium mineral-water oxygen isotope fractionation factors 1000lnα(mineral-water) = f(T) is set
539 at 100°C for all phases. The δ¹⁸O composition of the fluid phase (blue bar) is calculated using
540 the equilibrium serpentine-water fractionation factor (Scicchitano et al., 2023). Leveraging this
541 fluid composition, the equilibrium δ¹⁸O values for magnesite (brown bar), dolomite (orange
542 bar), and calcite (yellow bar) are estimated at 100°C using appropriate fractionation factors:
543 dolomite-water (Horita, 2014), magnesite-water (Zheng, 1999), and calcite-water (Freidman
544 and O'Neil, 1977). The close agreement between these calculated values at 100°C and observed
545 δ¹⁸O carbonate compositions support equilibrium precipitation, except for calcite which
546 requires a temperature range of 0 to 100°C. The broader range of δ¹⁸O and Δ¹⁷O values observed
547 in calcite grains compared to dolomite and magnesite remains unclear within this framework
548 (e.g., different clasts and/or different degrees of aqueous alteration), suggesting further
549 investigation is needed. The estimated fluid isotopic compositions (blue bar, Fig. 7) is
550 nevertheless consistent with other estimates (blue diamond, Fig. 7, see Kita et al., 2024) and
551 notably the composition of water in equilibrium with magnetite and dolomite pairs in C0002-
552 C1001 from Nagashima et al. (2022) estimated to be δ¹⁸O = +8‰ and Δ¹⁷O = + 0.6‰ by
553 applying the temperature of 92°C (point N in Figure 7). Notably, this low-temperature
554 mineralization is analogous to the microscale interplay of serpentinization (lizardite) and
555 carbonation (dolomite) that generates alkaline fluids during Quaternary low-temperature
556 alteration (<100°C) in New Caledonia ophiolites (Corre et al. 2023).
557
558
559 **5. Conclusion**
560
561 In this study, we show that high resolution cathodoluminescence helps to shed a light on
562 hydrothermal activity in Ryugu. While evaporation or freezing of water-rich fluids cannot be
563 ruled out, we propose as an alternative that brine and salt formation occurs as a byproduct of
564 serpentinization. While more robust chemical models are necessary to address the conditions
565 of precipitations of the secondary phases present, the low temperature serpentinization scheme
566 presented here is consistent with aqueous alteration model, including the starting materials and
567 precipitation mineral sequence proposed for Ryugu (Nakamura et al. 2022, 2023; Matsumoto
568 et al., 2024; see recap in Fig. 15 from Tsuchiyama et al., 2024). This agreement suggests that
569 within the Ryugu parent body, serpentinization preceded carbonatation, a sequence akin to
570 retrograde processes seen in terrestrial ocean floor serpentinites. This offers an alternative
571 explanation to the purely prograde (increase) temperature evolution previously inferred for
572 aqueous alteration in primordial asteroids (Kita et al., 2024 and references therein). Similar
573 serpentinization processes may also explain mineral sequence in CM chondrites (Suttle et al.,
574 2021). Such an interpretation is compatible with oxygen and carbon isotope measurements,
575 suggesting that Ryugu dolomite formed during retrograde cooling in the late stages of aqueous
576 alteration (Fujiya et al., 2024). A concomitant decrease in the water/rock ratio and a subsequent
577 increase in the ionic strength of the aqueous brines is caused by water and element incorporation
578 of hydrated silicates (Fig. 6). These factors are all critical in controlling the nature, chemistry,
579 and isotopic composition of the resulting evaporite minerals, including dolomite.
580

5. Conclusion

560

561 In this study, we show that high resolution cathodoluminescence helps to shed a light on
562 hydrothermal activity in Ryugu. While evaporation or freezing of water-rich fluids cannot be
563 ruled out, we propose as an alternative that brine and salt formation occurs as a byproduct of
564 serpentinization. While more robust chemical models are necessary to address the conditions
565 of precipitations of the secondary phases present, the low temperature serpentinization scheme
566 presented here is consistent with aqueous alteration model, including the starting materials and
567 precipitation mineral sequence proposed for Ryugu (Nakamura et al. 2022, 2023; Matsumoto
568 et al., 2024; see recap in Fig. 15 from Tsuchiyama et al., 2024). This agreement suggests that
569 within the Ryugu parent body, serpentinization preceded carbonatation, a sequence akin to
570 retrograde processes seen in terrestrial ocean floor serpentinites. This offers an alternative
571 explanation to the purely prograde (increase) temperature evolution previously inferred for
572 aqueous alteration in primordial asteroids (Kita et al., 2024 and references therein). Similar
573 serpentinization processes may also explain mineral sequence in CM chondrites (Suttle et al.,
574 2021). Such an interpretation is compatible with oxygen and carbon isotope measurements,
575 suggesting that Ryugu dolomite formed during retrograde cooling in the late stages of aqueous
576 alteration (Fujiya et al., 2024). A concomitant decrease in the water/rock ratio and a subsequent
577 increase in the ionic strength of the aqueous brines is caused by water and element incorporation
578 of hydrated silicates (Fig. 6). These factors are all critical in controlling the nature, chemistry,
579 and isotopic composition of the resulting evaporite minerals, including dolomite.
580

580
Comparative analyses of samples from Bennu (Lauretta et al., 2024; Connolly et al., 2025; McCoy & Russell et al., 2025; Zega et al., 2025) have already revealed several mineralogical similarities with Ryugu and CI chondrites. These comparisons will be crucial for validating our model's predictions and refining our understanding of brine alteration processes in primitive asteroids. This includes the nature of the precursor to serpentinization, particularly in Ryugu and Bennu where (chondrule) olivine pseudomorphs are scarce, the mechanisms of hydration and carbonation, the potential local and temporal variability of the brine chemistry, and the overall duration of brine-driven alteration. By addressing these points, comparative studies will deepen our understanding of the specific mechanisms and environmental conditions involved in water-rock interactions on primitive asteroids.

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All data needed to evaluate the conclusions in the paper are present in the paper and in the supplementary information. Additional data related to this paper may be requested from the authors.

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## Acknowledgements

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We thank the entire Hayabusa2 Team for making possible the return of samples from the asteroid Ryugu. This project was financially supported by ANR O-Return ANR-21-CE49-0005 (GL and MP). CRHEA and Lagrange laboratories are also acknowledged for their support. We thank Lucie Delauche for his help in preparing the samples and Wataru Fujiya for sharing with us his chemical analyzes of the carbonates. We also thank the Centre National d'Etudes Spatiales (CNES) for its full support, in the frame of the OSIRIS-REx and MIAMI-H2 projects. We acknowledge the use of the SEM-FIB facility of the Institut des Sciences Moléculaires d'Orsay, and the CAMPARIS facility at Sorbonne Université, where parts of this work were conducted. This work is, in part, in support of NASA contract NNM10AA11C, the OSIRIS-REx asteroid sample return mission (D. S. Lauretta, principal investigator), as a co-investigator (G.L.) through CNES.

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G.L. and C.E designed the study. C.C. made the EDX map and V.G, M.P, V.B, G.L. made the CL measurements on sample provided by C.E. G.L., V.G., M.P., N.V., C.E. reduced the data, performed the calculations and models. G.L., M.P., V.G., V.B, C.C., N.V., T.N., S.T., H.Y., C.E. contributed to discussions, including comparison with terrestrial samples and reviewed the manuscript. G.L., M.P., V.G. and C.E. wrote the paper under the supervision of T.N., S.T., H.Y. G.L., M.P., V.G., V.B, C.C., N.V., T.N. W.F., S.T., H.Y., C.E. participated to the reading, the final interpretation and correction of the manuscript.

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## Competing interests

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The authors declare no competing interests.

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## Additional information

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**Fig. 1** Multi-elemental chemical map of Ryugu sample A0064-FO013-17-02. This particle is dominated by a mixture of serpentine + saponite hydrated phases (dark green). Notice the abundance of dolomites (brown). The fine-grained matrix also contains numerous euhedral pyrrhotite crystals (pink) and isolated/framboidal magnetites (white). Modal abundances (vol. %) of major mineral phases and cathodoluminescence panchromatic images of this particle are shown in Fig. S1 and Fig. S2 respectively.
<figure><img src="image_3.png" /><figcaption>790
791 **Fig. 2** Cathodoluminescence panchromatic images of several representative dolomites from various Ryugu samples. They exhibit varying Mn activated luminescence intensity, revealing intricate internal zoning at the micron scale, imperceptible with traditional BSE-SEM techniques. Most grains exhibit oscillatory zoning alternating equant/euhedral and fibrous/columnar crystal morphology. Notice that the fibrous morphology is growing almost perpendicular to the edge of the equant crystals. The outermost shell often shows a high CL intensity. This alternated pattern in dolomite suggests the formation of several concentric epitaxial overgrowths around a low CL intensity core. Note that some dolomites (e.g., a, b and f) bear the trace of the spot analysis left by the electron microprobe. Dolomite may also host some rounded inclusions of magnetite (a-c).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3 Dolomite panchromatic image and CL spectra from Ryugu sample with example of deconvolution. Mn²⁺-activated luminescence in dolomite is characterized by main gaussian peak at around 655 nm (≈1.90 eV). Point analyses revealed however a systematic broadening of the emission band from the core to the edge, accompanied by the emergence of a shoulder (black arrow) at shorter wavelengths (500-600 nm). CL in dolomite is thus characterized by two overlapping Gaussian peaks 655 nm (E₁) and at 578 nm (E₂), corresponding to Mn²⁺ substitution for Mg²⁺ in the Mg site and Mn²⁺ substitution for Ca²⁺ in the Ca site, respectively (see text).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 4** Mn partitioning into Mg (E₁) and Ca (E₂) sites vs Total Mn peak area in dolomite from Ryugu. Total Mn peak area = Mn²⁺ peak area in Mg site (E₁) + Mn²⁺ peak area in Ca site (E₂). Dolomites with a high Mg site to Ca-site manganese ratio (K_D ≥ 5) are consistent with evaporitic condition of formation. Numbers correspond to deconvoluted spectra from dolomite grains shown in Supplementary information Fig. S5. Errors on highest and lowest K_D values are around 30% and 10% relative, respectively. Some grains show an increase CL intensity (Total Mn peak area) as Mn content increases from core to rim in dolomites (see text).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5 a) High-resolution cathodoluminescence analysis of a dolomite grain from Ryugu sample A0064-FO013-17-02. Note the peculiar dolomite morphology: a regularly shaped equant core is overgrown by “perpendicular” fibrous/columnar crystals. The duplication of this pattern suggests that environmental conditions varied significantly during the aqueous alteration of the Ryugu parent body. b) Concentric epitaxial overgrowths as revealed by variation of Mn²⁺-activated luminescence using Image J processing along the A-B profile. Assuming that cathodoluminescence variations (a) and (b) are directly related to the conditions of precipitation, the oscillatory zoning in the dolomite of Ryugu samples indicates very likely cyclic variations in the degree of supersaturation of the brine. c) The Mullins-Sekerka instability (adapted from Mehes and Vicsek, 2016) explains how a smooth crystal interface becomes unstable during growth, leading to a change in morphology, with the fiber growth direction perpendicular to the surface of regularly-shaped equant core crystal. This instability is driven by supersaturation at the solid-liquid interface, where solute is delivered faster than it can be incorporated into the crystal. The Peclet number (Pe) quantifies this relationship by comparing the rate of solute transport by convection to that by diffusion. A high Peclet number signifies that convective flow is dominant. If this strong flow delivers high levels of supersaturation to the growing interface, it promotes interfacial instability, which, in turn, drives the formation of fibrous, dendritic, or other columnar crystal morphologies. Concentration is indicated by blue color, and selected concentrations by black contour lines while red arrows point the growing interface.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>871
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**Fig. 6** Schematic model of brine formation by serpentinization as an alternative to both evaporation and/or freezing. This process likely led to the drying out of the hydrothermal fluid (red arrows), increasing its salinity and driving the precipitation of an evaporite mineral sequence, with dolomite forming early and intimately associated with serpentine. Serpentine (and saponite) precipitation induced fluctuations in fluid saturation that drove the successive episodes of dolomite growth. These variations were likely linked to changes in pH or salinity (see text for explanation).

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<figure><img src="image_8.png" /><figcaption>**Fig. 7** Oxygen isotope compositions in Ryugu. a) $\Delta^{17}\text{O}$ versus $\delta^{18}\text{O}$ compositions for phyllosilicate-carbonate and magnetite in Ryugu samples (adapted from Kita et al., 2024). Blue diamond stands for estimated water isotopic composition by Kita et al. 2024 and Nagashima et al., 2022. b) Calculated $\delta^{18}\text{O}$ isotope compositions of the same minerals according to the presented brine formation model by serpentinization. Phases color code is the same in a) and b) (see text for explanation).</figcaption></figure>

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Low temperature brine formation by serpentinization on asteroid (162173) Ryugu

Guy Libourel¹,⁴, Marc Portail², Vincent Guigoz², Virginie Brändli², Cyrille Collin³, Nathalie Vigier⁵,
Tomoki Nakamura⁶, Hikaru Yabuta⁷, Shogo Tachibana⁸, Cecile Engrand¹².

¹Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France.

²Université Côte d'Azur, CNRS, CRHEA, Valbonne, France.

³PSL Research University, MINES Paris-PSL, CEMEF, Valbonne, France.

⁴Hawaiʻi Institute of Geophysics and Planetology, University of Hawaiʻi at Mānoa, Honolulu, USA.

⁵CNRS, Sorbonne Université, LOV, IMEV, Villefranche sur Mer, France.

⁶Department of Earth Science, Tohoku University, Sendai, Japan.

⁷Earth and Planetary Systems Science Program, Hiroshima University, Higashi-Hiroshima, Japan.

⁸UTokyo Organization for Planetary and Space Science, University of Tokyo, Tokyo, Japan.

⁹IJCLab, UMR 9012, Université Paris-Saclay, CNRS, Orsay, France.

Corresponding author: Guy Libourel, libou@oca.eu

Supplementary information

- Cathodoluminescence of carbonates

- Mass balance calculation of serpentinization reaction

Supplementary table

- Table S1: EPMA chemical composition of representative dolomites from Ryugu samples.

Supplementary figures

- Figure S1. Modal abundance of minerals in Ryugu sample A0064-FO013-17-02 (see Fig. 1) and C0002-FC016-04 as determined by XMapTools

- Figure S2. Comparison between BSE (left) and CL (right) panchromatic images of 3 particles of Ryugu, (a) A0064-FO013-17-02, (b) C0040-FC025 and (c) A0064-FO013-17.

- Figure S3. Example of spectra and of hyperspectral line scan of dolomite from Ryugu sample A0064-FO013-17-02.

- Figure S4. BSE, EDX and CL analyses of two calcites grains from Ryugu.

- Figure S5. Dolomites from polished grains at Ryugu's A and C sampling sites were analyzed using CL spectroscopy

- Figure S6. Location maps of polished grains from Ryugu sampling sites A and C used in this study, as detailed in Table S1.
962

963 Cathodoluminescence of carbonates

964

965 Cathodoluminescence, the emission of photons (ultraviolet to near-infrared) following
966 injection of high-voltage electrons, has long been recognized as a powerful technique to
967 explore many fundamental properties of carbonates including insights into crystal growth,
968 zonation, deformation, trace elements, and defect structure, as well as its modes and its
969 physico-chemical conditions of formation. The luminescence of carbonate minerals is
970 primarily determined by the concentration of Mn²⁺ activators. However, Fe²⁺ acts as a
971 "quencher," suppressing luminescence by creating non-radiative energy dissipation
972 pathways. This effect is evident in magnesite from Ryugu, where the high Fe²⁺ content
973 significantly reduces the overall luminescence. More specifically, the different emission's
974 center of luminescence in carbonates are caused by: (i) substitutions of Mn²⁺ for Ca²⁺
975 and/or Mg²⁺ (ii) substitutions of rare earth elements (REE) for Ca²⁺; (iii) dislocations of the
976 [CO₃]²⁻ anions; (iv) oxygen vacancies and broken Ca-O bonds (Sommer, 1972a, 1972b;
977 El Ali et al., 1993;).

978

979 Panchromatic CL images reveal grayscale variations representing the total photon emission
980 across all wavelengths, namely ~200 / 950nm. Brighter areas indicate higher photon
981 reception by the photomultiplier. High-sensitivity CL system and high-resolution FEG-
982 SEM enable the detection of subtle compositional features within each dolomite crystal,
983 which would be invisible using standard backscattered electron (BSE) imaging. In-depth
984 spectroscopic study has also been undertaken to better characterize what is responsible for
985 the distinct CL zoning in carbonates. For quantifying the spectral response of the
986 carbonates, the panchromatic luminescence radiation coming from the scanned sample
987 have been analyzed by directing it towards the monochromator of the CL set up. CL
988 characterization all together provides insights into detailed crystal chemistry and growth
989 modes of carbonates, not available by other research techniques.

990

991 The CL spectral analysis of Ryugu samples allows to go further, since two types of
992 emission centers have been distinguished for Mn : a yellow-green emission band (≈ 570
993 ±10 nm) linked to Mn-Ca ion substitution in the Ca octahedral site of the dolomite and a
994 red emission at lower energy (≈ 655 ±10 nm) in response to Mn-Mg ion substitution in the
995 corresponding Mg site (El Ali et al., 1993). When the wavelength axis (nm) is converted
996 in energy (eV), the plot can be deconvoluted in Gaussian peaks associated to different
997 emission centers of the structure, as shown for the dolomite grain. Each spectrum was thus
998 fitted using 6 peaks, similarly as previous reports with fitted CL spectra on carbonates
999 (Chapoulie et al., 2005). Two prominent peaks in the cathodoluminescence spectrum of
1000 dolomite occur between 1.5 and 2.5 electron volts (eV) and are attributed to the substitution
1001 of Mn²⁺ for Mg²⁺ (E₁) and Ca²⁺ (E₂) within the CaMg[CO₃]₂ lattice, respectively (Sommer
1002 1972a and 1972b). The corresponding peak area E₁ and E₂ are then used for K_D calculation
1003 to evaluate the partitioning of Mn²⁺ between the two octahedral sites. The remaining peaks,
1004 observed between 2.5 and 4 eV, are generally associated with intrinsic defects in dolomite,
1005 potentially arising from lattice distortions, anion vacancies, or electron-hole recombination
1006 involving (CO₃)⁻ as the electron and Ca⁺ or Mg⁺ as the hole. These intrinsic defects are not
1007 the primary focus of this study.

1008

1009
1010 **Mass balance calculation of serpentinization reaction**

1011 Calculations for the serpentinization of pure forsterite (Reaction 1) demonstrate that NaCl
1012 precipitation is possible at a water/rock ratio of 0.15 and 1% initial salinity. Although
1013 factors such as sodium sequestration in saponite or higher initial dilution may limit salt
1014 precipitation in natural settings, our model provides a first-order proof of concept: mineral
1015 hydration can drive brine saturation by consuming solvent water, even in the absence of
1016 evaporation.

1017 The calculation is based on the following assumptions and steps:

1018 Balanced Reaction:

1019 The serpentinization of forsterite (Mg₂SiO₄) is represented by the following balanced
1020 reaction:

$$
1021 \quad 2\text{Mg}_2\text{SiO}_4 \text{ (forsterite)} + 3\text{H}_2\text{O (water)} \rightarrow \text{Mg}_3\text{Si}_2\text{O}_5(\text{OH})_4 \text{ (serpentine)} + \text{Mg(OH)}_2 \text{ (brucite)}
$$

1022 This reaction shows that 3 moles of water are consumed for every 2 moles of forsterite.

1023 NaCl Solubility:

1024 The saturation concentration of NaCl in a brine is a key variable. The solubility of NaCl in
1025 water at standard conditions is approximately 35.9 g per 100 g of water, which corresponds
1026 to a saturation concentration of 26.4% by mass. Precipitation will begin once the
1027 concentration of the dissolved NaCl reaches this point.

1028 Calculation:

1029 *Initial conditions:* For a water/rock ratio of 0.15, 150 g of water are present for every 1000 g of rock. With an initial salinity of 1%, the mass of dissolved NaCl is 1.5 g.

1031 *Water consumption:* To reach the 26% saturation concentration, the initial 1.5 g of NaCl must be dissolved in a final water mass of only 5.77 g. This means that 150 g-5.77 g=144.23 g of water must be consumed by the reaction.

1034 *Degree of advancement:* This amount of water corresponds to approximately 8.01 moles. Based on the reaction stoichiometry, the consumption of 8.01 moles of water requires the consumption of 5.34 moles of forsterite. Since the initial 1000 g of rock contains 7.11 moles of forsterite, the reaction must advance to a degree of 5.34/ 7.11, which is approximately 75%, for NaCl to precipitate.

1039 Results:

1040 The degree of advancement for the serpentinization reaction to cause NaCl precipitation,
1041 given an initial 1% salinity and a water/rock ratio of 0.15, is 75%.

1042
<table><thead><tr><th rowspan="2"></th><th rowspan="2">DataSet/Point</th><th rowspan="2">Comment</th><th rowspan="2">Ech</th><th rowspan="2">Obj</th><th colspan="7">Weight%</th></tr><tr><th>Mg</th><th>Ca</th><th>Mn</th><th>Fe</th><th>O</th><th>Total</th></tr></thead><tbody><tr><td>558</td><td>105/9.</td><td>HY2ST03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>11,51</td><td>22,16</td><td>3,98</td><td>2,15</td><td>18,81</td><td>59,25</td></tr><tr><td>559</td><td>105/10.</td><td>HY2ST03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>11,16</td><td>21,77</td><td>5,28</td><td>2,12</td><td>18,69</td><td>59,61</td></tr><tr><td>74</td><td>37/1.</td><td>HY2_SP03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>11,34</td><td>17,07</td><td>3,75</td><td>1,97</td><td>17,95</td><td>54,20</td></tr><tr><td>75</td><td>37/2.</td><td>HY2_SP03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>10,83</td><td>20,75</td><td>4,49</td><td>1,56</td><td>17,44</td><td>55,32</td></tr><tr><td>76</td><td>37/3.</td><td>HY2_SP03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>10,69</td><td>19,75</td><td>3,12</td><td>1,35</td><td>16,49</td><td>51,67</td></tr><tr><td>77</td><td>37/4.</td><td>HY2_SP03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>11,52</td><td>20,12</td><td>4,06</td><td>1,74</td><td>17,77</td><td>55,63</td></tr><tr><td>78</td><td>37/5.</td><td>HY2_SP03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 9</td><td>10,94</td><td>18,83</td><td>3,94</td><td>1,60</td><td>18,25</td><td>55,58</td></tr><tr><td>80</td><td>37/7.</td><td>HY2_SP03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 Objet 8</td><td>11,52</td><td>18,14</td><td>2,39</td><td>1,58</td><td>17,58</td><td>52,66</td></tr><tr><td>600</td><td>109/1.</td><td>HY2ST03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 "B"</td><td>12,58</td><td>18,09</td><td>2,02</td><td>1,16</td><td>17,17</td><td>51,99</td></tr><tr><td>601</td><td>109/2.</td><td>HY2ST03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 "B"</td><td>11,76</td><td>22,58</td><td>3,22</td><td>1,90</td><td>18,65</td><td>58,50</td></tr><tr><td>543</td><td>104/4.</td><td>HY2ST03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 "D"</td><td>12,03</td><td>20,35</td><td>3,80</td><td>1,54</td><td>18,46</td><td>57,11</td></tr><tr><td>545</td><td>104/6.</td><td>HY2ST03_03</td><td>A0064-FO013-17-02</td><td>SP03_03 "D"</td><td>12,28</td><td>21,10</td><td>4,04</td><td>1,86</td><td>18,78</td><td>58,63</td></tr><tr><td>329</td><td>90/10.</td><td>HY2ST03_07</td><td>C0046-FO004-010-05</td><td>SP03_07 "A"</td><td>12,29</td><td>21,45</td><td>3,91</td><td>2,74</td><td>18,86</td><td>59,54</td></tr><tr><td>82</td><td>37/1.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05 Object 1</td><td>11,90</td><td>20,33</td><td>3,49</td><td>2,06</td><td>18,70</td><td>57,61</td></tr><tr><td>83</td><td>37/2.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05 Object 1</td><td>11,55</td><td>17,86</td><td>3,10</td><td>1,73</td><td>17,83</td><td>53,76</td></tr><tr><td>84</td><td>37/3.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05 Object 1</td><td>11,45</td><td>18,01</td><td>3,03</td><td>1,95</td><td>17,85</td><td>53,92</td></tr><tr><td>256</td><td>76/1.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>12,03</td><td>21,73</td><td>2,89</td><td>1,91</td><td>18,73</td><td>58,04</td></tr><tr><td>257</td><td>77/1.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>11,52</td><td>22,83</td><td>5,03</td><td>1,76</td><td>19,56</td><td>61,60</td></tr><tr><td>258</td><td>78/1.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>11,87</td><td>20,39</td><td>3,43</td><td>1,58</td><td>18,32</td><td>56,46</td></tr><tr><td>62</td><td>27/4.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>10,32</td><td>18,58</td><td>3,71</td><td>2,54</td><td>18,44</td><td>56,08</td></tr><tr><td>63</td><td>27/5.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>11,38</td><td>20,22</td><td>2,43</td><td>2,20</td><td>17,63</td><td>54,54</td></tr><tr><td>64</td><td>27/6.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>9,37</td><td>20,11</td><td>2,74</td><td>1,80</td><td>16,36</td><td>51,20</td></tr><tr><td></td><td>45/3.</td><td>HY2_SP02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>11,44</td><td>20,44</td><td>4,93</td><td>1,44</td><td>18,56</td><td>58,00</td></tr><tr><td></td><td>45/4.</td><td>HY2_SP02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>12,02</td><td>18,85</td><td>2,54</td><td>2,15</td><td>18,74</td><td>56,20</td></tr><tr><td></td><td>45/5.</td><td>HY2_SP02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 2</td><td>11,36</td><td>19,24</td><td>3,31</td><td>1,19</td><td>17,09</td><td>52,85</td></tr><tr><td>259</td><td>79/1.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 7</td><td>12,50</td><td>22,05</td><td>6,04</td><td>2,02</td><td>20,11</td><td>63,44</td></tr><tr><td>260</td><td>80/1.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 7</td><td>12,41</td><td>20,88</td><td>4,45</td><td>1,94</td><td>18,84</td><td>58,99</td></tr><tr><td>67</td><td>27/9.</td><td>HY2ST02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 7</td><td>10,91</td><td>21,94</td><td>4,32</td><td>1,58</td><td>18,38</td><td>57,86</td></tr><tr><td>20</td><td>45/8.</td><td>HY2_SP02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 7</td><td>12,68</td><td>20,04</td><td>3,12</td><td>1,38</td><td>18,65</td><td>56,90</td></tr><tr><td>21</td><td>45/9.</td><td>HY2_SP02_05</td><td>A0064-FO013-17-01</td><td>SP02_05_obj 7</td><td>12,44</td><td>20,22</td><td>3,42</td><td>1,53</td><td>18,35</td><td>56,51</td></tr><tr><td>263</td><td>83/1.</td><td>HY2ST02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 3</td><td>11,37</td><td>21,68</td><td>4,85</td><td>2,61</td><td>18,83</td><td>59,90</td></tr><tr><td>264</td><td>83/2.</td><td>HY2ST02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 3</td><td>10,90</td><td>22,38</td><td>4,49</td><td>2,27</td><td>18,43</td><td>58,83</td></tr><tr><td>265</td><td>83/3.</td><td>HY2ST02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 3</td><td>10,57</td><td>22,51</td><td>4,50</td><td>2,44</td><td>18,36</td><td>58,79</td></tr><tr><td>266</td><td>83/4.</td><td>HY2ST02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 3</td><td>10,53</td><td>21,57</td><td>5,17</td><td>2,56</td><td>18,02</td><td>58,03</td></tr><tr><td>27</td><td>46/3.</td><td>HY2_SP02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 3</td><td>11,74</td><td>20,21</td><td>5,48</td><td>1,35</td><td>18,69</td><td>58,40</td></tr><tr><td>28</td><td>46/4.</td><td>HY2_SP02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 3</td><td>10,74</td><td>21,14</td><td>4,88</td><td>1,60</td><td>17,62</td><td>56,19</td></tr><tr><td>36</td><td>46/12.</td><td>HY2_SP02_06</td><td>C0040-FC025-01</td><td>SP02_06_obj 2</td><td>11,12</td><td>18,92</td><td>4,50</td><td>1,42</td><td>17,69</td><td>54,66</td></tr><tr><td>38</td><td>47/2.</td><td>HY2_SP02_09</td><td>C0046-FO004-010-03</td><td>SP02_09_obj 5</td><td>11,67</td><td>20,28</td><td>4,39</td><td>1,35</td><td>17,98</td><td>56,33</td></tr><tr><td>50</td><td>48/5.</td><td>HY2_SP02_10</td><td>C0046-FO004-010-03</td><td>SP02_10_obj 4</td><td>11,86</td><td>18,06</td><td>3,57</td><td>1,61</td><td>18,51</td><td>55,84</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>

1044

1045

1046

1047

<caption>Table S1: EPMA chemical composition and ternary diagram of representative dolomites from Ryugu samples.</caption>

1048
<figure><img src="image_10.png" /><figcaption>Modal proportions</figcaption></figure>

<table><tr><td>21%</td><td>Saponite</td></tr><tr><td>50%</td><td>Serpentine</td></tr><tr><td>16%</td><td>Dolomite</td></tr><tr><td>4%</td><td>Magnetite</td></tr><tr><td>4%</td><td>Iron Sulphite</td></tr><tr><td>2%</td><td>Whitlockite</td></tr><tr><td>3%</td><td>Sodium chloride</td></tr></table>

<figure><img src="image_11.png" /></figure>

**Fig. S1.** Modal abundance of minerals in Ryugu sample (up) A0064-FO013-17-02 (see Fig. 1) and (down) C0002-FC016-04 as determined by XMapTools (https://xmaptools.ch/). Serpentine and saponite are the dominant hydrated phases, while dolomite abundance varies between the two particles. Calcite is rare and has only been observed in C0002-FC016-04. Distinction between serpentine and saponite is made on Mg/Si and Al content of each pixel. Since serpentine and saponite form intimate mixtures at the nano- and micron-scale, areas labeled as 'serpentine' or 'saponite' represent only the predominant phyllosilicate.
<figure><img src="image_12.png" /></figure>

1062
1063

1064 **Fig. S2** Comparison between BSE (left) and CL (right) panchromatic images of 3 particles of
1065 Ryugu, (a) A0064-FO013-17-02, (b) C0040-FC025 and (c) A0064-FO013-17. In these
1066 samples, the panchromatic CL images allow us to distinguish the dolomites from the rest of the
1067 weakly or non-luminescent matrix and to highlight their complex zoning. Each dolomite grain
1068 exhibits a zoning (see details in Fig. 2, 3 and 5). Numbers refer to dolomite grains shown in
1069 Fig. 2 and 3.
<figure><img src="image_13.png" /><figcaption>**Fig. S3.** Example of spectra and of hyperspectral line scan of dolomite from Ryugu sample A0064-FO013-17-02. a) Dolomites from the Ryugu samples exhibit two primary spectral luminescence centers commonly attributed to intrinsic structural defects within the dolomite lattice and to extrinsic centers resulting from Mn ion substitutions, respectively. b) Variations in panchromatic cathodoluminescence intensity, indicative of core-edge differences, are observed within a dolomite grain. c) CL Hyperspectral line scan recording the core-edge differences of the dolomite shown in b).</figcaption></figure>

1070
1071
1072
1073
1074
1075
1076
1077

1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
<figure><img src="image_14.png" /><figcaption>HY2_SP02_04</figcaption></figure>

1088

<figure><img src="image_15.png" /><figcaption>HY2_SP02_04</figcaption></figure>

1089

<figure><img src="image_16.png" /><figcaption>HY2_SP02_04</figcaption></figure>

1090
HY2_SP02_04

<figure><img src="image_17.png" /></figure>

1091

HY2_SP02_04

<figure><img src="image_18.png" /></figure>

1092

<figure><img src="image_19.png" /></figure>

1093

1094

1095

1096

1097

**Fig. S4.** BSE, EDX and CL analyses of two calcites grains from Ryugu. Notice the relatively low luminescence intensity in the orange-red wavelength range suggesting the low Mn concentration of calcite by comparison with dolomite (see Fig. 3 and Fig. S3 and text).
SP02_06

Files #11/12/13 – Object #1

<figure><img src="image_20.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

161 / 11

205 / 20

294 / 46

1098

SP02_06

Files #13/14/15 – Object #2

<figure><img src="image_21.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

194 / 20

355 / 36

245 / 35

1099
SP02_06

Files #17 à 25 – Object #3

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

211 / 17

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

304 / 27

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

252 / 24

1100
1101
1102

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

283 / 40

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

456 / 23

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /></figure>

513 / 41

1103
<figure><img src="image_40.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

377 / 29

<figure><img src="image_41.png" /></figure>

699 / 82

<figure><img src="image_42.png" /></figure>

920 / 150

1104
1105
1106

SP02_06

Files #26 à 29 - Object #4

<figure><img src="image_43.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

482 / 56

<figure><img src="image_44.png" /></figure>

735 / 48

<figure><img src="image_45.png" /></figure>

944 / 119

1107
1108
1109

<figure><img src="image_46.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

632 / 78

1110
1111
<figure><img src="image_47.png" /></figure>

<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

323 / 28

<figure><img src="image_50.png" /></figure>

<figure><img src="image_51.png" /></figure>

<figure><img src="image_52.png" /></figure>

725 / 121

<figure><img src="image_53.png" /></figure>

<figure><img src="image_54.png" /></figure>

<figure><img src="image_55.png" /></figure>

649 / 62

1112

1113

<figure><img src="image_56.png" /></figure>

<figure><img src="image_57.png" /></figure>

<figure><img src="image_58.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

782 / 108

<figure><img src="image_59.png" /></figure>

<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /></figure>

484 / 53

1114

1115

1116
<figure><img src="image_62.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

445 / 43

451 / 50

215 / 24

1117
1118

<figure><img src="image_63.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

451 / 42

833 / 45

1119
1120
<figure><img src="image_64.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

1740 / 192

<figure><img src="image_65.png" /></figure>

1329 / 196

<figure><img src="image_66.png" /></figure>

646 / 88

1121
1122

<figure><img src="image_67.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

1005 / 142

<figure><img src="image_68.png" /></figure>

1270 / 179

<figure><img src="image_69.png" /></figure>

674 / 100

1123
1124
1125
1126
SP03

Files #31 à 36 - Object #8

<figure><img src="image_70.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

715 / 95

<figure><img src="image_71.png" /></figure>

745 / 97

<figure><img src="image_72.png" /></figure>

1154 / 138

1127

1128

1129

<figure><img src="image_73.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

1011 / 89

<figure><img src="image_74.png" /></figure>

1022 / 164

1130

1131

1132
SP03

Files #37 à 42 - Object #9

<figure><img src="image_75.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

823 / 79

1053 / 111

822 / 90

1133

1134

1135

1136

<figure><img src="image_76.png" /></figure>

Mn²⁺ peak area in Mg site / Mn²⁺ peak area in Ca site

1297 / 255

509 / 360

802 / 75

1137

1138
<figure><img src="image_77.png" /></figure>

**Fig. S5.** Dolomites from polished grains at Ryugu's A and C sampling sites were analyzed using CL spectroscopy. The locations of spectral analysis are shown on the corresponding SE and panchromatic CL images. Each spectrum was fitted with 5 Gaussian peaks to identify emission centers. This study focuses on the two peaks between 1.5 and 2.5 eV (Fig. 3), attributed to Mn²⁺ substitution for Mg²⁺ (E₁) and Mn²⁺ substitution for Ca²⁺(E₂). Their relative areas E₁/E₂ were calculated and plotted in Fig. 4 (see text).

1139

1140

1141

1142

1143

1144

1145

1146

1147

1148

1149

1150

1151

1152

1153
<figure><img src="image_78.png" /></figure>

<figure><img src="image_79.png" /></figure>

**Fig. S6.** Location maps of polished grains from Ryugu sampling sites A and C used in this study, as detailed in Table S1.