Modeling of aqueous alteration in the parent body of the asteroid Ryugu

1A. Tsuchiyama1,2, J. Matsuno1 and M. Matsumoto3
Research Organization of Science and Technology, 2CAS, Guangzhou Institute of Geochemistry, 3Tohoku University

The mineralogy, petrology and chemical properties of the samples collected from the asteroid 162173 Ryugu by theHayabusa2 spacecraft show that the samples correspond to CI meteorites not affected by terrestrial weathering (e.g., [1-3]). The
parent body of Ryugu were formed by accretion of minerals, organics, and ice, and aqueous alteration occurred by reaction ofmelted ice with the minerals and organics [1]. The major lithology of the Ryugu samples consists mostly of hydrous
phyllosilicates (serpentine and saponite), carbonates, FeS and magnetite which precipitated from liquid (melted ice) during theaqueous alteration. Least- and less-altered lithologies, where some anhydrous minerals, such as olivine, remained unaltered orpartially altered, were also observed [1]. Model chemical equilibrium of the aqueous alteration was calculated in an appropriate
condition (e.g., 40oC based on oxygen isotope equilibrium [2] and ice of H2O + 0.08 CO2 based on CO2-bearing fluidinclusion[1]) as a function of the initial water/rock ratio (in mass), W/R, and found that the mineral assemblages are almost
consistent with the least-altered, less-altered and altered major lithologies at W/R of 0.06 to 0.1, 0.1 to 0.2, and 0.2 to 0.9,
respectively [1]. The order of precipitation of minerals in the major lithology was also estimated from the included and enclosed
relations among these minerals [4].
We understood the outline of the aqueous alteration by the Ryugu sample initial analysis as mentioned above. However, westill do not know how the aqueous alteration occurred and ceased; for example, how liquid was present, and precipitation
occurred from the liquid. Therefore, in this study, we performed a modeling of the aqueous alteration by reference to the minerals
and their assemblages and the chemical composition of the Ryugu samples.
In the present model, we made the following assumptions to simplify the problem; (1) the system Mg-Fe-Si-Ca-O-S-C-H istaken into consideration with the bulk chemical composition of CI [5] that is unchanged during the alteration except for O(oxidation or reduction permitted). (2) The starting material is composed of anhydrous minerals, amorphous silicate, organics,and CO2-H2O ice. Olivine, metallic Fe, and FeS are considered for the anhydrous minerals. We adopted Mg# (Mg/(Mg+Fe) inmole) of 0.99 for olivine, which was observed in least- and less-altered lithologies [6]. Material similar to GEMS (glassembedded with metal and sulfide) is considered for amorphous silicate based on the observation in the least-altered lithology [1].
This GEMS-like material (GLM) is assumed to have the mean GEMS composition of [7]. We consider relatively oxidized GLM,
where only nanoFeS particles (without nanoFe) are embedded in FeO-bearing amorphous silicate. Thus, the Mg# of theamorphous silicate becomes 0.612 to maintain the mean GEMS composition. The density of GLM is assumed to be 3 g/cm3. (3)We consider only IOM as organics. The chemical composition of IOM is adopted from [8] and its density is assumed to be 1.3
g/cm3. IOM is not changed by the alteration. (4) The ice CO2/H2O ratio is 0.08 as proposed by [1]. The ice melts to form CO2bearing water. The temperature is 40oC proposed by [2] and the pressure is less than ~50 bar for a 50-km-radius parent body [1].
In these conditions, the melted ice is a liquid state (not fluid) and the densities of H2O and CO2 are almost constant (1 g/cm3 [9]and 1.04 g/cm3 [10], respectively). (5) The aqueously altered products are serpentine, saponite, FeS, magnetite, and dolomite.
The Mg#’s of serpentine and saponite are 0.85 based on the Ryugu sample analysis [1]. Dolomite is only considered as acarbonate although other carbonates (magnesite and calcite) are present in the Ryugu samples. (6) GLM stats to react with liquid
to form serpentine and saponite first, and simultaneously the same proportion of Fe as that of altered GLM becomes magnetite.
If the GLM is completely altered, olivine starts to react with liquid. (7) GLMs and the phyllosilicates (serpentine and saponite)
have nano-sized pores (nanopores) based on the TEM observation of the Ryugu samples [1]. Any other pores are not present in
the initial state, but the space that used be as ice acts as micron-sized pores (micropores). (8) The water/rock ratio, W/R, is definedas the mass ratio of water (or ice) to whole of the other materials including IOM.
The amounts of the formula weights of the phases (minerals, IOM and liquid) were calculated to have elemental balanceamong the chemical compositions of the phases and the bulk chemical compositions, and their volumes were obtained from thedensities of the phases. This calculation is similar to the CIPW norm calculation, and chemical equilibrium was not considered.
We made calculation for a case where the initial IOM volume is 10% with respect to solid phases and the porosities of thenanopores in both GLM and phyllosilicates are 40%.
In the initial state, the modes of olivine, Fe, FeS (nanoFeS in GLM not included), GLM with nanopores, and IOM arecalculated to be 17.2, 2.5, 4.9, 67.4, and 8.1 vol.%, respectively, to have the elementary balance (Fig.1 at W/R = 0). We changed
the initial water/rock ratio (in mass), W/R, as a parameter. The final mineral assemblages after the complete reaction with liquid
were calculated with different W/R. Changes in the volumes of the phases are shown in Fig. 1. In the range of W/R = 0 to 0.10
(Stage-1 in Fig. 1), mainly saponite and minor amounts of serpentine and magnetite form from GLM and Fe, and H2O in liquidis completely consumed. CO2 in liquid is also completely consumed to form dolomite, and therefore no liquid remains in thisstage after the aqueous alteration. GLM and Fe are completely consumed at W/R = 0.10. With further increase in W/R, olivine
starts to react with water, and serpentine forms by consuming some saponite together with olivine to accommodate their Mg/Siratios (W/R = 0.10 to 0.12, Stage-2 in Fig. 1). Olivine is completely consumed at W/R = 0.12, and no liquid remains in this stage
too. In W/R > 0.12, some liquid remains after complete reaction with GLM, Fe and olivine, and the volumes of magnetite,saponite, serpentine, and FeS are constant (W/R = 0.12 to 0.19, Stage-3 in Fig. 1). The volume of dolomite increases byconsuming CO2 in liquid, and no CO2 remains in liquid in this stage. In W/R > 0.19 (Stage-4 in Fig. 1), some CO2 remains inliquid because [C] > 2[Ca] and the volume of dolomite is constant. Stage-1, Stage-2 and Stages-3 and -4 roughly correspond theleast-altered, less-altered, and altered major lithologies, respectively, in terms of their mineral assemblages. The modes ofdolomite, magnetite, saponite with nanopores, serpentine with nanopore, FeS, and IOM in Stage-4 are 2.0, 4.3, 31.3, 48.1, 8.1,and 6.2 vol.%, respectively. These are similar to those of the main lithology [1]. The relation of W/R and stages are also roughly
consistent with the chemical equilibrium calculation of [1]. The boundary between Stages-2 and -3 (W/R = 0.12) corresponds to
W/R = 0.18 in [1]. This difference is derived from the rough calculation in the present model. The broad similarities suggest thatthe present model roughly reflects the equilibrium although the model itself does not consider equilibrium.
As a next step, we calculated the proportions of liquid filling pores. The total volume of the bulk material consisting ofminerals, IOM, and pores (nanopore in GLM and/or phyllosilicates and micropore after ice) increases by the aqueous alterationfrom the initial volume (minerals, IOM, nanopore in GLM, and ice) by ~10 – 20%. Because such volume increase would notoccur for asteroids, we assumed that the initial bulk volume maintains constant by collapsing some of micropores. Liquid (ifpresent) occupies the remaining pores. Fig. 2 shows the liquid proportion filling pores, f, and the total porosity including liquid,f, as a function of W/R for the initial state before aqueous alteration. As we assumed liquid preferentially fills nanopores due tocapillarity, with increasing W/R, f(nanopore) increases first, and then f(micropore) increases after f(nanopore) reaches 100 %.
For the final state after aqueous alteration (Fig. 3), similar behavior of f’s can be seen, but f(nanopore) and f(micropore) are 0 %in Stages-1, and -2. These results showed that liquid partially occupies only nanopores if W/R is small, and even if W/R is large,
macropores are not fully filled with liquid, suggesting that pores are not fully filled with liquid during aqueous alteration. The
bulk density of Ryugu samples was determined to be 1.79 g/cm3, indicating the porosity of 31 % if we compared with the grain
density of CI meteorites [1]. If the porosity has not changed from the end of aqueous alteration to present, f of 31 % corresponds
to W/R of 0.19 (Fig. 3), where 100 % of nanopores and 61 % of micropores are filled with liquid, respectively, before thealteration (Fig. 2), and 55 % of nanopores and 0 % of micropores are filled with liquid, respectively, after the alteration (Fig. 3).
These suggest that aqueous alteration in the Ryugu parent body should occur through dissolution and reprecipitation in conditions
where not so much water present.

Figure 1. The volumes of phases after
aqueous alteration at different water/rockratio, W/R.

Stage-2
Stage-1 Stage-3

100

80

80

60

60

40

40

20
00micropore
nanoporepore (average)
porosity

0.2

0.4
0.6
W/R (mass)

0.8

2001

Figure 2. Liquid proportion filling poresand porosity before aqueous alteration.

Stage-4
f-III_ver4b_10_99100100808060604040

2000micropore
nanoporepore (average)
porosity

0.2

0.4
0.6
W/R (mass)

0.8

200
1

Porosiry (including liquid), φ (%)

100

Liquid proportion filling pore, f (%)

Stage-4

Porosiry (including liquid), φ (%)

Liquid proportion filling pore, f (%)

Stage-2
Stage-1 Stage-3

Figure 3. Liquid proportion filling poresand porosity after aqueous alteration.

References[1] Nakamura T. et al. 2022. Science. 10.1126/science.abn8671. [2] Yokoyama T. et al. 2022. Science
10.1126/science.abn7850. [3] Ito M. et al. 2022. Nature Astron. 10.1038/s41550-022-01745-5. [4] Tsuchiyama A. et al. 2022.
85th Annual Meeting of The Meteoritical Society 2022 6221.pdf. [5] Lodders K. et al. 2009. Landolt–Bornstein (Berlin:Springer), 42. [6] Mikouchi T. et al. 2022. 85th Annual Meeting of The Meteoritical Society 2022 6180.pdf. [7] Keller L.P.
and Messenger S. 2011. Geochim. Cosmochim. Acta 75:5336. [8] Alexander C.M.O’D. et al. 2007. Geochim. Cosmochim.
Acta 71:4380. [9] Xiaoyan Ji. et al. 2005. Ind. Eng. Chem. Res. 44:8419. [10] Longhi J. et al. 2005. Geochim. Cosmochim.
Acta 6: 529.