Thermodynamic modeling of water-rock reactions in the parent body of ryugu

Introduction: The recent remote-sensing observation by Hayabusa2 is providing a large amount of data to unravel the origin of Ryugu. The Near Infrared Spectrometer (NIRS3) onboard Haybusa2 revealed that the IR reflectance of the global surface of Ryugu is extremely low (~0.02) and the spectra include small but clear absorption at 2.72 μm. These findings indicate the great abundance of dark materials and hydrous minerals in the surface rocks, respectively. Furthermore, the presence of absorption at 3.05 μm derived from NH4-bearing minerals is still uncertain but its possibility cannot be excluded. These materials on Ryugu are important clues to constrain the conditions of aqueous alteration such as the temperature experienced by the parent body and the original volatile compositions during accretion stage. In this work, we conducted thermodynamic modeling of chondrite-water reactions under various conditions to establish a model explaining the aqueous alteration of the parent body. Modeling Methods: In the thermodynamic calculations, a mean composition of CV chondrites was assumed for the initial bulk rock (minor amount of carbon, nitrogen and chlorine are also included) [1, 2]. For the initial fluid, four cases were assumed; CO2 concentration is 0, 1, 3 and 10 mol% (Cases 1–4, respectively) relative to water while the latter three cases also include NH3 (0.5%) and H2S (0.5%) additionally [3]. The equilibrium temperature and pressure were assumed to be 0, 100, 200, 300 and 350 °C, and vapor pressure of water. In the calculations, pyrene was considered as a representative of polycyclic aromatic hydrocarbon while C1 compounds except CH4 were included as soluble species [4]. The thermodynamic modeling of water-chondrite reactions were conducted with EQ3/6 computer code [5]. The thermodynamic database required for the calculations was generated by SUPCRT92 [6], with thermodynamic data for mineral, aqueous species and complexes [7–13]. Thermodynamic parameters for a series of smectites were estimated by using the procedure of Wilson et al [14]. During the simulated seawater-chondrite reactions, incorporation of water into hydrated minerals substantially condenses dissolved species in reacted fluids under conditions of low water to rock mass ratio (W/R), which elevate salinity and ionic strength beyond the appropriate values of modeling (2–3 molal). Therefore, the initial W/R was assumed to be 0.2–10 in the modeling. Results: The calculations showed that stabilities of hydrous minerals, carbonate, pyrene and NH4-bearing minerals change with temperature and W/R value, as detailed below. Stability of hydrous minerals. In Case 1 (CO2-free), the altered chondrite consists of serpentine, troilite and subordinate amount of hydrous/unhydrous minerals (e.g., magnetite, Na-saponite, gibbsite and chlorite) at 0–300 °C. However, with increasing temperature above 300 °C, olivine and clinopyroxene become major phases as the amounts of serpentine, chlorite and magnetite decrease. At 350 °C, olivine becomes the most abundant minerals in conjunction with disappearance of serpentine. Similar temperature dependencies of hydrous mineral stabilities were also shown at low W/R in Cases 2–4 (CO2=1–10%) whereas carbonate is predominant at high W/R (Fig. 1).