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

Introduction: The recent remote-sensing observation by Hayabusa2 provided a large amount of data to unravel the origin of Ryugu [1–3]. 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. The findings indicate the great abundance of dark materials and the subordinate amount of hydrous minerals in the surface, respectively [3]. These materials 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) [4, 5]. 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 [6]. 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 [7]. In the water-chondrite reactions, molecular hydrogen is generated through reduction of water by metal iron and FeO in chondrite, which elevate fH2 of fluid to H2 saturation level in some cases. Therefore, it was assumed that fH2 of fluid does not exceed water pressure (PH2O). Considering that the ice melting and subsequent water-chondrite reactions of the parent body starts from its center, the water-chondrite reactions that fH2 reaches PH2O potentially supply excess H2 to outer part of the parent body as ice melting proceeds outward. Therefore, water-chondrite reactions likely start under H2-rich conditions in the outer part of the parent body. Thus, two initial fH2 conditions were assumed; fH2 (initial) = 0 and fH2 (initial) = PH2O. These two fH2 (initial) conditions qualitatively reflect the inner and outer parts of the parent body, respectively. The thermodynamic calculations of waterchondrite reactions were conducted with EQ3/6 computer code [8]. The thermodynamic database required for the calculations was generated by SUPCRT92 [9] with thermodynamic data for mineral, aqueous species and complexes [10–16]. Thermodynamic parameters for a series of smectites were estimated by using the procedure of Wilson et al. [17]. Results and discussion: The calculations showed that stabilities of hydrous/anhydrous minerals, carbonate, pyrene change with temperature and water/rock mass ratio (W/R). In Case 1 (CO2-free), the altered chondrite consists of serpentine, troilite and subordinate amount of hydrous/anhydrous minerals (e.g., magnetite, 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 decrease in the amount of serpentine.