Experimental chondrite–water reactions under reducing and low-temperature hydrothermal conditions: Implications for incipient aqueous alteration in planetesimals

Sakiko Kikuchi, Takazo Shibuya, Mariko Abe, Katsuyuki Uematsu

Geochimica et Cosmochimica Acta
Available online 11 November 2021


“The presence of hydrous minerals in carbonaceous chondrites has been considered as an important evidence for the former presence of liquid water in parent asteroids. However, the evolution of water–rock reactions in hydrous asteroids remains not well constrained. Here, we conduct water–rock type experiments and chemical equilibria calculations under low-temperature hydrothermal and reducing conditions to investigate the alteration process and secondary mineral assemblages of chondritic rock in the earliest alteration stage. Using synthetic chondrite (mixtures of olivine (forsterite95), orthopyroxene (enstatite95), silicate glass, troilite and Femetal) as a starting material, our experiments were conducted at temperatures of 25°C–80°C for time periods between 1 to 460 days at a water-to-rock mass ratio of 10. A combination of X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses revealed that the primary secondary phases consisted of pyrrhotite, an amorphous SiO2-rich phase and saponite at 80°C, while the secondary phase consisted of an amorphous SiO2-rich phase and saponite at 25°C. At both temperatures, the SiO2-rich phases and saponite densely covered the surface of the primary phases. The Fe/Mg ratios of saponite and amorphous SiO2-rich phases showed clear difference between 80°C and 25°C. Saponite that was formed at 80°C was richer in Fe than the initial silicate phases, and the highest Fe/Mg ratios were obtained in the saponite encrusting the troilite and Femetal. These results suggest that the Fe distributed from the troilite and Femetal induced the formation of Fe-rich saponite. Some of the secondary minerals observed from our alteration experiments were consistent with those expected by chemical equilibria calculations. However, the formation of serpentine, the dominant secondary mineral expected from chemical equilibria calculations, was not observed in our experiments up to 460 days, probably because the preferential dissolution of SiO2-rich silicate glass in the earliest stage of alteration induced the formation of saponite rather than serpentine. The secondary mineral assemblage and its morphological characteristics, as observed by our alteration experiments, showed similarities with carbonaceous chondrites such as CM2 and CO3 chondrites. This alteration might be explained by water–rock reactions at low temperatures and by the short time alteration. These findings better constrain the temperatures and timescales of aqueous alterations in hydrous asteroids, as well as the role of water in the early solar system bodies.”