[P-PS24_1PM1] Origin and evolution of materials in space

[PPS24-04] Hydration of amorphous forsterite grains in protoplanetary disks

*Daiki YAMAMOTO¹, Minami KURODA¹, Shogo TACHIBANA² (1.Department of Natural History Sciences, Hokkaido University, 2.Department of Natural History Sciences, Hokkaido University)

Keywords:protoplanetary disk, Mg-silicate, crystalline, amorphous, forsterite, hydrous mineral

Hydrous silicate dust can be thermodynamically stable at low temperatures ( _~ 200K) in protoplanetary disks with the solar-system abundance of elements. Theoretical and experimental investigation (Fegley and Prinn, 1989; Imae et al., 1999) have shown that the hydration reaction between crystalline Mg-silicates and water vapor is too sluggish to occur during a lifetime of protoplanetary disks. However, infrared spectroscopic observation and investigation of extraterrestrial materials have shown that both crystalline and amorphous Mg-silicates may be present in protoplanetary disks and in the protosolar disk. Amorphous silicates are thermodynamically unstable, and thus could be hydrated more rapidly at higher temperatures than crystalline silicates (Nagahara and Ozawa, 2011). If hydration of amorphous silicates occurs within a disk lifetime, hydrated dust could be a source of water to terrestrial planets. In this study, in order to investigate hydration of amorphous Mg-silicates, we conducted closed-system reaction experiments of amorphous forsterite and water vapor in sealed glass tubes at temperatures of 1023-423K and P_H2O of 0.05 -50 bar for 2-1344 hours. Run products were analyzed with FT-IR and XRD. We found that samples reacted at temperatures below 723K and P_H2O of 5-50 bar showed sharp 3μm absorption features, clearly suggesting the formation of hydrous phase(s). XRD analyses of some run products showed that the hydrous phase was serpentine. Because the stable temperature of hydrous phase(s) at P_H2O of 5-50 bar is _~ 523K, the present results indicate that hydrous Mg-silicates can be formed metastably from amorphous forsterite at higher temperatures than the thermodynamic prediction. Moreover, hydration of amorphous forsterite occurred after 2-hour heating at 623K and P_H2O of 5 and 50 bar, implying that the activation energy for hydration of amorphous forsterite is much smaller than that for crystalline forsterite. Hydration reaction seems to have little P_H2O dependence at P_H2O of 5-50 bar, indicating that hydration is controlled by a reaction between amorphous forsterite with saturated adsorbing water molecules. No hydration occurred, however, at 523K and P_H2O <1 bar, which could be due to less effective adsorption of water molecules at low-pressure conditions.