[M-IS12] Planetary Volcanology

[MIS12-02] Laboratory investigations of salt precipitation upon decompression and degassing of volatile-containing brines: Implication for carbonate mineral formations on Solar system icy bodies

*Masahiro Yoda^1,2, Yasuhito Sekine^2,3 (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Earth-Life Science Institute of Tokyo Tech, 3.Institute of Nature and Environmental Technology, Kanazawa University)

Keywords:Cryovolcanic activities, Salts mineralization on the icy bodies, Degassing upon decompression

Chemical compositions of subsurface oceans on Europa, Ceres, Enceladus and other icy bodies can be constrained based on observations of non-ice materials on the surfaces. Carbonate mineral (i.e., sodium carbonate (Na₂CO₃)) is indeed found on bright spots of Ceres (De Sanctis et al., 2016), suggesting the presence of carbonate-rich subsurface brine reservoirs and precipitations of the carbonate salts upon cryovolcanic activities. The mineralogical and chemical compositions of the salts would reflect not only the water chemistry of liquid reservoirs, but also eruption dynamics and degassing processes. For instance, the previous experimental studies showed that rapid freezing of brine results in formation of Na₂CO₃, compared with NaHCO₃ precipitation in slow freezing (e.g., Vu et al., 2017; Thomas et al., 2018). In addition to rapid freezing, decompression upon cryovolcanic activities could affect salt precipitations through degassing of dissolved volatiles from brine. However, no experimental studies have been conducted to investigate the effects of rapid decompression and degassing of volatile on salt precipitation.

Here, we perform laboratory experiments to investigate the effect of decompression on salt mineralization. To this end, we newly constructed an experimental system, which can simulate rapid decompression and degassing of volatile from brine. Through evacuation of gas in a vacuum chamber, volatile-containing brine evaporates and subsequently freezes. We collected salt residues after the experiments and analyzed their size and structure using SEM and mineralogical and chemical compositions using both of SEM and micro XRD. We also observed dynamics of eruption using a high-speed camera as a function of dissolved CO₂ gas in starting brines.

In the experiments, we find a clear dependence of precipitated mineralogical compositions on speed of decompression. Precipitations of Na₂CO₃ tend to be suppressed in slow decompression and ineffective degassing, resulting in precipitation of NaHCO₃. To form more Na₂CO₃, rapid decompression and effective degassing are favored. Together with the results of the previous studies (Vu et al., 2017; Thomas et al., 2018), the occurrence of Na₂CO₃ on the bright spots of Ceres implies that occurrence of explosive eruption of cryomagma with degassing and rapid freezing, rather than effusive eruption.