09:30 〜 09:45
[PPS01-03] Laboratory experiments using sulfate solutions and basaltic rocks: Implications for hydrothermal reactions in Europa
キーワード：熱水活動、エウロパ、水岩石反応、室内実験、海洋化学
The subsurface ocean of Jovian moon Europa is a prime candidate for life in our Solar system. Hydrothermal environments could exist on the seafloor and subseafloor. Hydrothermal activity would work as an effective source of chemical components, including materials essential for life (i.e., CHONPS and trace elements). However, the composition of hydrothermal fluids has been mainly estimated by numerical studies, not by laboratory experiments. In especially, trace elements, such as bioessential metallic elements (Fe, Co, Ni, and Mo), have not been investigated.
We performed hydrothermal experiments to simulate chemical reactions in hydrothermal environments in Europa. We used powders of an eucrite, an extraterrestrial basaltic rock, as the simulant for seafloor rocks. The starting rocks consist of orthopyroxene, Ca-rich clinopyroxene, Ca-rich plagioclase, and minor phases of iron sulfide (pyrrhotite) and iron oxides. Since Europa’s seawater would contain SO4 as its characteristics, we used solutions containing 0.01 and 0.1 mol/L of H2SO4 as simulant seawater. The reaction condition was set at 300 °C and 50 MPa.
In our experiments, fluids reach steady states at pH around 6, nearly neutral pH. Major changes during experiments are decreases in SO4 and Mg concentrations and increases in Ca concentrations. Based on analyses of rocks after experiments, the fluid chemistry would be controlled by dissolutions of Ca-rich silicates and formations of sulfate (CaSO4) and phyllosilicate. Hydrogen sulfide (H2S) is dissolved in fluids, possibly due to sulfide dissolutions and/or reduction of SO4. The fluid chemistry is consistent with our numerical estimates for Europa (Tan et al., 2021).
Fe is the richest metallic element in fluids, reaching 10-4 mol/L at the steady state. After experiments, Fe is found in silicates and sulfides (pyrrhotite, pyrite). Considering H2S in fluids, Fe could be controlled by dissolution or formation of sulfides. On the other hand, other metals (Co, Ni, Mo) are poor at 10-7 mol/L in fluids. They are mainly found in sulfides and oxides at less than 0.5 wt% in rocks. Their trace abundances in rocks could limit the dissolutions. After experiments, CaSO4 also contains Mo, suggesting that Ca-bearing molybdate (CaMoO4) formation suppresses Mo dissolutions.
Rocks in the experiments, eucrite, are considered as the crust of asteroid Vesta. Vesta experienced differentiation of the rocky core, forming crustal rocks rich in Ca and poor in metals. Based on our results, if Europa experienced differentiation, SO4 in seawater would be removed by CaSO4 formation and reduction of SO4. In addition, hydrothermal fluids would be poor in metals essential for life, except Fe. If the rocky core is thermally more active, higher-temperature fluids could leach out more abundant metals from rocks as Earth. Inversely, in the thermally inactive case, seafloor rocks would be undifferentiated and more mafic compositions, providing fluids poorer in metals, as observed in alkaline hydrothermal systems on Earth and possibly in Enceladus (Tan et al., in revision).
We performed hydrothermal experiments to simulate chemical reactions in hydrothermal environments in Europa. We used powders of an eucrite, an extraterrestrial basaltic rock, as the simulant for seafloor rocks. The starting rocks consist of orthopyroxene, Ca-rich clinopyroxene, Ca-rich plagioclase, and minor phases of iron sulfide (pyrrhotite) and iron oxides. Since Europa’s seawater would contain SO4 as its characteristics, we used solutions containing 0.01 and 0.1 mol/L of H2SO4 as simulant seawater. The reaction condition was set at 300 °C and 50 MPa.
In our experiments, fluids reach steady states at pH around 6, nearly neutral pH. Major changes during experiments are decreases in SO4 and Mg concentrations and increases in Ca concentrations. Based on analyses of rocks after experiments, the fluid chemistry would be controlled by dissolutions of Ca-rich silicates and formations of sulfate (CaSO4) and phyllosilicate. Hydrogen sulfide (H2S) is dissolved in fluids, possibly due to sulfide dissolutions and/or reduction of SO4. The fluid chemistry is consistent with our numerical estimates for Europa (Tan et al., 2021).
Fe is the richest metallic element in fluids, reaching 10-4 mol/L at the steady state. After experiments, Fe is found in silicates and sulfides (pyrrhotite, pyrite). Considering H2S in fluids, Fe could be controlled by dissolution or formation of sulfides. On the other hand, other metals (Co, Ni, Mo) are poor at 10-7 mol/L in fluids. They are mainly found in sulfides and oxides at less than 0.5 wt% in rocks. Their trace abundances in rocks could limit the dissolutions. After experiments, CaSO4 also contains Mo, suggesting that Ca-bearing molybdate (CaMoO4) formation suppresses Mo dissolutions.
Rocks in the experiments, eucrite, are considered as the crust of asteroid Vesta. Vesta experienced differentiation of the rocky core, forming crustal rocks rich in Ca and poor in metals. Based on our results, if Europa experienced differentiation, SO4 in seawater would be removed by CaSO4 formation and reduction of SO4. In addition, hydrothermal fluids would be poor in metals essential for life, except Fe. If the rocky core is thermally more active, higher-temperature fluids could leach out more abundant metals from rocks as Earth. Inversely, in the thermally inactive case, seafloor rocks would be undifferentiated and more mafic compositions, providing fluids poorer in metals, as observed in alkaline hydrothermal systems on Earth and possibly in Enceladus (Tan et al., in revision).