16:30 〜 16:45
[SIT20-11] Detection of water in the Martian mantle by electrical conductivity measurement
キーワード：火星、マントル、水、電気伝導度
Water in the Martian mantle has had profound effects on its geological history, volcanic activity, and potential habitability. Although Martian meteorites contain hydrous minerals and suggest the mantle may hold 50–300 ppm H2O, similar to Earth's upper mantle, little is known about how water is distributed throughout the Martian mantle. Estimating the water content of the Martian mantle provides important constraints on the water inventory to understand its sources, sinks, and secular variation.
Electrical conductivity is useful in studying the composition, mineralogy and temperature of the planetary deep interior, if electrical conductivity is well constrained. The electrical conductivity of the mantle constituent minerals is mostly influenced by proton (H) and small polaron conduction (electron holes hopping between ferrous and ferric iron) mechanisms. In other words, conductivity is sensitive to small amounts of hydrogen and iron [1]. Since the Martian mantle has FeO-rich composition compared to the Earth’s mantle, the effect of proton conduction may be masked by small polaron conduction. Therefore, to estimate the water content of the Martian mantle, the contributions of small polaron must be considered to separately determined to reach a full understanding of the electrical conductivity of olivine and its high-pressure polymorphs.
In this study, we measured electrical conductivity of hydrous olivine and ringwoodite aggregates at high pressure to constrain the water content in the FeO-rich Martian mantle. The contributions of proton conduction significantly raise the conductivity against those of small polaron conduction, and ringwoodite conductivity is considerably higher than that of olivine. Water is expected to exist in the lithosphere shallower than 200 km. On the other hand, at depths of more than 200 km, the dry model can explain well the currently available conductivity–depth profile predicted from electromagnetic studies [2].
Reference: [1] Yoshino T. and Katsura T. (2013) Ann. Rev. Earth Planet. Sci., 41, 605–628. [2] Civet F. and Tarits P. (2014) Earth Planet Space, 66, 85.
Electrical conductivity is useful in studying the composition, mineralogy and temperature of the planetary deep interior, if electrical conductivity is well constrained. The electrical conductivity of the mantle constituent minerals is mostly influenced by proton (H) and small polaron conduction (electron holes hopping between ferrous and ferric iron) mechanisms. In other words, conductivity is sensitive to small amounts of hydrogen and iron [1]. Since the Martian mantle has FeO-rich composition compared to the Earth’s mantle, the effect of proton conduction may be masked by small polaron conduction. Therefore, to estimate the water content of the Martian mantle, the contributions of small polaron must be considered to separately determined to reach a full understanding of the electrical conductivity of olivine and its high-pressure polymorphs.
In this study, we measured electrical conductivity of hydrous olivine and ringwoodite aggregates at high pressure to constrain the water content in the FeO-rich Martian mantle. The contributions of proton conduction significantly raise the conductivity against those of small polaron conduction, and ringwoodite conductivity is considerably higher than that of olivine. Water is expected to exist in the lithosphere shallower than 200 km. On the other hand, at depths of more than 200 km, the dry model can explain well the currently available conductivity–depth profile predicted from electromagnetic studies [2].
Reference: [1] Yoshino T. and Katsura T. (2013) Ann. Rev. Earth Planet. Sci., 41, 605–628. [2] Civet F. and Tarits P. (2014) Earth Planet Space, 66, 85.