46. Temperature and Electrical Conductivity in the Upper Mantle

The temperature in the upper mantle is estimated on the basis of the mineralogical behaviour of mantle materials. Three assumptions are made in the estimation: i.e. (1) Kanamori's Vp-model is the representative of normal upper mantle in a tectonically active region, (2) the upper mantle is composed of olivine-rich peridotite and Fe/Mg ratio of mantle olivine is 1/9, and (3) the cause of the low velocity zone is partial melting. The upper mantle can be divided into two classes in a tectonically active region: (1) hot upper mantle, and (2) normal upper mantle. Two temperature distributions of a different kind are obtained for these mantles. Accordingly, two kinds of electrical conductivity distributions in the upper mantle are determined corresponding to the temperature distributions. The electrical conductivity in the hot upper mantle is two orders of magnitude, at least, higher than that in the normal upper mantle at depths shallower than several tens of kilometers. The high conductivity in the hot upper mantle might possibly correspond to "conductivity anomaly" in the upper mantle. The electrical conductivity thus estimated is lower than 10-13 emu even in the anomalous regions, as far as Fe/Mg ratio in mantle olivine [(Mg, Fe)2SiO4] is assumed to be 1/9. To explain the electrical conductivity higher than 10-12 emu as has been estimated in anomalous regions in some cases, the Fe/Mg ratio in mantle olivine should be assumed to be higher than 1/9 (e.g., 1.5/8.5) in the uppermost part of the hot upper mantle, not in the whole upper mantle. The point that a Fe-rich mantle should be assumed in hot regions seems important.|最近の,高温高圧実験から得られた,岩石や鉱物の,物理的性質や,鉱物学的性質に関するデータと,地震学的研究によつて得られた,上部マントルに関する諸事実とを,組合せて,上部マントルの温度を推定した.さらに,推定された温度分布と,最近得られた造岩鉱物,特に,オリビンの,電気伝導度のデータとを用いて,上部マントルの電気伝導度の分布を推定し,さらに,上部マントルの,電気伝導度異常について,その成因を考えた