Kentaro Omura

Experimental study of pressure dependence of electrical conductivity of olivine at high temperatures

Abstract The electrical conductivity of sintered polycrystalline olivines (Mg 1 − x Fe x ) 2 SiO 4 ( x = 10, 28, 52, 76 and 100%) has been measured at high pressures (2.9–7.0 GPa) and high temperatures (900–1900 K). Measurements at constant temperature and various pressures show that electrical conductivity increases with pressure in the case of high Fe content, whereas it decreases in the case of low Fe content. The results are interpreted as being due to two competing conduction mechanisms: ionic and electronic, which have different pressure dependences.

Change of electrical conductivity of olivine associated with the olivine−spinel transition

Abstract The electrical conductivity σ of (Mg1−x,Fex)2SiO4 was measured under the stable conditions for olivine (α-phase) and spinel phases (β- and γ-phases). The present measurements cover much lower iron-content olivines (X = 0.28 and 0.10) and higher temperatures (∼ 1000 to 1200°C) than previous works. Conductivity changes associated with the phase transitions ( σ γ σ α and σ β σ α ) were observed. They were ∼ 10 for σ γ σ α and ∼ 2 for σ β σ α at about 1200°C, independent of Fe content. These values are significantly smaller than those obtained so far for iron-rich samples at low temperatures in previous works. The activation energy E* of electrical conductivity of the spinel phase was slightly smaller by about 0.1 eV than that of the olivine phase of ∼ 0.8 to 1.4 eV. It indicates that the conductivity change depends only slightly on temperature. These data can be applied to the condition at the base of the Earths upper mantle. The measured conductivity change of one order of magnitude owing to the olivine-spinel transition is much smaller than the steep increase of conductivity of two orders of magnitude at the mantle transition zone estimated from geomagnetic observations.

Carbon-forming reactions under a reducing atmosphere during seismic fault slip

Graphite is a well-known solid lubricant and can be as important as clay minerals in reducing the frictional strength of faults. Some natural fault zones contain carbonaceous material (CM) even where host rocks do not contain it, and seismic fault motion can promote the graphitization of low-grade CM. Thus, the origin of CM in fault zones is an important issue in fault mechanics. Previous high-velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion, but most experiments have been conducted in an atmosphere under oxic conditions. Here we report experimental results on Carrara marble (free of CM), conducted under N2 or H2 atmospheres at a slip rate of 1.3 m/s and normal stresses of 2.0–3.1 MPa. A small amount of blackish material formed in generated gouge only under reducing conditions with the H2 atmosphere, and Raman spectroscopic analysis revealed the presence of CM (amorphous carbon) in the material. The CM is attributable to (1) the generation and pyrolitic dissociation of CH4, and/or (2) a reduction reaction of emitted CO2 due to calcite decomposition. We confirmed the formation of CH4 using gas chromatography. The CM produced in experiments resembles CM in the Nojima fault (Japan) gouge in terms of Raman spectra. The granitic host rock of this fault is free of CM, and calcite is precipitated close to the CM; therefore, the CM probably formed through processes similar to those simulated in our experiments. Future research should investigate the amount and origin of CM in natural fault zones.

Fission-track analysis of the Atotsugawa Fault (Hida Metamorphic Belt, central Japan): fault-related thermal anomaly and activation history

Abstract Fission-track (FT) thermochronology was applied to the Atotsugawa Fault in the Hida Metamorphic Belt, central Japan, to detect any ancient thermal anomaly associated with fault displacements. Apatite and zircon grains from gouges (c. 2 cm wide) and fractured rocks (c. 10 cm) at six fracture zones within a 15 m-wide fault zone were dated. Most of the zircon (120–150 Ma) and apatite (44–60 Ma) ages agree well with emplacement ages for the granites that intrude the Hida Belt. The discordance in zircon and apatite FT ages is interpreted to reflect cooling due to regional uplift and associated erosion. A thermal anomaly was identified at one of the gouge zones that showed an exceptionally young apatite age (c. 32 Ma) with an unimodal FT length distribution. It presumably indicates secondary heating induced by frictional slip during an earthquake, possibly giving a younger limit of the initiation of the activity in the Hida Belt.