Extensive normal faulting during exhumation revealed by the spatial variation of phengite K–Ar ages in the Sambagawa metamorphic rocks,central Shikoku,SW Japan

Metamorphic rocks experience change in the mode of deformation from ductile flow to brittle failure during their exhumation. We investigated the spatial variation of phengite K–Ar ages of pelitic schist of the Sambagawa metamorphic rocks (sensu lato) from the Saruta River area, central Shikoku, to evaluate if those ages are disturbed by faults or not. As a result, we found that these ages change by ca 5 my across the two boundaries between the lower‐garnet and albite–biotite, and the albite–biotite and upper‐garnet zones. These spatial changes in phengite K–Ar ages were perhaps caused by truncation of the metamorphic layers by large‐scale normal faulting at D₂ phase under the brittle‐ductile transition conditions (ca 300°C) during exhumation, because an actinolite rock was formed along a fault near the former boundary. Assuming that the horizontal metamorphic layers and a previously estimated exhumation rate of 1 km/my before the D₂ phase, the change of 5 my in phengite K–Ar ages is converted to a displacement of about 10 km along the north‐dipping, low‐angle normal fault documented in the previous study. Phengite ⁴⁰Ar–³⁹Ar ages (ca 85 to 78 Ma) in the actinolite rock could be reasonably comparable to the phengite K–Ar ages of the surrounding non‐faulted pelitic schist, because the K–Ar ages of pelitic schist could have been also reset at temperatures close to the brittle–ductile transition conditions far below the closure temperature for thermal retention of argon in phengite (about 500–600°C).     

Electrical conductivity jump produced by the α-β-δ transformation in Mn2GeO4

The electrical conductivity of three polymorphs of Mn₂GeO₄ was measured under high pressures in the temperature range of 300–1200 K. It was found that the electrical conductivity increases discontinuously due to the transformation both from olivine structure (α) to modified spinel structure (β) and from β to strontium plumbate structure (δ). The amount of discontinuous change is about one half order of magnitude from α to β and one third order of magnitude from β to δ at 1200 K. In order to see the effect of the presence of iron ions, the electrical conductivity of the solid solution of (Mn_0.9Fe_0.1)₂GeO₄ was also measured. It was found that at low temperatures, where impurity conduction may be dominant, the solid solution is more conductive than the pure Mn₂GeO₄, but at high temperature no significant differences were observed between the solid solution and pure Mn₂GeO₄ in all polymorphs.A phase transformation from modified spinel structure to strontium plumbate structure is considered to be one of the plausible transformations occurring at a depth around 650 km in the earth's mantle. These experiments suggest that if this kind of transformation occurs in the mantle, some degrees of discontinuous change in electrical conductivity may be expected around 650 km.     

Experimental study on pumice and obsidian

Thermal experiment on the pumice at atmospheric pressure shows that welding begins at about 900° C and complete melting occurs at 1350° C. It is noticed that FeO content of pumice decreases first with increasing temperature, attaining minimum value at 1100° C, and then again increases with increasing temperature. Therefore, an equilibrium is expected in the presence of liquid phase as follows:

$$4FeO + O_2 = 2Fe_2 O_3 .$$     

Field measurement and modeling of near-bed sediment transport processes with fluid mud layer in Tokyo Bay

Tokyo Bay is one of the estuaries in Japan with a high population of almost 26 million people in the basin area. One of the major concerns for the environment in this water area is the decreasing ecosystem functions including the deterioration of water and sediment qualities caused by various anthropogenic activities. Since the bottom sediments around almost the entire area of the inner bay consist of fine materials with a high organic content, which cause the deterioration of water quality through processes such as hypoxia, an understanding of the fine sediment dynamics in the Bay is crucial for an environmental assessment of the water area. This paper proposes a model for the key processes of fine sediment dynamics, which reflects field data about muddy bed structures and their dynamics obtained during the monitoring campaign in 2007. One of the specific features of the sediment in the Bay at present is the persistent existence of fluid mud layers (water content over 300?%) with a thickness of around a few decimeters, which might be caused by deposition of abundant organic particles due to eutrophication. The present study shows that diffusion flux model delivers quite reliable results for estimating erosion flux from the top of fluid mud layers after calibrating the model parameter against the time series data of vertical flux measured by an acoustic Doppler velocimeter system. This study also derives analytical solutions, based on the Bingham fluid concept, of advection flux in the fluid mud layer on which external shear stress force is applied.     

CaPtO3 as a novel post-perovskite oxide

Structural, morphological, magnetic, and thermal properties have been investigated for a novel post-perovskite oxide CaPtO₃ synthesized under high pressure. By comparing obtained structural parameters with those for known post-perovskite compounds, we argue that the chemical bond has a strong covalent character. Precise measurements of the Langevin susceptibility χ ₀ = −9.6 × 10⁻⁵ emu/mol and Debye temperature θ ∼ 470 K provide a good opportunity to confirm the reliability of first-principle calculations on predicting physical properties of the Earth’s D” layer.     

Crystallization temperature of wyomingite from Leucite Hills

Wyomingite collected from Leucite Hills is composed mainly of leucite, diopside, phlogopite, and small amounts of apatite, calcite, magnetite and rare amphibole, and is characterized by very high content of potash. Thermal experiments at atmospheric pressure indicate that the liquidus phase is always diopside with liquidus temperature of 1320 °C, and solidus temperature is about 1000 °C. Various kinds of melt inclusions are abundant in all constituent minerals. They comprise mono-phase (glass only), two-phase (gas+glass), three-phase (gas+glass+one crystalline phase) and multi-phase (gas+glass+more than two crystalline phases) inclusions. Thermal experiments have been made on these inclusions in phlogopite, diopside, and leucite in order to estimate the temperature of crystallization by homogenizing these inclusions. The results show that the crystallization of wyomingite began with formation of phlogopite accompanied by diopside at 1270 °C. Although diopside ceased crystallization at 1220 °C recurrent crystallization of phlogopite was noticed between 1120 ° and 1040 °C. Leucite crystallized out abundantly between 1250 ° and 1150 °C. Complete solidification of wyomingite occurred at about 1000 °C.     

Compressibility of calcium ferrite-type MgAl2O4

 In situ X-ray diffraction experiments of calcium ferrite-type MgAl₂O₄ have been carried out using a diamond anvil cell combined with synchrotron radiation and an imaging plate X-ray detector under hydrostatic pressures up to 9 GPa. The observed unit-cell volumes at various pressures were fitted to the Birch-Murnaghan equation of state, yielding a bulk modulus of K _{T 0}= 241(3) GPa when K′ _{T 0}=4 is assumed. This relatively large bulk modulus of calcium ferrite-type MgAl₂O₄ is consistent with that expected from the systematic relation between bulk modulus and molar volume for the most compounds possessing fcc oxygen packing. Received March 5, 1996/Revised, accepted October 15, 1996