Carbon isotopic thermometry and geobarometry of sillimanite isograd in thermal aureoles: the depth of emplacement of upper crustal granitic bodies

Carbon isotope fractionation between coexisting calcite and graphite (C ) has been studied in metamorphosed limestones from three thermal aureoles around Cretaceous granitic bodies (i.e., Tanohata, Tono, and Senmaya aureoles) in the Kitakami Mountains, Northeast Japan. C in each aureole decreases toward the granitic bodies, and becomes virtually uniform near the sillimanite isograd for metapelites, although calcite has variable isotopic ratios reflecting the original sedimentary compositions. The relationships indicate that isotopic equilibrium has been attained in metamorphosed limestone of sillimanite grade. Estimated C at the sillimanite isograd is similar in the Tanohata and Tono aureoles, but different in the Senmaya aureole with smaller carbon isotopic fractionations. From the temperature dependence of C and the negative dP/dT of andalusite–sillimanite equilibrium, we conclude that the sillimanite isograd in the Senmaya aureole was under higher temperature and lower pressure than in the other two localities. Temperatures at the sillimanite isograd are estimated by using existing calibrations of carbon isotopic exchange between calcite and graphite, whereas pressures are estimated from carbon isotopic temperatures and the andalusite–sillimanite equilibrium (Holdaway and Mukhopadhyay 1993a). Consistency of the P–T estimates is examined in the light of phase equilibria in the pelitic system. The estimated pressures at the sillimanite isograd are at about 2.1–2.7(±0.2) kbar for the Tanohata and Tono aureoles and less than 1 kbar for the Senmaya aureole, respectively. Geobarometry of sillimanite isograd in thermal aureoles indicates a marked difference in the depth of solidification of upper crustal granitoids: the Senmaya pluton has intruded and solidified at a very shallow level of less than 4 km whereas the Tanohata and Tono plutons are more deep-seated (ca. 8–10 km). The method can also be an effective tool in studying low-pressure type metamorphism in which geothermobarometry using garnet is not always applicable.Editorial responsibility: J. Hoefs     

Topography of the Seismic Velocity Discontinuities beneath the Chugoku and Shikoku Districts,Southwest Japan

The first P-arrival-time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the Conrad and Moho discontinuities beneath the Chugoku and Shikoku districts, southwest Japan, as well as to determine earthquake hypocenters and P-wave station corrections. The depth to the discontinuity was estimated by minimizing the travel-time residuals of more than 8700 first P arrivals observed at 55 seismic stations. The Conrad and Moho discontinuities are located within depth ranges of 15–25 km and 30–40 km, respectively. The Moho is deeper under the mountain area than under the Seto Inland Sea area, and especially deep under the Pacific Coast of the Shikoku district and the mountain area in the Chugoku district. The depth variation of the Moho is quite similar to the Bouguer gravity anomaly distribution and the lateral variations of the P-wave velocity. The deep Moho under the southern Shikoku is located at the portion in which the continental Moho under the island arc meets the oceanic Moho that is the boundary interface between the oceanic crust and the Philippine Sea (PHS) plate dipping toward the back arc. Although there are high mountains in the northern and middle Shikoku, the Moho is not so deep because subduction of the PHS plate prevents the Moho from getting deep, while the Moho is deep due to isostatic balance under the mountain area in the Chugoku district. In addition, we indicated the possibility that the upper boundary of the oceanic crust just above the high-velocity PHS plate is in contact with the deep Moho under the western Chugoku. The contact of the Moho with the oceanic crust can explain the markedly negative gravity anomaly observed in the western Chugoku and the later phase that appears just after the first P arrival from local earthquakes.     

Transformation functions of soil color and climate

Measurements on modern soil color suggest well functional relationships between the soil formation process and the present climatic factors. The redness and yellowness of soil are chiefly caused by the contents of hematite and fullonite, and their correlations to climate are the best in humid regions in tropic and warm temperate regions. The lightness of soil mainly correlates to the organic accumulation, humification and carbonatization processes, and its correlation to climate can only be found in the humid-arid extratropical belt. The humidity and surface roughness of soil have so strong influence on soil color that there are great errors on the measurement of colorness in the field. The study on soil colors of typical loess sections shows that soil color can record the characteristics of Asia monsoon and the global climatic fluctuations well at millennial and ten-thousand-year scales. It can also indicate the pedogenesis and the climatic characteristics which magnetic susceptibility could not be reflected in humidity areas. Therefore, soil color can be used as a new climatic proxy which is easy and quick to measure, and will make an active influence on the study of global changes, geomorphology and Quaternary.

     

Relation between hard X-ray spectra and electron energy spectra

The relationship between hard X-ray spectra and energetic electron spectra in solar X-ray bursts is investigated, and a simplified cross-section for bremsstrahlung which is applicable to the region of mildly relativistic energies is proposed. Using the proposed cross-section, we solve an integral equation to obtain the electron energy spectrum. The validity of the proposed cross-section is checked by comparing the spectrum calculated by the exact Bethe-Heitler formula. A good agreement between two calculated spectra is obtained up to 10 MeV energy with an accuracy of 20 %.     

SELENE project status

SELENE (Selenological and Engineering Explorer) project started as a joint mission of the former ISAS (Institute of Space and Astronautical Science) and the former NASDA (National Space Development Agency: the two organizations were merged into JAXA in 2002) of Japan in 1998. The launch target is rescheduled for 2006 due to delay of completion of launch vehicle, H-IIA. The SELENE project is now under a sustained design phase. The flight model components were manufactured, and the interface tests between the bus-system and the mission instruments were completed by the end of March 2004. The functional checks and calibration for the flight model components are being carried out at present. From the beginning of 2005, the final assembly tests will start.     

High-pressure transformations of ilmenite to perovskite, and lithium niobate to perovskite in zinc germanate

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO₃. ZnGeO₃ ilmenite transformed into perovskite at 30.0 GPa and 1300±150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite–perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO₆ octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO₆ octahedra in the ilmenite–perovskite transition. The correlation of quenchability with rotation angle of GeO₆ octahedra for other germanate perovskites is also discussed.     

Long-term high-frequency measurements of dibromomethane in the atmosphere at algae-rich and algae-poor coastal sites

Dibromomethane (CH₂Br₂), a natural stratospheric ozone depleting substance, is mostly emitted from the ocean, but the relative importance of coastal (or macroalgae) and open ocean emissions is unknown. We made long-term high-frequency measurements of CH₂Br₂ concentrations at two remote coastal sites in Japan, on the subtropical Hateruma Island (poor in macroalgae) and at Cape Ochiishi (rich in macroalgae). CH₂Br₂ concentrations at Hateruma showed prominent seasonal variation, being lower in summer (around 0.94 ppt) than in winter (around 1.23 ppt). In contrast, CH₂Br₂ concentrations at Ochiishi were highly variable, often exceeding 2 ppt in the summer but with minimum baseline concentrations close to those from Hateruma; in the winter the concentrations were almost constant at about 1.3 ppt. Analysis of the data suggested that (1) emissions from macroalgae were not likely to extend offshore, but instead were localized near the shore, (2) strong macroalgal emissions of CH₂Br₂ were almost limited to the summer, but it was not reflected in the seasonality of the baseline concentrations of CH₂Br₂ in the atmosphere, and therefore (3) macroalgal or coastal emissions of CH₂Br₂ in the temperate zone might have a rather limited contribution to the global CH₂Br₂ sources. These findings are especially important for the understanding of the tropospheric and stratospheric bromine budget.     

Geochemical relation between gas hydrates and water venting in the seismic blanking zone on the northern Cascadia continental margin offshore Vancouver Island, Canada

The isotopic composition (δD and δ¹⁸O) and chloride concentration (Cl⁻) of pore waters from the northern Cascadia continental margin offshore Vancouver Island were measured to characterize the relations between the water flow regime and the distribution, formation and dissociation of gas hydrates. The δD values of pore waters in gas hydrate-bearing sediments are slightly higher ( 1‰) than those of seawater as the result of gas hydrate dissociation during core recovery and handling. Within the seismic blanking zone, the δD values were slightly lower (− 1‰) than values measured from sites outside the blanking area (0‰). We attribute these differences to 1) distillation of D-rich water during hydrate formation in the center of the blanking zone and 2) limited migration of pore water between inside and outside of the blanking zone due to different fluid fluxes. In contrast, the δ¹⁸O values and Cl⁻ concentrations do not show significant spatial variation due to decreased isotopic fractionation of oxygen and small fraction of chloride relative to hydrogen isotope during gas hydrate formation. The δD value of pore water, therefore, appears to be a sensitive indicator of gas hydrate occurrence. We estimate that gas hydrate occupied at least 2.0 to 6.3% of sediment pore space using δD distribution in this area.