Pressure and temperature dependence of cation distribution in Mg-Mn olivine

Synthetic (Mg_0.51, Mn_0.49)₂SiO₄ olivine samples are heat-treated at three different pressures; 0, 8 and 12 GPa, all at the same temperature (～500° C). X-ray structure analyses on these single crystals are made in order to see the pressure effect on cation distribution. The intersite distribution coefficient of Mg and Mn in M1 and M2 sites, K _D = (Mn/Mg) _M1/(Mn/Mg) _M2, of these samples are 0.192 (0 GPa), 0.246 (8 GPa) and 0.281 (12 GPa), indicating cationic disordering with pressure. The small differences of cell dimensions between these samples are determined by powder X-ray diffraction. Cell dimensions b and c decrease, whereas a increases with pressure of equilibration. Cell volume decreases with pressure as a result of a large contraction of the b cell dimension. The effect of pressure on the free energy of the cation exchange reaction is evaluated by the observed relation between the cell volume and the site occupancy numbers. The magnitude of the pressure effect on cation distribution is only a fifth of that predicted from the observed change in volume combined with thermodynamic theory. This phenomenon is attributed to nonideality in this solid solution, and nonideal parameters are required to describe cation distribution determined in the present and previous experiments. We use a five-parameter equation to specify the cationic equilibrium on the basic of thermodynamic theory. It includes one energy parameter of ideal mixing, two parameters for nonideal effects, one volume parameter, and one thermal parameter originated from the lattice vibrational energy. The present data combined with some of the existing data are used to determine the five parameters, and the cation distribution in Mg-Mn olivine is described as a function of temperature, pressure, and composition. The basic framework of describing the cationic behavior in olivine-type mineral is worked out, although the result is preliminary: each of the determined parameters is not accurate enough to enable us to make a reliable prediction.     

Fine structure beneath the Tohoku district, northeastern Japan arc, as derived by an inversion of P-wave arrival times from local earthquakes

Three dimensional P-wave velocity structure beneath the Tohoku district, northeastern Japan arc, is investigated by an inversion of arrival times from local earthquakes using the method originally due to Aki and Lee (1976).In the crust (0–32 km depth) a low-velocity region is found along the volcanic front and its vicinity. Velocities at depths of 32–65 km are low beneath the regions where many Quaternary volcanoes and geothermal areas are distributed. In the region deeper than 65 km, the subduction of the Pacific plate is clearly revealed, and the mantle structure above the descending plate is rather uniform. These features suggest that volcanic activities have relation to the upper mantle structure. The results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone.     

Fracture strength of dry silicate rocks at high confining pressures and activity of acoustic emission

Three dry silicate rocks, gabbro, dunite and eclogite, were triaxially compressed up to a confining pressure of 3 GPa at room temperature. These rocks exhibited brittle fracture behavior up to the highest confining pressure. The change of the mechanism of fracture in the brittle region is suggested from the measurement of the compressive fracture strength and the activity of acoustic emission. The existence of the “high-pressure brittle-fracture” phase is proposed. The fracture strength increased with increase of confining pressure. The increasing rate of strength was lowered at a value of confining pressure: at about 0.8 GPa on gabbro; at about 1.0 GPa on dunite; and at about 1.5 GPa on eclogite. At lower confining pressures than the above value, the acoustic emission rate began to increase at the onset of dilatancy and increased rapidly followed by fracture as the axial stress was increased. At the higher confining pressures, however, the acoustic emission rate did not increase rapidly before final fracture, and stayed constant to the fracture. The similar behavior was shown on the granite studied previously. It is interesting that the frictional strength forms the boundary between “low- and high-pressure brittle-fracture” phases.     

A New Coastal Marine Ecosystem Model Study Coupled with Hydrodynamics and Tidal Flat Ecosystem Effect

A new coastal marine ecosystem model was developed, which was composed of pelagic and benthic ecosystems, and was applied to Mikawa Bay, Japan. This model deals with variations of biochemical and physical interactions among dissolved oxygen and C–N–P species (composition formed out of carbon, nitrogen and phosphorus elements) so that it resolves the flux dynamics of carbon, nitrogen, phosphorus and oxygen elements. The physical and biochemical mechanism figured in this model is constructed for the purpose of simulating the estuarine lower trophic ecosystem, in areas where the sea was too deep for light to reach the sea-bottom. As a result of coupling the benthic with pelagic system, the effect of process of sedimentation and nutrient diffusion back to the pelagic system could be indicated. In addition, by implementing the tidal flat ecosystem model's calculation result, the integrated model can include the effect of water purification in tidal flats where the light can reach the sea-bottom, and where sea-weed, sea grass and benthic algae exist. In this study, the model indicates that oxygen-depleted water exists at the sea-bottom especially in summer mainly caused by an increase of oxygen consumption in the benthic system and a decrease of the vertical mixing water process. Furthermore, by comparing the case – with the tidal flat ecosystem model and the case without it, the effect of water purification of tidal flat estuaries was indicated. From the viewpoint of a short time scale, the tidal flat has the potential to restrict red tide (rapid increase of phytoplankton), and from the viewpoint of a long time scale, it restricts the sedimentation of detritus. Restricting the sedimentation prevents oxygen-depleted water occurring in the coastal marine system of Mikawa Bay.     

Interferometer observation of pulsating sources associated with a type IV solar radio burst

An extremely complex outburst, part of which showed unsually rapid intensity fluctuations of a few second interval, was observed on 1970 November 5 with the 160 MHz interferometer of the Nobeyama Solar Radio Station. The pulsating source, which was stable in position and strongly circularly polarized ( 60 %), had an extension as large as 17 (7.5 × 10⁵ km) in the east-west direction. The structure of the source remained unchanged while the source darkened and brightened repeatedly. The change of the source brightness occurred in a time shorter than a second.Two alternative mechanisms responsible for the pulsating phenomenon are suggested; (1) gyroresonance absorption of continuum radiation by a fast particle beam injected in a quasi-periodic manner into a large region of weak magnetic field, or (2) magnetohydrodynamic oscillation of the continuum source itself, which is intrinsically much smaller than observed. It is observed as a large source as a consequence of scattering of the emitted radiation in a region situated above the source.     

Double-planed deep seismic zone and anomalous structure in the upper mantle beneath northeastern Honshu (Japan)

Seismicity located by using the most recent data obtained from the high-gain seismograph network of Tohoku University shows that the deep seismic zone beneath northeastern Honshu, Japan, is composed of two thin planes which are parallel to each other and are 30–40 km apart. Focal mechanisms derived from the earthquakes in the upper plane are reverse-faulting, or, some of them, down-dip compression. As a contrast, those in the lower plane are down-dip extension. The location of the upper boundary of the descending lithospheric slab, inferred from the arrival-time difference between ScS and ScSp waves and from the travel-time anomaly of intermediate-depth earthquakes observed at the small-scale seismic array, coincides exactly with the upper plane of the double-planed deep seismic zone. Anelasticity (1/Q) structure of the upper mantle consists of three distinct zones: a high-Q (Qs− 1500) inclined lithospheric slab, an intennediate-Q (Qs−350) land-side mantle between the Pacific coast and the volcanic front, and a low-Q (Qs − 100) land-side mantle between the volcanic front and the coast of the Japan Sea.The evidence obtained here provides valuable information as to the definition of the type of mechanism producing the plate motion beneath island arcs.     

Future projections of Indian Summer Monsoon under multiple RCPs using a high resolution global climate model multiforcing ensemble simulations

Climate Dynamics - Present-day simulations (1983–2003) of a global climate model of 60-km resolution with three deep convection schemes are analysed to find the best scheme for simulation of...     

Determination of Molybdenum, Antimony and Tungsten at sub μg g−1 Levels in Geological Materials by ID-FI-ICP-MS

We have developed a rapid and accurate method for the determination of Mo, Sb and W in geological samples using isotope dilution inductively coupled plasma-mass spectrometry with a flow injection system (ID-FI-ICP-MS). The chemical procedure requires HF digestion of the sample with a Mo-Sb-W mixed spike, subsequent evaporation and dissolution of Mo, Sb and W from Mg and Ca fluorides with HF. Recovery yields of Mo, Sb and W in the extraction were > 94% for samples of peridotite, basalt and andesite composition, with the exception of W in samples of peridotite composition for which recovery was 81%. No matrix effects were observed in the determination of the isotope ratios of Mo, Sb and W in solutions prepared from peridotite, basalt and andesite samples down to a dilution factor of 100. Detection limits of Mo, Sb and W in silicate materials were at the several ng g⁻¹ level. Analysis of the silicate reference materials PCC-1, DTS-1, BCR-1, BHVO-1, AGV-1 from the US Geological Survey and JP-1, JB-1, -2, -3, JA-1, -2, and -3 from the Geological Survey of Japan as well as the Smithsonian reference Allende powder yielded reliable Mo, Sb and W concentrations. The repeatability in the analysis of basalts and andesites was < 9%. This technique requires only 0.2 ml sample solution, and is therefore suitable for analyzing small and/or precious samples such as meteorites, mantle peridotites and their mineral separates.     

Determination of Zirconium, Niobium, Hafnium and Tantalum at ng g-1 Levels in Geological Materials by Direct Nebulisation of Sample HF Solution into FI-ICP-MS

We have developed a rapid and accurate method to determine Zr, Nb, Hf and Ta (denoted as HFSE) in geological samples by inductively coupled plasma-mass spectrometry fitted with a flow injection system (FI-ICP-MS). The method involves sample decomposition by HF followed by HF dissolution of HFSE coprecipitated with insoluble M and Ca fluoride residues formed during the initial HF attack. This HF solution was directly nebulized into an ICP mass spectrometer. An external calibration curve method and an isotope dilution method (ID) were applied for the determination of Nb and Ta, and of Zr and Hf, respectively. Recovery yields of HFSE were > 96% for peridotite, basalt and andesite compositions, apart from Zr and Hf for peridotite (> 85%). No matrix effects for either signal intensities of HFSE or isotope ratios of Zr and Hf were observed in basalt, andesite and peridotite solutions down to a dilution factor of 100. Detection limits in silicate rocks were 40, 2, 1 and 0.1 ng g-1 for Zr, Nb, Hf and Ta, respectively. This technique required only 0.1 ml of sample solution, and thus is suitable for analysing small and/or precious samples such as meteorites, mantle peridotites and their mineral separates. We also present newly determined data for the Zr, Nb, Hf and Ta concentrations in USGS silicate reference materials DTS-1, PCC-1, BCR-1, BHVO-1 and AGV-1, GSJ reference materials JB-1, -2, -3, JA-1, -2 and -3, and the Smithsonian reference Allende powder.