Sound velocities of Na0.4Mg0.6Al1.6Si0.4O4 NAL and CF phases to 73 GPa determined by Brillouin scattering method

The sound velocities of two aluminum-rich phases in the lower mantle, hexagonal new Al-rich phase (NAL) and its corresponding high-pressure polymorph orthorhombic Ca-ferrite-type phase (CF), were determined with the Brillouin scattering method in a pressure range from 9 to 73 GPa at room temperature. Both NAL and CF samples have identical chemical composition of Na_0.4Mg_0.6Al_1.6Si_0.4O₄ (40 % NaAlSiO₄–60 % MgAl₂O₄). Infrared laser annealing in the diamond anvil cell was performed to minimize the stress state of the sample and obtain the high-quality Brillouin spectra. The results show shear modulus at zero pressure G ₀ = 121.960 ± 0.087 GPa and its pressure derivative G’ = 1.961 ± 0.009 for the NAL phase, and G ₀ = 129.653 ± 0.059 GPa and G’ = 2.340 ± 0.004 for the CF phase. The zero-pressure shear velocities of the NAL and CF phases are obtained to be 5.601 ± 0.005 km/sec and 5.741 ± 0.001 km/sec, respectively. We also found that shear velocity increases by 2.5 % upon phase transition from NAL to CF at around 40 GPa.     

Contributions of slab fluid and sediment melt components to magmatism in the Mariana Arc–Trough system: Evidence from geochemical compositions and Sr,Nd, and noble gas isotope systematics

This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, ⁸⁴Kr/⁴He and ¹³²Xe/⁴He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of ³He/⁴He and ⁴⁰Ar/³⁶Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in ²⁰Ne/²²Ne and ²¹Ne/²²Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess ¹²⁹Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.     

Groundwater vulnerability assessment in shallow aquifer of Kathmandu Valley using GIS-based DRASTIC model

In this paper, groundwater aquifer vulnerability map has been developed by incorporating the major geological and hydro-geological factors that affect and control the groundwater contamination using GIS based DRASTIC model. This work demonstrates the potential of GIS to derive a map by overlying various spatially referenced digital data layers that portrays cumulative aquifer sensitivity ratings across the Kathmandu Valley, Nepal, providing a relative indication of groundwater vulnerability to contamination. In fact, the groundwater is the major natural resources in Kathmandu for drinking purpose. The decline in groundwater levels due to the over exploitation and thus extracted water from shallow aquifer has been contaminated by the infiltration of pollutants from polluted river and land surface is continuous and serious. As the demand for water for human and industrial use has escalated and at the same time, the engineering and environmental costs are much higher for new water supplies than maintaining the existing sources already in use. Management of groundwater source and protecting its quality is therefore essential to increase efficient use of existing water supplies. Aquifer vulnerability maps developed in this study are valuable tools for environmental planning and predictive groundwater management. Further, a sensitivity analysis has been performed to evaluate the influence of single parameters on aquifer vulnerability assessment such that some subjectivity can be reduced to some extent and then new weights have been computed for each DRASTIC parameters.     

A consideration on the mechanism of generation,stagnation and disappearance of the kuroshio Meander

A mechanism of the Kuroshio Meander is discussed by comparing some observed characteristics of the Kuroshio path with short- and long-term variations of the wind field over the North Pacific. It is suggested that the meander is caused by the blocking of the Kuroshio current by the Izu-Ogasawara Ridge. The blocking occurs when the depth of the main current increases or when the vertical shear becomes weak. These structural variations are closely related to the supposed baroclinic response of the North Pacific Subtropical Gyre to long-term variations of the wind field with a period of about 56 years. The Kuroshio Meander is initiated by a trigger meander at the offiing of Shikoku Island. The trigger meander is closely related to the supposed barotropic response of the gyre to short-term variations of the wind field with a period of about 34 months.The barotropic response of the North Pacific Subtropical Gyre to the short-term variation of the wind field yields the rapid change of the vertical structure of the Kuroshio current. This change generates the trigger meander in combination with the complicated pattern of the continental slope at the offing of Shikoku Island. The trigger meander is carried away toward the Izu-Ogasawara Ridge by the Kuroshio current. When the baroclinic response of the gyre is favourable for the blocking of the main current, the trigger meander and the cold eddy grow fed by the upwelling of the deep water of the Kuroshio which is blocked at the west of the ridge. The growing stops when the scale of the trigger meander reaches to the size of the steady Rossby wave which corresponds to the over-all mean velocity of the Kuroshio at that time, because the meander exceeding the size of the steady Rossby wave moves west-ward and separates from the ridge. Then the deep water of the Kuroshio at the west of the ridge which has been under the hard constraint of the cyclonic circulation in the form of the cold eddy becomes possible to flow arround the ridge. The upwelling stops and there remains only the general dissipation process of the available potential energy in the cold eddy. Then the meander gradually decreases its size and returns to the ridge when the meander becomes smaller than the steady Rossby wave at that time. It is blocked and begins to grow there again. In this way, the Kuroshio Meander behaves as a quasi-steady Rossby wave and stagnates at the west of the ridge until the baroclinic response of the gyre becomes unfavourable for the blocking of the Kuroshio current by the ridge.     

A 100 m laser strainmeter system installed in a 1 km deep tunnel at Kamioka, Gifu, Japan

We have installed a laser strainmeter system in a deep tunnel about 1,000 m below the ground surface at Kamioka, Gifu, Japan. The system consists of three types of independent interferometers: (1) an EW linear strainmeter of the Michelson type with unequal arms, (2) an NS-EW differential strainmeter of the Michelson type with equal arms and (3) a NS absolute strainmeter of the Fabry–Perot type. These are configured in L-shaped vacuum pipes, each of which has a length of 100 m. (1) and (2) are highly sensitive (order of 10⁻¹³ strain) and have wide dynamical range (10⁻¹³–10⁻⁶). Observations with strainmeters (1) and (2) started on June 11, 2003. (3) is a new device for absolute-length measurements of the order of 10⁻⁹ of a long-baseline (100 m) Fabry–Perot cavity by the use of phase-modulated light. This third strainmeter will be ready for operation before the end of 2004. The laser source of strainmeters (1) and (2) is a frequency-doubled YAG laser with a wavelength of 532 nm. The laser frequency is locked onto an iodine absorption line and a stability of 2 × 10⁻¹³ is attained. The light paths of the laser strainmeter system are enclosed in SUS304 stainless steel pipes. The inside pressure is kept to be 10⁻⁴ Pa. Consequently, quantitative measurement of crustal strains of the order of 10⁻¹³ can be attained by employing the laser strainmeter system of (1) and (2) at Kamioka. This resolving power corresponds to that of a superconducting gravimeter. Using the laser strainmeter system, we expect to determine parameters related to fluid core resonance, core modes and core undertone as well as other geodynamic signals such as slow strain changes caused by silent earthquakes or slow earthquakes.     

Prograde eclogites from the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt, central Shikoku, southwest Japan

Abstract   Prograde eclogites occur in the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt of central Shikoku. The Tonaru mass is considered to be a metamorphosed layered gabbro, and occurs as a large tectonic block (approximately 6.5 km × 1 km) in a high-grade portion of the Sambagawa schists. The Tonaru mass experienced high- P /low- T prograde metamorphism from the epidote-blueschist facies to the eclogite facies prior to its emplacement into the Sambagawa schists. The estimated P – T conditions are T  = 300–450°C and P  = 0.7–1.1 GPa for the epidote-blueschist facies, and the peak P – T conditions for the eclogite facies are T  = 700–730°C and P  ≥ 1.5 GPa. Following the eclogite facies metamorphism, the Tonaru mass was retrograded to the epidote amphibolite facies. It subsequently underwent additional prograde Sambagawa metamorphism, together with the surrounding Sambagawa schists, until the conditions of the oligoclase–biotite zone were reached. The high- P /low- T prograde metamorphism of the eclogite facies in the Tonaru mass and other tectonic blocks show similar steep d P /d T geothermal gradients despite their diverse peak P – T conditions, suggesting that these tectonic blocks reached different depths in the subduction zone. The individual rocks in each metamorphic zone of the Sambagawa schists also recorded steep d P /d T geothermal gradients during the early stages of the Sambagawa prograde metamorphism, and these gradients are similar to those of the eclogite-bearing tectonic blocks. Therefore, the eclogite-bearing tectonic blocks reached greater depths in the subduction zone than the Sambagawa schists. All the tectonic blocks were ultimately emplaced into the hanging wall side of the later-subducted Sambagawa high-grade schists during their exhumation.     

The Behaviour of a Man-Made Island During the Great Hanshin Earthquake,Japan

This paper describes the behaviour of Port Island in Kobe city during and after the Great Hanshin Earthquake of 17 January 1995. The island is near the city centre and was constructed on the soft seabed in Kobe Port. A vertical array of four seismometers and pore water pressure sensors recorded the response of the ground during and after the earthquake. This study shows how the stiffness of the ground dropped and recovered during and after the earthquake.     

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km² and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?     

A new high-pressure and high-temperature polymorph of FeS

A new polymorph of FeS has been observed at pressures above 30 GPa at 1,300 K by in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. It is stable up to, at least, 170 GPa at 1,300 K. The new phase (here called FeS VI) has an orthorhombic unit cell with lattice parameters a = 4.8322 (17) Å, b = 3.0321 (6) Å, and c = 5.0209 (8) Å at 85 GPa and 300 K. Its topological framework is based on the NiAs-type structure as is the case for the other reported polymorphs (FeS I-V). The unit cell of FeS VI is, however, more distorted (contracted) along the [010] direction of the original NiAs-type cell. For example, the c/b axial ratio is ～1.66 at 85 GPa and 300 K, which is considerably smaller than that of orthorhombic FeS II (～1.72) and NiAs-type hexagonal FeS V (=√3 ≈ 1.73). The phase boundary between FeS IV and VI is expected to be located around 30 GPa at 1,300 K. The phase transition is accompanied by gradual and continuous changes in volume and axial ratios and may be second order. At room temperature, FeS VI becomes stable over FeS III at pressures above 36 GPa. It is, therefore, suggested that the phase boundary of FeS III–VI and/or FeS IV–VI has negative pressure dependence.