Enriched back-arc basin basalts from the northern Mariana Trough: implications for the magmatic evolution of back-arc basins

The composition of basalts erupted at the earliest stages in the evolution of a back-arc basin permit unique insights into the composition and structure of the sub-arc mantle. We report major and trace element chemical data and O-, Sr-, Nd-, and Pb- isotopic analyses for basalts recovered from four dredge hauls and one ALVIN dive in the northern Mariana Trough near 22°N. The petrography and major element chemistry of these basalts (MTB-22) are similar to tholeiites from the widest part of the Trough, near 18°N (MTB-18), except that MTB-22 have slightly more K₂O and slightly less TiO₂. The trace element data exhibit a very strong arc signature in MTB-22, including elevated K, Rb, Sr, Ba, and LREE contents; relatively lowK/Ba and highBa/La andSr/Nd. The Sr- and Nd- isotopic data plot in a field displaced from that of MTB-18 towards Mariana arc lavas, and the Pb-isotopic composition of MTB-22 is indistinguishable from Mariana arc lavas and much more homogeneous than MTB-18. Mixing of 50–90% Mariana arc component with a MORB component is hypothesized. We cannot determine whether this resulted from physical mixing of arc mantle and MORB mantle, or whether the arc component is introduced by metasomatism of MORB-like mantle by fluids released from the subducted lithosphere. The strong arc signature in back-arc melts from the Mariana Trough at 22°N, where the back-arc basin is narrow, supports general models for back-arc basin evolution whereby early back-arc basin basalts have a strong arc component which diminishes in importance relative to MORB as the back-arc basin widens.     

Heat budget of air passing through a hot tunnel in northeastern Japan

Summary A geothermal and meteorological observation was carried out inside a hot tunnel (named Torigoe tunnel) penetrating through a volcanic zone in northeastern Japan, just after it has been penetrated. Rock wall surface temperature, some thermal properties of rock samples, temperature profile in slender pits drilled into the rock body, temperature of water flowing on the floor and the meteorological elements of air passing through the tunnel are contained in the observation.Introducing the coefficient of heat transfer determined from the Reynolds analogy for turbulent forced convection in a circular tube, the heat budget of the air-tunnel system is investigated with those observed data. The results show that the net sensible heat of 58.61 kW is drawn out from the tunnel by air passing through it and seems to be overcompensated by the net heat supply of 67.21 kW from the rock wall to air, while for the net latent heat removal of 101.56 kW by air the apparent undercompensation of 80.01 kW is found from the water flowing on the floor. Consideration is carried out on such excess and deficiency of heat supplies.

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Zusammenfassung In einem heißen Tunnel (dem Torigoe-Tunnel), der durch vulkanisches Gebiet im nordöstlichen Japan verläuft, wurden unmittelbar nach Tunneldurchstoß geothermische und meteorologische Untersuchungen durchgeführt. Die Temperatur der Gesteinsoberfläche, spezifische thermische Eigenschaften der Gesteinsproben, das Temperaturprofil von dünnen Bohrkernen, als auch die Temperatur des Wassers am Tunnelgrund und meteorologische Daten der Durchzugsluft werden in dieser Untersuchung berücksichtigt.Durch Einführung des durch die Reynolds-Analogie determinierten Wärmeaustauschkoeffizienten für turbulente, erzwungene Konvektion in einer kreisförmigen Röhre wird der Wärmehaushalt der Luft im Tunnelsystem unter Heranziehung der angeführten Daten untersucht. Die Resultate zeigen, daß 58.61 kW fühlbare Wärme dem Tunnel durch die Luft entzogen werden, die jedoch durch eine Zufuhr von 67.21 kW von der Felswand an die Luft abgegebener fühlbarer Wärme überkompensiert zu werden scheint, während der dauernden latenten Wärmeabfuhr durch die Luft von 101.56 kW eine Unterkompensierung von 80.01 kW durch das am Tunnelgrund fließende Wasser gegenübersteht. Die Folgen dieses Überschusses bzw. Defizites werden genauer diskutiert.

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With 5 Figures     

Synthesis and crystal-chemical characterization of MgSiO3 perovskite

The orthorhombic MgSiO₃ perovskite has been synthesized with the aid of a double-stage split-sphere-type high-pressure apparatus at about 280 kbar and 1000°C. The unit cell dimensions are: a = 4.7754(3)Å, b = 4.9292(4)Å and c = 6.8969(5)Å with the probable space group Pbnm. Calculated density is 4.108 g cm^?3. Crystal structure determination has been carried out by means of both the geometrical simulation (DLS) technique and the ordinary powder X-ray analysis. The results indicate that the MgSiO₃ perovskite is closer to the ideal perovskite than ScAlO₃ perovskite.     

A simple, rheological model for plate motion to explain episodic volcanism and tectonism: An application of whitehead and gans' fault model

A simple, rheological model is presented for plate motions to explain episodic volcanism and tectonism. A lithospheric plate is modelled by a gliding block and the basal edge of the sinking plate is modelled by a stick-slip-like fault system. The friction force is assumed to have a maximum at a critical sliding velocity. When the sliding velocity at the basalt edge exceeds the critical velocity, the basal edge system becomes unstable and the plate accelerates. Conditions that the model system has a periodic solution are obtained and parameters of the system are evaluated from rheological properties of the mantle and configurations of the plate. Results suggest that a plate moves nearly steadily over a long period of time but it accelerates sharply during a very short period of time. The accelerating period is considered to correspond to the episodes of active volcanism and tectonism.     

High-pressure X-ray diffraction studies on β- and γ-Mg2SiO4

Pressure effects on the lattice parameters of β- and γ-Mg₂SiO₄ have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V₀) at 90 kbar are 5.4 · 10^?2 and 4.2 · 10^?2 for β- and γ-Mg₂SiO₄, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg₂SiO₄, respectively. The α → γ transition obeys Wang's linear V_φ?ρ relation but the α → β transition does not.     

High-pressure synthesis of ZnSiO3 ilmenite

High-pressure phase relations in ZnSiO₃ and Zn₂SiO₄ were investigated at about 1000°C and in the pressure range of 100–500 kbar, using a double-staged split-sphere type of high-pressure apparatus.Clinopyroxene-type ZnSiO₃ transforms directly into a polymorph with the ilmenite structure at 120 kbar. The hexagonal unit cell dimensions of the ZnSiO₃ ilmenite are determined to be $\text{a = 4.746 ± 0.001}\text{A}\text{?}\text{and}\text{c = 13.755 ± 0.002}\text{A}\text{?}$ under ambient conditions.The following reactions are also recognized at about 1000°C:

and:

The stabilities of silicate ilmenites, especially the absence of ilmenite of transition metal silicate composition, is discussed. It is pointed out that data on phase relations in zinc silicates may be informative for the consideration on those in magnesium silicates under very high pressures. It is suggested that the silicate ilmenite may be a major constituent in the lower mantle.     

Sequential processes in a landslide hazard at a slate quarry in Okayama,Japan

The 12 March 2001 landslide at a slate quarry in Okayama, Japan killed three workers. Composite studies based on field surveys of the landslide slope, interviews with local residents and quarry workers, and inspections of hydrological and seismological data have been used to clarify the causes of this slide and its movements. The results indicate that the landslide was enabled firstly by the steepness of the slope, which had been undercut by river; secondly, the structure was that of a dip-slope that was prone to deep-seated slides along bedding planes; thirdly, numerous joints and faults were present. Surprisingly, rainfall, earthquakes, and explosions do not appear to have played any role in the triggering of this slide. The interviews demonstrated that the frequency of precursory failures increased over a period of several hours before the 12 March 2001 landslide. Inspection of the seismograph records and the eyewitness evidence both indicate that the main part of the landslide consisted of two phases of slope failure within 23 s. After the slide, the frequency of the failures gradually decreased with time over a period of several days. Three new terms are proposed for landslides: foreslide, mainslide, and afterslide, following the terms foreshock, main shock, and aftershock used in seismology.     

The 1995 Northern Niigata Earthquake with special attention to a geopressured hydrothermal system

The 1995 Northern Niigata Earthquake of magnitude (M) 5.5 occurred at the eastern margin of the Niigata seismic gap and might have been a precursor of a large destructive earthquake. The anomaly areas in temperature, electrical conductivity and Cl^- concentration of groundwater were approximately coincident with the area of the seismic intensity 6 on the Japan Meteorological Agency scale, and convincingly demonstrated the presence of a buried active fault beneath the epicentral area, as was suggested by a linear distribution of seismic intensity 6. These anomalies of groundwater were created by the expulsion of geopressured hydrothermal water along an active fault. Anomalies in local groundwater and hotspring systems associated with the earthquake and the proximity of the earthquake to the Niitsu oil field led to an interpretation that the earthquake might have been triggered by activity within the geopressured hydrothermal system. The accumulation of geopressured hydrothermal water in combination with high rock temperature might reduce fracture strength of the rock, and trigger earthquake occurrence.