Calorimetric study of high-pressure phase transitions among the CdGeO3 polymorphs (pyroxenoid,garnet, ilmenite,and perovskite structures)

At high pressures, CdGeO₃ pyroxenoid transforms to garnet, then to ilmenite, and finally to perovskite. Enthalpies of transition among the four phases were measured by high temperature calorimetry. The entropies of transition and slopes of the boundaries were calculated using the measured enthalpies and free energies calculated from the phase equilibrium data. Pyroxenoid and garnet are very similar energetically. However garnet is a high pressure phase because of its lower entropy and smaller volume. The pyroxenoid-garnet transition has a small positiveP-T slope. Ilmenite is intermediate in enthalpy between garnet and perovskite, but is lower in entropy than both phases. Therefore the garnet-ilmenite transition has a positivedP/dT, while a negativedP/dT is calculated for the ilmenite-perovskite transition. The thermochemical data for the CdGeO₃ phases are generally consistent with the observed high pressure phase relations. The high entropy of perovskite relative to ilmenite, observed in several ABO₃ comounds including CdGeO₃, is related to the structural features of perovskite, in which relatively small divalent cations occupy the large sites of 8–12 fold coordination. The thermochemistry of the CdGeO₃ polymorphs shows several similarities to that of the CaGeO₃ system.     

Partitioning of elements between majorite garnet and melt and implications for petrogenesis of komatiite

Partitioning of elements between majorite garnet and ultrabasic melt has been studied at 16 GPa and 1950° C. Ca, Ti, La, Sm, Gd, Zr, Hf, Fe, Ni, Mn, K, and Na are enriched in the melt, whereas Al, Cr, V, Sc and Yb are concentrated in majorite garnet. Thus, majorite garnet fractionation by partial melting could produce chemical heterogeneities in these elements deviating from chondritic abundance. Using the partitioning behaviour of elements between majorite garnet and ultrabasic melt, the petrogenesis of komatiite is discussed. A simple model to explain the chemical varieties of komatiites is as follows. Aluminadepleted komatiite was generated by partial melting of the primitive mantle at 200–650 km depth, and alumina-enriched komatiite is the product of remelting of the residual solid at the same depths, whereas alumina-undepleted komatiite was formed by partial melting of the primitive upper mantle at depths shallower than 200 km. We suggest the possibility of large-scale chemical layering or heterogeneity in the early Archean upper mantle as an alternative model for komatiite genesis; shallower mantle depleted in majorite garnet and the underlying mantle enriched in majorite garnet. Alumina-depleted and alumina-enriched komatiites in the early Archean might be generated by a high degree of partial melting of the layered mantle. Such chemical layering could have been homogenized by the late Archean. This explains the observations that alumina-depleted and alumina-enriched komatiites were generally formed in the early Archean but alumina-undepleted komatiite was erupted in the late Archean.     

Coexisting sodic augite and omphacite in a Sanbagawa metamorphic rock,Japan

Coexisting sodic augite and omphacite were found in a zoisite amphibolite from the Iratsu epidote amphibolite mass in the Sanbagawa metamorphic terrain of central Shikoku, Japan. The occurrences of the sodic augite-omphacite pairs are classified into four types by texture: independent, composite, intergrowth and exsolution types. Sodic augite and omphacite of the independent and composite types (pair A) have X _Na (=Na/(Na + Ca)) = 0.15 and 0.35, respectively, and were stable in the epidote amphibolite facies during the Sanbagawa progressive metamorphism. On the other hand, X ^Na values of sodic augite and omphacite of the intergrowth and exsolution types (pair B) are 0.10 and 0.44, respectively. The Na-poor augite and Na-rich omphacite of the pair B were formed by re-equilibration of the pair A at lower temperature. The pair A of the Iratsu sample suggests that a compositional gap lies between sodic augite and C2/c omphacite under epidote amphibolite facies conditions, and is in marked contrast to the coexistence of sodic augite and P2/n omphacite reported from some low-grade, high-pressure metamorphic terrains. A possible phase diagram to explain the chemistry and mode of occurrence of the coexisting sodic pyroxenes is proposed.     

Relationship between mid-Cretaceous (upper Albian–Cenomanian) ammonoid facies and lithofacies in the Yezo forearc basin, Hokkaido, Japan

An examination of inner shelf, outer shelf, and slope deposits in the Yezo forearc basin, northern Japan, provides new insights into the relationship between mid-Cretaceous ammonoid facies and lithofacies. Although undergoing post-mortem transport to some degree, the ammonoids were not moved to areas outside of their original habitat. This assumption is based on the condition of the outer shell surface, general absence of fragmentation, and sedimentary structures. Desmoceras predominates in the upper Albian–Cenomanian succession regardless of lithofacies, the family Gaudryceratidae is the second-most dominant group in each lithofacies, the abundance of Zelandites decreases offshore, and other groups, including Acanthoceratidae, are uncommon but occur in both inshore and offshore facies. External shell ornamentation does not necessarily vary according to lithofacies differences, while the shape of the whorl section does vary with lithofacies as a reflection of ambient environments. The smooth, slender Zelandites and the compressed morph of the smooth Desmoceras predominate in high-energy regimes represented by frequent hummocky cross-stratification and current ripple marks of an inner shelf. In contrast, the depressed morph of Desmoceras predominates in low-energy, offshore, muddy sea-floor regimes.     

Garnet-ilmenite-perovskite transitions in the system Mg4Si4O12-Mg3Al2Si3O12 at high pressures and high temperatures: phase equilibria, calorimetry and implications for mantle structure

Phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000 °C using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle. Both of glass and crystalline starting materials were used for the experiments. At 1000 °C, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg₄Si₄O₁₂-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg₃Al₂Si₃O₁₂-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000 °C supersede preliminary phase relations determined at about 900 °C in the previous study. The phase relations at 1000 °C are quite different from those reported previously at 1600 °C where garnet s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg₄Si₄O₁₂. The stability field of Mg₃Al₂Si₃O₁₂ ilmenite was determined at 800-1000 °C and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al₂O₃ content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2±10.4 and 168.6±8.2 kJ/mol, respectively, for Mg₄Si₄O₁₂, and 150.2±15.9 and 98.7±27.3 kJ/mol, respectively, for Mg₃Al₂Si₃O₁₂. Thermodynamic calculations using these data give rise to phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ at 1000 and 1600 °C that are generally consistent with those determined experimentally, and confirm that the single-phase field of ilmenite expands from Mg₄Si₄O₁₂ to Mg₃Al₂Si₃O₁₂ with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg₂SiO₄-Fe₂SiO₄ system are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600 °C lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for garnet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs.     

Water masses and currents of deep circulation southwest of the Shatsky Rise in the western North Pacific

We conducted full-depth hydrographic observations in the southwestern region of the Northwest Pacific Basin in September 2004 and November 2005. Deep-circulation currents crossed the observation line between the East Mariana Ridge and the Shatsky Rise, carrying Lower Circumpolar Deep Water westward in the lower deep layer (θ<1.2 °C) and Upper Circumpolar Deep Water (UCDW) and North Pacific Deep Water (NPDW) eastward in the upper deep layer (1.3–2.2 °C). In the lower deep layer at depths greater than approximately 3500 m, the eastern branch current of the deep circulation was located south of the Shatsky Rise at 30°24′–30°59′N with volume transport of 3.9 Sv (1 Sv=10⁶ m³ s⁻¹) in 2004 and at 30°06′–31°15′N with 1.6 Sv in 2005. The western branch current of the deep circulation was located north of the Ogasawara Plateau at 26°27′–27°03′N with almost 2.1 Sv in 2004 and at 26°27′–26°45′N with 2.7 Sv in 2005. Integrating past and present results, volume transport southwest of the Shatsky Rise is concluded to be a little less than 4 Sv for the eastern branch current and a little more than 2 Sv for the western branch current. In the upper deep layer at depths of approximately 2000–3500 m, UCDW and NPDW, characterized by high and low dissolved oxygen, respectively, were carried eastward at the observation line by the return flow of the deep circulation composing meridional overturning circulation. UCDW was confined between the East Mariana Ridge and the Ogasawara Plateau (22°03′–25°33′N) in 2004, whereas it extended to 26°45′N north of the Ogasawara Plateau in 2005. NPDW existed over the foot and slope of the Shatsky Rise from 29°48′N in 2004 and 30°06′N in 2005 to at least 32°30′N at the top of the Shatsky Rise. Volume transport of UCDW was estimated to be 4.6 Sv in 2004, whereas that of NPDW was 1.4 Sv in 2004 and 2.6 Sv in 2005, although the values for NPDW may be slightly underestimated, because they do not include the component north of the top of the Shatsky Rise. Volume transport of UCDW and NPDW southwest of the Shatsky Rise is concluded to be approximately 5 and 3 Sv, respectively. The pathways of UCDW and NPDW are new findings and suggest a correction for the past view of the deep circulation in the Pacific Ocean.     

Isotopic equilibration ages for the Miyanohara tonalite from the Higo metamorphic belt in central Kyushu, Southwest Japan: Implications for the tectonic setting during the Triassic

Abstract Miyanohara tonalite occurs in the middle part of the Higo metamorphic belt in the central Kyushu, Southwest Japan. This tonalite intrudes into early Permian Ryuhozan metamorphic rocks in the south and is intruded by Cretaceous Shiraishino granodiorite in the north. The Miyanohara tonalite yielded three mineral ages: (i) 110–100 Ma for Sm–Nd and Rb–Sr internal isochrons and for K–Ar hornblende; (ii) 183 Ma for Sm–Nd internal isochron; and (iii) 211 Ma for Sm–Nd internal isochron. The ages of 110–100 Ma may indicate cooling age due to the thermal effect of the Shiraishino granodiorite intrusion. The ages of 183 Ma and 211 Ma are consistent with timing of intrusion of the Miyanohara tonalite based on geologic constraints. The hornblende in the sample which gave 183 Ma shows discontinuous zoning under microscope, whereas the one which gave 211 Ma does not show zonal structure. These mineralogical features suggest that the 183 Ma sample has suffered severely from later tectonothermal effect compared with the 211 Ma sample. Therefore, the age of 211 Ma is regarded as near crystallization age for the Miyanohara tonalite. The magmatic process, geochronology and initial Sr and Nd isotope ratios for the Miyanohara tonalite are similar to those of early Jurassic granites from the Outer Plutonic Zone of the Hida belt that constitutes a marginal part of east Asia before the opening of the Japan Sea. Intrusion of the Miyanohara tonalite is considered to have taken place in the active continental margin during the late Triassic.     

Calculation of Interannual Variations of Sea Level in the Subtropical North Pacific

The interannual variations of sea level at Chichi-jima and five other islands in the subtropical North Pacific are calculated for 1961–95 with a model of Rossby waves excited by wind. The Rossby-wave forcing is significant east of 140°E. Strong forcing of upwelling (downwelling) Rossby wave occurs during El Niño (La Niña) and warm (cold) water anomaly in the eastern equatorial Pacific. The first and second baroclinic modes of Rossby wave are more strongly generated than the barotropic mode in the study area. A higher vertical mode of Rossby wave propagates more slowly and is more decayed by eddy dissipation. The best coefficient of vertical eddy dissipation is determined by comparing the calculated sea level with observation. The variation in sea level at Chichi-jima is successfully calculated, in particular for the long-term change of the mean level between before and after 1986 with a rise in 1986 as well as the variations with periods of two to four years after 1980. It is concluded that variations of sea level at Chichi-jima are produced by wind-forced Rossby waves, the first baroclinic wave primarily and the barotropic wave secondly. The calculation for other islands is less successful. Degree of the success in calculation almost corresponds to a spatial difference in quantity of wind data, and seems to be determined by quality of wind data.     

Effects of frozen soil and snow cover on cold‐season soil water dynamics in Tokachi,Japan

Despite the potential impact of winter soil water movements in cold regions, relatively few field studies have investigated cold‐season hydrological processes that occur before spring‐onset of snowmelt infiltration. The contribution of soil water fluxes in winter to the annual water balance was evaluated over 5 years of field observations at an agricultural field in Tokachi, Hokkaido, Japan. In two of the winters, soil frost reached a maximum depth of 0·2 m (‘frozen’ winters), whereas soil frost was mostly absent during the remaining three winters (‘unfrozen’ winters). Significant infiltration of winter snowmelt water, to a depth exceeding 1·0 m, occurred during both frozen and unfrozen winters. Such infiltration ranged between 126 and 255 mm, representing 28–51% of total annual soil water fluxes. During frozen winters, a substantial quantity of water (ca 40 mm) was drawn from deeper layers into the 0–0·2 m topsoil layer when this froze. Under such conditions, the progression and regression of the freezing front, regulated by the thickness of snow cover, controlled the quantity of soil water flux below the frozen layer. During unfrozen winters, 13–62 mm of water infiltrated to a depth of 0·2 m, before the spring snowmelt. These results indicate the importance of correctly evaluating winter soil water movement in cold regions. Copyright © 2010 John Wiley & Sons, Ltd.     

Rapid development of arctic cyclone in June 2008 simulated by the cloud resolving global model NICAM

In this study, we conducted a numerical simulation of a rapid development of an arctic cyclone (AC) that appeared in June 2008 using a cloud resolving global model, Nonhydrostatic ICosahedral Atmospheric Model (NICAM). We investigated the three dimensional structure and intensification mechanism of the simulated AC that developed to the minimum sea level pressure of 971 hPa in the model. According to the result, the AC indicates a barotropic structure with a warm core in the lower stratosphere and a cold core in the troposphere. The development of the AC is accompanied by an intense mesoscale cyclone (MC) showing baroclinic structure with a marked local arctic front. The upper level warm core of the AC is formed by an adiabatic heating associated with the downdraft in the lower stratosphere. The rapid development of the AC is caused by the combination of the intensification of the upper level warm core and the merging with the baroclinically growing MC in the lower level. The merging of the AC and MC and the vertical vortex coupling with the upper air polar vortex are the most important mechanisms for the rapid development of the arctic cyclone.