Granulite formation in a Gondwana fragment: petrology and mineral equilibrium modeling of incipient charnockite from Mavadi, southern India

We report a new occurrence of incipient charnockite from Mavadi in the Trivandrum Granulite Block (TGB), southern India, and discuss the petrogenesis of granulite formation in an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium modeling. In Mavadi, patches and lenses of charnockite (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Opx?+?Ilm?+?Mag) of about 30 to 220 cm in length occur within Opx-free Grt-Bt gneiss (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Ilm). The application of mineral equilibrium modeling on the charnockite assemblage in the NCKFMASHTO system to constrain the conditions of charnockitization defines a P–T range of 800 °C at 4.5 kbar to 850 °C at 8.5 kbar, which is broadly consistent with the results from the conventional geothermobarometry (810–880 °C at 7.7–8.0 kbar) on these rocks. The P–T conditions are lower than the peak metamorphic conditions reported for the ultrahigh-temperature granulites from this area (T?>?900 °C). The heterogeneity in peak P–T conditions within the same crustal block might be related to local buffering of metamorphic temperatures by the Opx-Bt-Kfs-Qtz assemblage. The result of T versus mole H₂O (M(H₂O)) modeling demonstrated that the Opx-free assemblage in the Grt-Bt gneiss is stable at M(H₂O)?=?0.3 to 1.5 mol%, and orthopyroxene occurs as a stable mineral at M(H₂O) <0.3 mol%, which is consistent with the petrogenetic model of incipient charnockite related to the lowering of the water activity and stabilization of orthopyroxene through the breakdown of biotite by dehydration caused by the infiltration of CO₂-rich fluid from external sources. We also propose a possible alternative mechanism to form charnockite from Grt-Bt gneiss through slight variations in bulk-rock chemistry (particularly for the K- and Fe-rich portion of Grt-Bt gneiss) that can enhance the stability of orthopyroxene rather than that of biotite, with K-metasomatism playing a possible role.     

Experimental studies on debris flow with logs focusing on specific weight difference of log species

There are many experimental approaches,field investigations and numerical calculations for movements of woods in a clear water and debris flow.However,kinematic conditions for accumulated logs and the interactions between a main flow and logs have not been fully evaluated.Mitigations for woods need taking into account the characteristics of tree species such as conifer and broad-leaf trees and of shapes such as root swells and crown.In the present study,we focus on the differences in specific weight of conifer and broad-leaf trees with some moisture in a sediment-water mixture flow with narrow flow width,and consider that conifer and broad-leaf tree are floating and submerged solid phase,respectively.Flume tests are conducted in steady flow of clear and debris flow over a rigid bed in order to evaluate conifer and broad-leaf tree movement in clear water and debris flow.Experimental data indicates that dimensionless transverse diffusion coefficient can be 0.1 to 0.4 and 0.3 to 0.9 in flow direction.Those diffusive characteristics seem to be independent of Reynolds number and Froude number,but dependent of bed slope,i.e.,gravity,though detailed considerations are needed to discuss about flow characteristics such as spatial eddy structures,momentum transfer induced by interactions of logs and so on.     

Sensitivity of Salt-leached Clay Sediments in the Ariake Bay Area,Japan

This study aims to determine whether the principle that “divalent cation predominance in the pore water precludes quick clay development” applies to the Ariake Bay sediments. The chemical and geotechnical properties of an Ariake clay sediment are presented, and sensitivity is discussed with a focus on pore-water salinity and cation composition. In recent years, reduction of pore-water salinity has occurred due to permeation of river water through the sediments because of overpumping of groundwater. Sodium remains the dominant pore-water cation in an upper zone, whereas divalent cations are dominant in the deeper zone. Divalent cation domination in the deeper zone is ascribed to Ca release from nearby cement-stabilized sediments and to Mg increase in response to a change in river water quality. The upper zone's sensitivity ranged from 15 to 77, and the remolded strength was mostly <0.5 kPa, such that quick clay was present over much of its depth. In contrast, the deeper zone's sensitivity was <40, and its remolded strength exceeded 0.5 kPa; quick clay was not present despite the <2 g/L salinity. The absence of quick clay is ascribed to the high remolded strength caused by the pore-water divalent to monovalent cation ratio being greater than 0.25.     

Modeling interaction of fluid and salt in an aquifer/lagoon system

To simulate the dynamic interaction between a saline lagoon and a ground water system, a numerical model for two-dimensional, variable-density, saturated-unsaturated, and coupled flow and solute transport (saltwater intrusion by finite elements and characteristics [SIFEC]) was modified to allow the volume of water and mass of salt in the lagoon to vary with each time step. The modified SIFEC allows the stage of a lagoon to vary in accordance with a functional relation between the stage and water volume of the lagoon, and also allows the salt concentration of the lagoon to vary in accordance with the salt budget of the lagoon including chemical precipitation and dissolution of salt. The updated stage and salt concentration of the lagoon are in turn used as transient boundary conditions for the coupled flow and solute transport model. The utility of the modified model was demonstrated by applying it to the eastern Mediterranean coastal region of Turkey for assessing impacts of climate change on the subsurface environment under scenarios of sea level rise, increased evaporation, and decreased precipitation.     

Assessing flood disaster impacts in agriculture under climate change in the river basins of Southeast Asia

Natural Hazards - This study focused on flood damage assessment for future floods under the impact of climate change. Four river basins of Southeast Asia were selected for the study. They included...     

Crustal structure beneath Aso Caldera,Southwest Japan,as derived from receiver function analysis

Aso Volcano experienced a huge pyroclastic eruption 90 thousand years ago, and formed a large caldera (18 km × 25 km). In order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically, we investigated seismic velocity discontinuities and velocity structure beneath Aso Caldera using receiver functions and a genetic algorithm inversion. We confirm the existence of the Moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath Aso Caldera, from imaging of receiver functions observed only at stations outside the caldera. As a result of a more detailed examination with GA inversion, a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera. S-wave velocity of the layer is estimated to be 2.0–2.4 km/s. We estimate that the low velocity layer contains at most 15% melt or 30% aqueous fluid. The layer exists near the Conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle-eastern part of the caldera. Fluid contained in the layer might be related with huge pyroclastic eruptions of Aso Volcano.     

Long-period ground motions from the 2003 Tokachi-oki earthquake

We study characteristics of long-period ground motions from the 2003 Tokachi-oki earthquake (Mj 8.0), a large interplate earthquake, based on spatial distribution maps and attenuation relationships for four kinds of peak ground velocity (PGV) value. The first kind (PGV(WB)) is obtained from a maximal value of vector sum of the three-component, wide-band velocity seismograms, and the other three kinds (PGV(BP10), PGV(BP20), and PGV(BP30)) are obtained from a maximal value of vector sum of the three-component, narrow band-pass filtered velocity seismograms (the central periods are 10, 20, and 30 s). The spatial distribution maps for all kinds of PGV value show azimuth dependence; the PGV values in Hokkaido, northern side of the epicenter are larger than those in Tohoku, southwestern side of the epicenter, when compared at a comparable distance. We find that the features result from the radiation pattern of long-period surface waves, that is, the source effect. The attenuation relationships show the following trends: The PGV(WB) values are larger than the sum of the PGV(BP10), PGV(BP20), and PGV(BP30) at distances (D) less than 200 km, while the PGV(WB) values are comparable to the sum of the PGV(BP20) and PGV(BP30) at D > 200 km. This indicates that the PGV(WB) values at D < 200 km are affected by ground motions with periods less than 10 s, while long-period surface waves mainly contribute to the PGV(WB) values at D > 200 km. The basin site effects generate a patchy pattern in the spatial distribution maps and a large scattering in the attenuation relationships for the PGV(WB) and PGV(BP10) values. Finally, we conclude that the PGV(WB) values from the 2003 Tokachi-oki earthquake are controlled by the radiation pattern of long-period S and surface waves and various basin site effects.     

Thermal metamorphism of the C,G, B,and F asteroids seen from the 0.7 μm, 3 μm,and UV absorption strengths in comparison with carbonaceous chondrites

Abstract Thermal metamorphism study of the C, G, B, and F asteroids has been revisited using their UV, visible, NIR, and 3 μm reflectance spectra. High-quality reflectance spectra of seven selected C, G, B, and F asteroids have been compared with spectra for 29 carbonaceous chondrites, including thermally-metamorphosed CI/CM meteorites. There are three sets of spectral counterparts, among which 511 Davida and B-7904 are the most similar to each other in terms of both spectral shape and brightness. By comparing the 0.7 μm and 3 μm absorption strengths of 21 C, G, B, and F asteroids and heated Murchison samples, these asteroids have been grouped into three heating-temperature ranges. These correspond to (1) <400 °C: phyllosilicate-rich; (2) 400–600 °C: phyllosilicates transformed to anhydrous silicates; and (3) >600 °C: fully anhydrous. A good correlation between the UV and 3 μm absorption strengths has been confirmed for the C, G, B, and F asteroids and the CI, CM, and CR meteorites. A plot of the UV absorption strength vs. the IRAS diameter for 142 C, G, B, and F asteroids shows that the maximum UV absorption strength decreases as the diameter increases for the asteroids >60 km, with a notable exception, Ceres. These relationships suggest that some of the larger asteroids may be the heated inner portions of once larger bodies and that common CI/CM meteorites may have come from the lost outer portions, which escaped extensive late-stage heating events.     

Interactions of uranium with bacteria and kaolinite clay

We assessed the accumulation of uranium (VI) by a bacterium, Bacillus subtilis, suspended in a slurry of kaolinite clay, to elucidate the role of microbes on the mobility of U(VI). Various mixtures of bacteria and the koalinite were exposed to solutions of 8 × 10^− 6 M- and 4 × 10^− 4 M-U(VI) in 0.01 M NaCl at pH 4.7. After 48 h, the mixtures were separated from the solutions by centrifugation, and treated with a 1 M CH₃COOK for 24 h to determine the associations of U within the mixture. The mixture exposed to 4 × 10^− 4 M U was analyzed by transmission electron microscope (TEM) equipped with EDS. The accumulation of U by the mixture increased with an increase in the amount of B. subtilis cells present at both U concentrations. Treatment of kaolinite with CH₃COOK, removed approximately 80% of the associated uranium. However, in the presence of B. subtilis the amount of U removed was much less. TEM–EDS analysis confirmed that most of the U removed from solution was associated with B. subtilis. XANES analysis of the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both revealed that it was present as U(VI). These results suggest that the bacteria have a higher affinity for U than the kaolinite clay mineral under the experimental conditions tested, and that they can immobilize significant amount of uranium.     

Petrochemistry of Volcanic Rocks in the Hishikari Mining Area of Southern Japan, with Implications for the Relative Contribution of Lower Crust and Mantle-derived Basalt

Abstract. This study presents the petrographical, mineralogical, and geochemical characteristics of Late Pliocene‐Pleistocene volcanic rocks distributed in the Hishikari gold mining area of southern Kyushu, Japan, and discusses their origin and evolution. The Hishikari volcanic rocks (HVR), on the basis of age and chemical compositions, are divided into the Kurosonsan (2.4–1.0 Ma) and Shishimano (1.7–0.5 Ma) Groups, which occur in the northern and southern part of the area, respectively. Each group is composed of three andesites and one rhyodacite. HVR are characterized by high concentrations of incompatible elements compared with other volcanic rocks in southern Kyushu, and have low Sr/Nd and high Th/U, Th/Pb, and U/Pb ratios compared with typical subduction‐related arc volcanic rocks. Modal and whole‐rock compositions of the HVR change systematically with the age of the rocks. Mafic mineral and augite/hypersthene ratios of the andesites decrease with decreasing age in the Kurosonsan Group, whereas in the Shishimano Group, these ratios are higher in the youngest andesite. Similarly, major and trace element compositions of the younger andesites in the former group are enriched in felsic components, whereas in the latter group the youngest andesite is more mafic than older andesites. Moreover, the crystallization temperature of phenocryst minerals decreases with younger age in the former group, whereas the opposite trend is seen in the latter group. Another significant feature is that rhyodacite in the Shishimano Group is enriched in felsic minerals and incompatible elements, and exhibits higher crystallization temperatures of phenocryst minerals than the rhyodacite of the Kurosonsan Group. Geochemical attributes of the HVR and other volcanic rocks in southern Kyushu indicate that a lower subcontinental crust, characterized by so‐called EMI‐type Sr‐Nd and DUPAL anomaly‐like Pb isotopic compositions, is distributed beneath the upper to middle crust of the Shimanto Supergroup. The HVR would be more enriched in felsic materials derived from the lower crust by high‐alumina basaltic magma from the mantle than volcanic rocks in other areas of southern Kyushu. The Kurosonsan Group advanced the degree of the lower crust contribution with decreasing age from 51 %, through 61 and 66 % to 77 %. In the Shishimano Group, the younger rhyodacite and andesite are derived from hotter magmas with smaller amounts of lower crust component (58 and 57 %) than the older two andesites (65 % and 68 %). We suggest that the Shishimano rhyodacite, which is considered to be responsible for gold mineralization, was formed by large degree of fractional crystallization of hot basaltic andesite magma with less lower crustal component.