Element transport by dehydration of subducted sediments: Implication for arc and ocean island magmatism

High-pressure experiments on a natural pelite have been conducted at 2–11-GPa pressures in order to evaluate contributions of subducted sediments to arc and ocean island magmatism. Obtained phase relations suggest that, at least in modern subduction zones, subsolidus dehydration of chlorite and phengitic muscovite in the subducted sediments, rather than partial melting, is a predominant process in overprinting sediment components onto the magma source region. Trace element compositions of sediment-derived fluids are estimated based on dehydration experiments at 5.5 GPa and 900/1300°C. Pb is effectively transported by fluids relative to other elements. This results in the Pb enrichment for arc basalts by fluids, generated by the dehydration of subducted sediments, together with altered mid-ocean ridge basalt (MORB), and complementary depletion of Pb in subducted sediments. Inferred arc magma compositions obtained by model calculations based on the present experimental results agree well with a natural primitive arc basalt composition. A large increase in the U/Pb ratio in the subducted sediments at deeper levels than major dehydration depths results in a high Pb isotopic ratio through radioactive decay after long periods of isolation. Combined with other isotopic ratios such as Sr and Nd, it is possible to produce the EM II source, one of the enriched geochemical reservoirs for ocean island basalt magmas, by mixing of a small amount of subducted sediments with depleted or primitive mantle.     

Seismic Wavefields in the Deep Seafloor Area from a Submarine Landslide Source

We use the finite difference method to simulate seismic wavefields at broadband land and seafloor stations for a given terrestrial landslide source, where the seafloor stations are located at water depths of 1,900–4,300 m. Our simulation results for the landslide source explain observations well at the seafloor stations for a frequency range of 0.05–0.1 Hz. Assuming the epicenter to be located in the vicinity of a large submarine slump, we also model wavefields at the stations for a submarine landslide source. We detect propagation of the Airy phase with an apparent velocity of 0.7 km/s in association with the seawater layer and an accretionary prism for the vertical component of waveforms at the seafloor stations. This later phase is not detected when the structural model does not consider seawater. For the model incorporating the seawater, the amplitude of the vertical component at seafloor stations can be up to four times that for the model that excludes seawater; we attribute this to the effects of the seawater layer on the wavefields. We also find that the amplification of the waveform depends not only on the presence of the seawater layer but also on the thickness of the accretionary prism, indicating low amplitudes at the land stations and at seafloor stations located near the trough but high amplitudes at other stations, particularly those located above the thick prism off the trough. Ignoring these characteristic structures in the oceanic area and simply calculating the wavefields using the same structural model used for land areas would result in erroneous estimates of the size of the submarine landslide and the mechanisms underlying its generation. Our results highlight the importance of adopting a structural model that incorporates the 3D accretionary prism and seawater layer into the simulation in order to precisely evaluate seismic wavefields in seafloor areas.     

Kinematic analysis of ultrahigh-pressure–high-pressure metamorphic rocks in the Chaglinka–Kulet area of the Kokchetav Massif, Kazakhstan

Abstract The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh-pressure–high-pressure (UHP–HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite- and diamond-bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneiss-dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large-scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north-westward extrusion of UHP Unit II.     

Stream temperature,specific conductance and runoff process in mountain watersheds

Stream temperature ranged from 3 to 4°C at an experimental site during snowmelt on Hokkaido Island, Japan, which provided direct evidence of major contributions of subsurface water to stream water. In contrast, stream temperatures during rainstorms in summer decreased gradually after stream flow peaked, attaining a nearly constant temperature ranging from 9 to 11°C. During storm flow recession, stream temperatures during summer or snowmelt were similar to the soil temperature at 1·8 m below the land surface, suggesting that subsurface water contributions to stream flow are derived from this depth. The hygrographs during two rainstorms, August 1987 and September 1989, were separated using temperature. The stream temperature was assumed to depend on the mixing of surface flow, having a temperature ranging from that of rainfall to that of shallow (50 cm deep) soil water, and subsurface flow, having the temperature of the soil at 1·8 m below the land surface. Subsurface flow was estimated to contribute 85–90% of the total stream flow during each rainstorm. A two‐component hydrograph separation was also evaluated using specific conductance. Runoff contributions from the two sources for the temperature and specific conductance analysis were similar. Analysis of the temperature and conductance–discharge hysteresis loop, and of individual flow components for storm hygrographs, provide a general picture of the runoff process in the experimental basin. Copyright © 1999 John Wiley & Sons, Ltd.     

Failure of double friction pendulum bearings under pulse‐type motions

Although the behavior of friction sliding bearings is well understood, the failure behavior has not been thoroughly investigated. However, predicting and understanding the failure of bearings is an important key in designing isolated structures to minimize their collapse in extreme events, and thus, this study is critical. Because of its relative simplicity and particular availability in certain markets, the failure of the double friction pendulum (DFP) bearing at its physical displacement limit is investigated. The bearing is modeled with a rigid body model including inertia for each of the bearing components. A nonlinear viscoelastic impact model is included to simulate the impact between bearing components. As isolation systems are particularly vulnerable to long‐period excitations, analytical pulses are used as input excitations to investigate the influences of pulse parameters on the failure of DFP. The influences of DFP design parameters are investigated as well. To confirm that the response to the analytical pulses correctly represents the behavior under long‐period ground motions, wavelet analysis to is performed on 14 pairs of pulse‐type ground motion records to extract their pulses, and the failure prediction made from the extracted analytical pulse is compared with the failure from the real ground motions. It is found that using the extracted pulses provides a good estimation for the failure prediction of the ground motions. Copyright © 2016 John Wiley & Sons, Ltd.     

Contribution of different functional groups to the diet of major predatory fishes at a seagrass meadow in northeastern Japan

We examined the variation in habitat use and diet of three dominant fish species (Myoxocephalus brandti, Pholidapus dybowskii, and Pholis crassispina) in a seagrass meadow in the Akkeshi-ko estuary in northeastern Japan, where broad and dense Zostera marina beds exist, using a semi-quantitative census of the fishes and analyses of their stomach contents. Differences among the three fish species in the temporal variation in abundance of each age class (mainly 1- and 2-year age classes) indicated that the temporal pattern of utilization of the seagrass meadow were different among them. In the semi-quantitative dietary analysis, two prey categories, i.e., taxonomic group (order and suborder) and functional group, were used to explain the variation in prey composition with size-dependent changes. The six prey functional groups were classified based on the ecological traits of the prey, i.e., trophic level, size, and life type (habitat and behavior). Ontogenetic shifts in prey of the three fish species could be fully explained by a combination of the two prey categories, and not by the use of only one category (taxonomic or functional group). The pattern of ontogenetic shifts in prey differed among the fish species and size (age) classes. These results indicate that segregation of habitat (seagrass meadow) and prey group (taxonomic and functional group) is performed among the three species, which may contribute to their coexistence in this estuary.     

Bimodal representation of the tropical intraseasonal oscillation

The tropical intraseasonal oscillation (ISO) shows distinct variability centers and propagation patterns between boreal winter and summer. To accurately represent the state of the ISO at any particular time of a year, a bimodal ISO index was developed. It consists of Madden-Julian Oscillation (MJO) mode with predominant eastward propagation along the equator and Boreal Summer ISO (BSISO) mode with prominent northward propagation and large variability in off-equatorial monsoon trough regions. The spatial–temporal patterns of the MJO and BSISO modes are identified with the extended empirical orthogonal function analysis of 31?years (1979–2009) OLR data for the December–February and June–August period, respectively. The dominant mode of the ISO at any given time can be judged by the proportions of the OLR anomalies projected onto the two modes. The bimodal ISO index provides objective and quantitative measures on the annual and interannual variations of the predominant ISO modes. It is shown that from December to April the MJO mode dominates while from June to October the BSISO mode dominates. May and November are transitional months when the predominant mode changes from one to the other. It is also shown that the fractional variance reconstructed based on the bimodal index is significantly higher than the counterpart reconstructed based on the Wheeler and Hendon’s index. The bimodal ISO index provides a reliable real time monitoring skill, too. The method and results provide critical information in assessing models’ performance to reproduce the ISO and developing further research on predictability of the ISO and are also useful for a variety of scientific and practical purposes.     

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources,plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and ⁸⁷Sr/⁸⁶Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.