Measurements of ocean bottom pressure with a quartz sensor

Year long measurements of bottom pressure were made at 2,036 m depth in Sagami Trough, at 2,538 m depth in Suruga Trough, and at 32 m depth in the south of Minami-Daitojima Island. Amplitudes and phase lags of the major constituents of tides were estimated by the response method, and they were compared with the observational results at several tide stations operated by the Japan Meteorological Agency. A comparison with Schwiderski's global models for the eight tidal constituents showed that the amplitudes were in good accordance to one another within 3 cm, and that the differences of phase lags were less than 15°. The largest portion of the variations of the bottom pressure was caused by the tides: the variance of the major eight constituents was more than 98.5% as large as the total variance. The measurements show that tidal waves can be recorded offshore with a sufficient accuracy by the quartz sensors. Drifts of indication of the pressure gauges were significant and they prevented detection of a long-term variation which might be caused by fluctuations of the ocean currents or by the eddies.     

Episyenite formation in the Toki granite,central Japan

Episyenite is a quartz-depleted vuggy rock resulting from hydrothermal alteration of granitic rocks. This is the first report of its existence in an island arc, which is identified in a deep drill core of the Toki Cretaceous granite distributed in central Japan. In order to understand the petrographical features of the episyenite, neutron porosity measurement, geochemical analysis, microscopic observation, and X-ray computed tomography scanning were carried out. The results show remarkably high porosity (35.4 %) due to interconnecting vugs and the removal of quartz, plagioclase, and biotite. The Rb–Sr isotopic results and the paragenetic sequence of secondary minerals in the vugs suggest that the hydrothermal alteration process can be divided into an episyenitization stage and a later hydrothermal stage. At the episyenitization stage (70.6 ± 3.1 Ma) ca. 6 million years after the emplacement of the unaltered granite (76.3 ± 1.5 Ma), dissolution of quartz, biotite, and plagioclase occurred and was followed by the precipitation of albite, vermicular chlorite, and platy calcite. The episyenitization is considered as a local alteration of the Toki granite in an isotopically closed system. At the later hydrothermal stage, illite and secondary quartz precipitated from circulating meteoric-derived water in the dissolution vugs. Superimposing alteration at the later hydrothermal stage is limited, which results in the preservation of the episyenite in an almost primitive condition.     

Peculiar Mg–Ca–Si metasomatism along a shear zone within the mantle wedge: inference from fine-grained xenoliths from Avacha volcano,Kamchatka

We found extremely high-Mg# (=Mg/(Mg + total Fe) atomic ratio) ultramafic rocks in Avacha peridotite suite. All the high-Mg# rocks have higher modal amounts of clinopyroxene than ordinary Avacha peridotite xenoliths, and their lithology is characteristically heterogeneous, varying from clinopyroxenite through olivine websterite to pyroxene-bearing dunite. The Mg# of minerals is up to 0.99, 0.98 and 0.97 in clinopyroxene, orthopyroxene and olivine, respectively, decreasing progressively toward contact with dunitic part, if any. The petrographical feature of pyroxenes in the high-Mg# pyroxenite indicates their metasomatic origin, and high LREE/HREE ratio of the metasomatic clinopyroxene implies that the pyroxenites are the products of reaction between dunitic peridotites and high-Ca, silicate-rich fluids. The lithological variation of the Avacha high-Mg# pyroxenites from clinopyroxenite to olivine websterite resulted from various degrees of fluid-rock reaction coupled with fractional crystallization of the high-Ca fluids, which started by precipitation of high-Mg# clinopyroxene. Such fluids were possibly generated originally at a highly reduced serpentinized peridotite layer above the subducting slab. The fluids can reach the uppermost mantle along a shear zone as a conduit composed of fine-grained peridotite that developed after continent-ward asthenospheric retreats from the mantle wedge beneath the volcanic front. The fluids are incorporated in mantle partial melts when the magmatism is activated by expansion of asthenosphere to mantle wedge beneath the volcanic front.     

Origin of Neoproterozoic ophiolitic peridotites in south Eastern Desert, Egypt, constrained from primary mantle mineral chemistry

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe²⁺)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A1₂O₃, 0.88 wt.% Cr₂O₃ and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93?94.5) than that in dunites (Fo_94.3?Fo_95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46?0.81 and Mg#s, 0.34?0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82?0.86). The chromian spinels in Arais peridotites are low in TiO₂, 0.05 wt.% and Y_Fe [= Fe³⁺/(Cr + Al + Fe³⁺)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25?30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al₂O₃ composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine?spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ?O₂?=?2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).     

Sequentially sampled gas hydrate water,coupled with pore water and bottom water isotopic and ionic signatures at the Kukuy mud volcano,Lake Baikal: ambiguous deep-rooted source of hydrate-forming water

The isotopic and ionic composition of pure gas hydrate (GH) water was examined for GHs recovered in three gravity cores (165–193 cm length) from the Kukuy K-9 mud volcano (MV) in Lake Baikal. A massive GH sample from core St6GC4 (143–165 cm core depth interval) was dissociated progressively over 6 h in a closed glass chamber, and 11 sequentially collected fractions of dissociated GH water analyzed. Their hydrogen and oxygen isotopic compositions, and the concentrations of Cl^– and HCO₃ ^– remained essentially constant over time, except that the fraction collected during the first 50 minutes deviated partly from this pattern. Fraction #1 had a substantially higher Cl^– concentration, similar to that of pore water sampled immediately above (135–142 cm core depth) the main GH-bearing interval in that core. Like the subsequent fractions, however, the HCO₃ ^– concentration was markedly lower than that of pore water. For the GH water fractions #2 to #11, an essentially constant HCO₃ ^–/Cl^– ratio of 305 differed markedly from downcore pore water HCO₃ ^–/Cl^– ratios of 63–99. Evidently, contamination of the extracted GH water by ambient pore water probably adhered to the massive GH sample was satisfactorily restricted to the initial phase of GH dissociation. The hydrogen and oxygen isotopic composition of hydrate-forming water was estimated using the measured isotopic composition of extracted GH water combined with known isotopic fractionation factors between GH and GH-forming water. Estimated δD of ?126 to ?133‰ and δ¹⁸O of ?15.7 to ?16.7‰ differed partly from the corresponding signatures of ambient pore water (δD of ?123‰, δ¹⁸O of ?15.6‰) and of lake bottom water (δD of ?121‰, δ¹⁸O of ?15.8‰) at the St6GC4 coring site, suggesting that the GH was not formed from those waters. Observations of breccias in that core point to a possible deep-rooted water source, consistent with published thermal measurements for the neighboring Kukuy K-2 MV. By contrast, the pore waters of core St6GC4 and also of the neighboring cores GC2 and GC3 from the Kukuy K-9 MV show neither isotopic nor ionic evidence of such a source (e.g., elevated sulfate concentration). These findings constrain GH formation to earlier times, but a deep-rooted source of hydrate-forming water remains ambiguous. A possible long-term dampening of key deep-water source signatures deserves further attention, notably in terms of diffusion and/or advection, as well as anaerobic oxidation of methane.     

12th International Symposium on River Sedimentation(ISRS2013)(Kyoto,Japan,Sep.2-5,2013)

Date:Sep.2-5,2013 Venue:KYOTO TERRSA,Kyoto,Japan Organizer: Research Center for Fluvial & Coastal Disasters Disaster Prevention Research Institute,Kyoto University Erosion and sedimentation processes in river catchments as well as their management are of global importance.The social,economic,environmental and political impacts of these processes are significant.They pose great challenges for our human society on the way to the mid-21st century.     

The CLIVAR C20C project: which components of the Asian–Australian monsoon circulation variations are forced and reproducible?

A multi-model set of atmospheric simulations forced by historical sea surface temperature (SST) or SSTs plus Greenhouse gases and aerosol forcing agents for the period of 1950–1999 is studied to identify and understand which components of the Asian–Australian monsoon (A–AM) variability are forced and reproducible. The analysis focuses on the summertime monsoon circulations, comparing model results against the observations. The priority of different components of the A–AM circulations in terms of reproducibility is evaluated. Among the subsystems of the wide A–AM, the South Asian monsoon and the Australian monsoon circulations are better reproduced than the others, indicating they are forced and well modeled. The primary driving mechanism comes from the tropical Pacific. The western North Pacific monsoon circulation is also forced and well modeled except with a slightly lower reproducibility due to its delayed response to the eastern tropical Pacific forcing. The simultaneous driving comes from the western Pacific surrounding the maritime continent region. The Indian monsoon circulation has a moderate reproducibility, partly due to its weakened connection to June–July–August SSTs in the equatorial eastern Pacific in recent decades. Among the A–AM subsystems, the East Asian summer monsoon has the lowest reproducibility and is poorly modeled. This is mainly due to the failure of specifying historical SST in capturing the zonal land-sea thermal contrast change across the East Asia. The prescribed tropical Indian Ocean SST changes partly reproduce the meridional wind change over East Asia in several models. For all the A–AM subsystem circulation indices, generally the MME is always the best except for the Indian monsoon and East Asian monsoon circulation indices.     

East Asian summer monsoon simulation by a 20-km mesh AGCM

East Asian summer monsoon climate simulated by a global 20-km mesh atmospheric general circulation model (AGCM) forced by the global sea surface temperature during the period 1979–1998 is investigated. In comparison with a lower resolution (180-km mesh) model experiment, it is revealed that the 20-km mesh AGCM shows the superiority in simulating orographic rainfall not only its location but also its amount. The Baiu frontal structure is also better simulated in the higher resolution model, which leads to stronger Baiu rainfall. The 20-km model also shows more intense extremes in precipitation. Interannual variability of June–August mean precipitation and seasonal march of the monsoon rain band are also investigated. This paper is a contribution to the AMIP-CMIP Diagnostic Sub-project on General Circulation Model Simulation of the East Asian Climate, coordinated by W.-C. Wang.     

Genetic Aspects of the Manto-type Copper Deposits Based on Geochemical Studies of North Chilean Deposits

Recent studies on mineralogy, geochronology, fluid inclusion and stable isotope (Pb, Os, S, C, O, Sr) characteristics were reviewed to determine constraints for genetic models of the Chilean manto‐type copper deposits. The Chilean manto‐type deposits are divided into the two geologic categories of the northern areas (Arica–Iquique, Tocopilla–Taltal) and the central areas (Copiapó, La Serena, Santiago). The former is distributed in the coastal range composed of Jurassic andesite‐dominated volcano‐sedimentary piles and younger plutonic intrusions, and yields chalcocite (‐digenite) and bornite as the principal hypogene copper sulfides. The latter is hosted mostly in Lower Cretaceous volcano‐sedimentary sequences, and has chalcopyrite‐rich mineral associations. The fluid inclusion data indicate that the primary copper mineralization was commonly generated in the temperature range 150–360°C under low‐pressure conditions near the boiling curve, mediated with relatively saline brines. Generally, homogeneous Pb and S isotope compositions for primary copper minerals imply direct magma source or leaching of igneous rocks. Pb and Os isotope data published for some deposits, however, suggest that ore‐forming metals were derived mainly from the volcano‐sedimentary host rocks. The noticeably negative isotope ratios of primary sulfide sulfur and hydrothermal calcite carbon of some central area deposits indicate influx of sedimentary rock components, and the high ⁸⁷Sr/⁸⁶Sr initial ratios of hydrothermal calcite from the Tocopilla–Taltal area deposits imply contribution of the contemporaneous seawater or marine carbonates. These isotopic constraints imply a formation mechanism in which the Chilean manto‐type copper deposits formed epigenetically in the process of hydrothermal interaction of non‐magmatic surface‐derived brine with the volcano‐sedimentary host rocks, which is inferred to have been induced by a deep‐seated plutonic complex as the possible heat source.