Takamoto Okudaira

Chemical characteristics of newly discovered black smoker fluids and associated hydrothermal plumes at the Rodriguez Triple Junction, Central Indian Ridge

Abstract The chemical characteristics of hydrothermal fluids in the Indian Ocean have been revealed for the first time. A hydrothermal field (called the Kairei field), including active black smoker chimneys, was discovered by ROV Kaiko dives at 25°19.17′S, 70°02.40′E (∼2450 m depth) on the southwestern flank of an off-axis knoll located ∼15 miles north of the Rodriguez Triple Junction, Central Indian Ridge, after detailed tow-yo surveys of the hydrothermal plume distribution. The temperature of the fluid expelled from the most active chimney was almost stable (359–360°C) throughout three successive days of fluid sampling. The endmember fluid has a chemical composition similar to the hydrothermal fluids sampled from sediment-starved mid-ocean ridges in the Pacific and the Atlantic oceans, suggesting typical interactions between hot fluid and mid-ocean ridge basalts in the subseafloor reaction zone.

First Hydrothermal Vent Communities from the Indian Ocean Discovered

Abstract Thriving chemosynthetic communities were located for the first time in the Indian Ocean between 2420 and 2450 m, on a volcanic knoll at the eastern crest of an axial valley, approximately 22 km north of the Rodriguez Triple Junction. The communities were distributed in a 40m by 80m field around the knoll. At least seven active vent sites, including black smoker complexes that were emitting superheated water at 360°C, were observed at the field. The faunal composition of the Indian Ocean hydrothermal vent communities had links to both Pacific and Atlantic vent assemblages. This discovery supports the hypothesis that there is significant communication between vent faunas in the Pacific and Atlantic Oceans via active ridges in the Indian Ocean.

A new estimate of the conditions for transition from basal 〈a〉 to prism [c] slip in naturally deformed quartz

Abstract Quartz c -axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [ c ], prism 〈 a 〉 and basal 〈 a 〉 slip systems were activated during the plastic deformation under high-temperature metamorphic conditions. With increasing temperature, quartz c -axis fabric patterns change from girdle type, through X -maximum with a faint crossed girdle, to X -maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature, closely correlated with the change in quartz c -axis fabric. The fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size. Moreover, observations with transmission electron microscopy (TEM) clearly identify the dominant activation of [ c ] slip through the presence of basal subgrain boundaries and free dislocations with the [0001] Burgers vector in the quartz from the metachert samples exhibiting X -maximum c -axis fabric. Based on these observations it is concluded that the mechanism switch from basal 〈 a 〉 to prism [ c ] slip systems occurred with increasing temperature in the metacherts. The transition temperature is petrologically estimated to be ca. 550–600°C at the natural strain rate.

Sm-Nd and Rb-Sr dating of amphibolite from the Nellore- Khammam schist belt, SE India: constraints on the collision of the Eastern Ghats terrane and Dharwar-Bastar craton

The Nellore‐Khammam schist belt, SE India, is sandwiched in between the Proterozoic Eastern Ghats terrane and the Archaean Dharwar‐Bastar craton. We report Sm‐Nd and Rb‐Sr mineral isochron ages of amphibolite from the schist belt. The Sm‐Nd and Rb‐Sr ages are 824 ± 43 Ma and 481 ± 16 Ma, respectively. The Sm‐Nd age indicates the timing of peak metamorphism, whereas the Rb‐Sr age indicates the Pan-African thermal overprint. The peak metamorphism was related to collision of the Eastern Ghats terrane with the Dharwar-Bastar craton, which occurred during early Neoproterozoic time.

Petrology and geochronology of Mesoproterozoic mafic–intermediate plutonic rocks from Mitwaba (D. R. Congo): implications for the evolution of the Kibaran belt in central Africa

Mesoproterozoic supracrustal rocks in the Kibaride belt (southeast Congo) were intruded by mafic–intermediate plutonic rocks. These igneous rocks were affected by greenschist- to amphibolite-facies metamorphism during the Mesoproterozoic Kibaran orogenesis. U–Pb single zircon dating of the Mitwaba mafic–intermediate plutonic rocks yields an emplacement age of 1.38 Ga. The compositions of the Mitwaba mafic–intermediate plutonic rocks range from gabbro to diorite and show the following elemental concentrations: SiO2: 49–58 wt %, TiO2: 0.53–0.92 wt %, Al2O3: 13.1–18.68 wt %, Zr: 45–142 ppm, Y: 13–43 ppm. Mg no. 40–66 indicates variable degrees of fractionation of the magmas. The rocks are marked by high and variable Th/Ta (3–14), La/Nb (2–5) and low Ce/Pb (0.3–12.8) and Ti/V (10–19). Chondrite-normalized REE patterns exhibit enrichment in LREE relative to HREE ((La/Yb)N = 2.9–5.8). Primordial mantle-normalized spider diagrams show negative slopes with gradual decrease from LIL to HFS elements and are marked by Nb and Ti negative anomalies. Immobile trace-element contents indicate a continental arc setting for these mafic–intermediate igneous rocks. They are inferred to have originated from a mantle wedge enriched by fluid from a subducting slab, with possibly an additional contribution from subducted sediments. Low, sub-chondritic Nb/Ta ratios in these mafic rocks support this interpretation. A model involving underplating of mafic–intermediate arc magma into the crust, triggering partial melting of Mitwaba group metasedimentary rocks during the accretionary stage of the Kibaran orogeny, is proposed to explain the coeval emplacement of mafic–intermediate arc magmas and peraluminous S-type granitoids in the Kibaride belt of central Africa.

U‐Pb SHRIMP Dating of Detrital Zircons from the Nzilo Group (Kibaran Belt): Implications for the Source of Sediments and Mesoproterozoic Evolution of Central Africa

The Kibaran Supergroup, a >3000‐km‐long belt of metasedimentary and igneous rocks in the southeastern Congo, is in a critical location between the Congo Craton (sensu stricto) and the Tanzania‐Bangweulu Block. Understanding its tectonic evolution will shed much‐needed light on the amalgamation history of sub‐Saharan Africa. This study presents U‐Pb SHRIMP age data for 150 detrital zircons from four metasedimentary formations of the Nzilo Group, the middle lithostratigraphic unit within the Kibaran Supergroup in Katanga Province. These samples yielded dates between \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Characteristic Differences in the Chemical Composition of Spheroidal Carbonaceous Particles in Japanese and Chinese Cities

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U-Pb SHRIMP Ages of Detrital Zircons from Hiriyur formation in Chitradurga Greenstone Belt and its Implication to the Neoarchean Evolution of Dharwar Craton, South India

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