Collisional bending of the western Paleo-Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

The Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (−9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc

Grain boundary diffusion of W in lower mantle phase with implications for isotopic heterogeneity in oceanic island basalts by core-mantle interactions

Tungsten isotopes provide important constraints on the ocean-island basalt (OIB) source regions. Recent analyses of μ182W in modern basalts with high 3He/4He originating from the core-mantle boundary region reveal two distinct features: positive μ182W in Phanerozoic flood basalts indicating the presence of primordial reservoir, and negative μ182W in modern OIBs. One possibility to produce large variations in μ182W is interaction between the mantle and outer core. Here, we report grain boundary diffusion of W in lower mantle phases. High pressure experimental results show that grain boundary diffusion of W is fast and strongly temperature dependent. Over Earth's history, diffusive transport of W from the core to the lowermost mantle may have led to significant modification of the W isotopic composition of the lower mantle at length scales exceeding one kilometer. Such grain boundary diffusion can lead to large variations in μ182W in modern basalts as a function of the distance of their source regions from the core mantle boundary. Modern oceanic island basalts from Hawaii, Samoa and Iceland exhibit negative μ182W and likely originated from the modified isotope region just above the core-mantle boundary, whereas those with positive μ182W could be derived from the thick Large Low Shear Velocity Provinces (LLSVPs) far from the core-mantle boundary (CMB). When highly-oxidized slabs accumulate at the CMB oxidizing the outer core at the interface, a large W flux with negative μ182W can be added to the silicate mantle. As a result, the source region of the OIB would be effectively modified to a negative μ182W

Disequilibrium REE compositions of garnet and zircon in migmatites reflecting different growth timings during single metamorphism (Aoyama area, Ryoke belt, Japan)

Chemical disequilibrium of coexisting garnet and zircon in pelitic migmatites (Aoyama area, Ryoke belt, SW Japan) is shown by microtextural evidence and their heavy rare earth element (HREE) patterns. In zircon, two stages of metamorphic rim growth is observed under cathodoluminescence image, although their SHRIMP UPb zircon ages are similar at ca. 92 Ma. Inner and outer rims of zircon tend to show steep HREE patterns irrespective of the UPb age. The inner rims tend to give higher U content than the outer rims; some rim analyses give various Th/U ratios of 0.02–0.07 compared to the very low (<0.02) values seen in the rest of rim analyses. The higher-Th/U values are ascribed to the mixed analyses between thin prograde domains and thick retrograde overgrowths. Zircon grains with inclusions similar to previously-reported melt inclusions are further enclosed in garnet, supporting the growth of thin zircon domains coexisting with garnet during the prograde metamorphism. Garnet rims are commonly replaced by biotite-plagioclase intergrowths, indicating a back reaction with partial melts. Garnet exhibits decrease in HREE and Y concentrations towards the rim, pointing to its prograde growth. The garnet cores have prograde xenotime inclusions, show steep HREE patterns, and yield growth temperature of ~530–570 °C by a YAG-xenotime thermometer. On the other hand, the garnet rims have no xenotime inclusion and show flat HREE patterns. Rare garnet domains including sillimanite needles also show flat HREE patterns and low Y concentrations, which is interpreted as a product of dehydration melting consuming biotite and sillimanite at near-peak P-T conditions (~800 °C and ~0.5 GPa). One such garnet domain gives nearly-equilibrium REE distribution pattern when paired with the matrix zircon rims. Retrograde xenotime is present in the cracks in garnet and in the biotite-plagioclase intergrowths, suggesting that retrograde breakdown of garnet released HREE and Y to form it. Considering the availability of HREE and Zr and presence of melt inclusions in zircon rims, most part of the zircon rims with positive HREE patterns likely grew during the melt crystallization stage, meaning that the zircon rims and presently-preserved garnet domains did not grow in equilibrium. The above scenario was tested by the array plot analysis and it gave a result consistent with microtextural and traditional REE distribution constraints. Combination of microtextural and the array plot analyses may become a powerful tool to reliably correlate the zircon ages to the P-T evolution of the high-grade metamorphic rocks

ON THE NAGELL-LJUNGGREN EQUATION (Analytic Number Theory : Distribution and Approximation of Arithmetic Objects)

We show that there exists an effective upper bound for the solutions to the Nagell-Ljunggren equation of the form --=y^{q} in 4 unknowns in integers x>1, y>1, m>2, q>1, when x is a cube of an integer. Our method relies on a refined estimate of linear forms in logarithms

Geochemical and radiogenic isotopic signatures of granitic rocks in Chanthaburi and Chachoengsao provinces, southeastern Thailand : Implications for origin and evolution

The Chanthaburi, Pliew, Klathing, Khao Cha Mao, and Khao Hin Son granitic bodies in Chanthaburi and Chachoengsao provinces in southeastern Thailand, which are located on the southwestern side of the Mae Ping Fault and eastern side of the Klaeng Fault, were investigated. In this study, magnetic susceptibility measurements, whole-rock chemical composition and Nd-Sr isotope analyses, and zircon U-Pb dating were conducted on these granitic bodies. The surveyed granitic rocks are classified as I- to A-type granites, are of the ilmenite series, and show clearly negative Eu anomalies, which suggest they formed under reducing conditions. Nd-Sr isotope ratios indicate continental crust material involvement in the formation of these granite bodies. The magnetic and geochemical signatures are similar to those of granite bodies in southwestern Cambodia. The study area is thus considered an extensional area of southwestern Cambodia, corresponding to the Sukhothai Zone (the Chanthaburi-Kampong Chhnang Zone). Zircon U-Pb dating yields ages of 208–214 Ma (the Late Triassic) for granite bodies except for the Khao Cha Mao granitic body, which dates to 55 Ma. The former age corresponds to the collision time of the Sibumasu and Indochina terranes, and the latter age is likely related to the collision time of the Indian and Eurasian continents

Sclerite formation in the hydrothermal-vent “scaly-foot” gastropod — possible control of iron sulfide biomineralization by the animal

A gastropod from a deep-sea hydrothermal field at the Rodriguez triple junction, Indian Ocean, has scale-shaped structures, called sclerites, mineralized with iron sulfides on its foot. No other organisms are known to produce a skeleton consisting of iron sulfides. To investigate whether iron sulfide mineralization is mediated by the gastropod for the function of the sclerites, we performed a detailed physical and chemical characterization. Nanostructural characterization of the iron sulfide sclerites reveals that the iron sulfide minerals pyrite (FeS2) and greigite (Fe3S4) form with unique crystal habits inside and outside of the organic matrix, respectively. The magnetic properties of the sclerites, which are mostly consistent with those predicted from their nanostructual features, are not optimized for magnetoreception and instead support use of the magnetic minerals as structural elements. The mechanical performance of the sclerites is superior to that of other biominerals used in the vent environment for predation as well as protection from predation. These characteristics, as well as the co-occurrence of brachyuran crabs, support the inference that the mineralization of iron sulfides might be controlled by the gastropod to harden the sclerites for protection from predators. Sulfur and iron isotopic analyses indicate that sulfur and iron in the sclerites originate from hydrothermal fluids rather than from bacterial metabolites, and that iron supply is unlikely to be regulated by the gastropod for iron sulfide mineralization. We propose that the gastropod may control iron sulfide mineralization by modulating the internal concentrations of reduced sulfur compounds

Análises in situ de U e Pb em zircão por SHRIMP II por controle remoto e de Hf por LA-ICP-MS: um exemplo de datação e da evolução genética de zircão através da razão 176Hf/177Hf em amostra da pedreira Ita no Complexo Atuba, SE Brasil

Operar o espectrômetro de massa SHRIMP à distância, remotamente via Internet, é sem dúvida uma técnica alternativa de grande interesse para a datação de cristais de zircão. Embora não represente avanço no método geocronológico U-Pb em zircão, a técnica de operação remota traz, além de facilidade, grande economia nesse tipo de análise. Foi executada pela primeira vez em análises espectrométicas envolvendo dois laboratoratórios internacionais (São Paulo, Brasil - Beijing, China) possibilitando a obtenção de resultados em tempo real. O procedimento foi aplicado em três amostras de rochas gnáissico-migmatíticas da pedreira Ita (próxima à cidade de Curitiba - Paraná - Brasil) pertencentes ao Complexo Atuba. Tais rochas, quando analisadas através do método U-Pb (TIMS) demonstraram uma evolução complexa, com idades bastante imprecisas. A presença de importantes heranças arqueanas e paleoproterozoicas, neste complexo, foram confirmadas nas zonas internas de cristais de zircão obtidas em leucossomas neoproterozoicos. Análises adicionais realizadas em rochas dioríticas indicaram ser intrusivas, e não encaixantes, e apresentaram idades relacionadas às colisões continentais (0.6 Ga) envolvidas durante a assembléia de Gondwana, no Neoproterozoico. A determinação das razões de isótopos de Hf em cristais de zircão, por meio de LA-ICP-MS, representa uma nova opção para checar a importância relativa da contribuição de material do manto (&#949;Hf >; 0) e crosta (&#949;Hf ; 0) and crustal contributions (&#949;Hf < 0) during the growth of the zircon crystals. While the archean component in the complex was derived from the mantle (&#949;Hf +1.5 to +8.7) the paleoproterozoic component had a crustal contribution (&#949;Hf-9.1 to -10.1)

A consumer’s guide to satellite remote sensing of multiple phytoplankton groups in the global ocean

Phytoplankton are composed of diverse taxonomical groups, which are manifested as distinct morphology, size and pigment composition. These characteristics, modulated by their physiological state, impact their light absorption and scattering, allowing them to be detected with ocean color satellite radiometry. There is a growing volume of literature describing satellite algorithms to retrieve information on phytoplankton composition in the ocean. This synthesis provides a review of current methods and a simplified comparison of approaches. The aim is to provide an easily comprehensible resource for non-algorithm developers, who desire to use these products, thereby raising the level of awareness and use of these products and reducing the boundary of expert knowledge needed to make a pragmatic selection of output products with confidence. The satellite input and output products, their associated validation metrics, as well as assumptions, strengths and limitations of the various algorithm types are described, providing a framework for algorithm organization to assist users and inspire new aspects of algorithm development capable of exploiting the higher spectral, spatial and temporal resolutions from the next generation of ocean color satellites