57Fe Mössbauer study of pumpellyiteokhotskite-julgoldite series minerals

Summary Fe and Mn distribution in the pumpellyite group minerals (W ₈ X ₄ Y ₈ Z ₁₂0_56-n (OH) _n ) has been studied by using⁵⁷Fe Mössbauer spectroscopy. The studied Fe-pumpellyites, belonging to the pumpellyite-julgoldite series, were collected from two localities; metabasites in the Tokoro belt, Hokkaido, and gabbroic sills in the Shimane Peninsula, Japan. Okhotskite, an Mn³⁺-dominant pumpellyite group mineral, was separated from the ores of metamorphosed manganiferous iron ore deposits in the Tokoro belt.⁵⁷Fe Mössbauer spectrum of Tokoro Fe-pumpellyite is composed of two Fe²⁺- and two Fe³⁺-doublets. On the basis of the single crystal structure refinements of Al-pumpellyites published so far, doublets were assigned to Fe _W ²⁺ (IS= 1.01 andQS = 2.73 mm/s), Fe _X ²⁺ (IS = 0.97 andQS = 3.18 mm/s), Fe _X ³⁺ (IS = 0.29 andQS =1.37 mm/s) and Fe _Y ³⁺ (IS = 0.36 andQS = 2.09 mm/s), whereIS is isomer shift relative to a metallic iron absorber andQS is quadrupole splitting. The Mössbauer spectrum of the Mitsu Fepumpellyite is composed of three doublets assigned to Fe _X ²⁺ (IS= 1.14 andQS = 3.20 mm/s), Fe _X ³⁺ (IS = 0.36 andQS =1.13 mm/s) and Fe _Y ³⁺ (IS = 0.37 andQS= 1.93 mm/s). These assignments show strong preference of Fe³⁺ in the X-site. The Mössbauer spectrum of the okhotskite is composed of two doublets by Fe _X ³⁺ (IS= 0.37 andQS = 1.13 mm/s) and Fe _Y ³⁺ (IS = 0.42 andQS = 2.18 mm/s). The area ratio shows that Fe _X ³⁺ :Fe _Y ³⁺ ratio is 94:6. On the basis of chemical and Mössbauer analyses, Mn _X ³⁺ :Mn _Y ³⁺ ratio is given as 19:81, indicating stronger Y-site preference of Mn³⁺ than Fe³⁺, what is consistent with Jahn-Teller theory. Al, Mn³⁺ and Fe³⁺ prefer the Y-site in this order.

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Eine⁵⁷Fe Mössbauer-Studie von Mineralen der Pumpellyit-Okhotskit-Julgoldit-Serie

Zusammenfassung Die Fe- und Mn-Verteilung in Mineralen der Pumpellyit-Gruppe (W ₈ X ₄ Y ₈ Z ₁₂O_56-n (OH)n) wurde mittels Mössbauer-Spektroskopie studiert. Die untersuchten Fe-Pumpellyite der Pumpellyit-Julgoldit-Serie stammen von zwei verschiedenen japanischen Lokalitäten: von Metabasiten des Tokoro-Gürtels, Hokkaido, und von Gabbro-Sills der Shimane Halbinsel. Okhotskit, ein Mn³⁺-dominiertes Mineral der Pumpellyit-Gruppe, wurde aus Erzen einer Mn-führenden Eisenerzlagerstätte des Tokoro-Gürtels separiert. Das⁵⁷Fe Mössbauer-Spektrum der Tokoro Fe-Pumpellyite zeigt zwei Fe²⁺- und zwei Fe³⁺-Doubletten. Auf Grund bisher publizierter verfeinerter Einkristall-Strukturuntersuchungen von Al-Pumpellyiten werden diese Doubletten folgendermaßen zugeordnet: Fe _W ²⁺ (IS = 1.01 undQS = 2.73 mm/s), Fe _X ²⁺ (IS = 0.97 undQS = 3.18 mm/s), Fe _X ³⁺ (IS = 0.29 undQS =1.37 mm/s) und Fe _Y ³⁺ (IS = 0.36 undQS = 2.09 mm/s).IS bezeichnet dabei die Isomer-Shift relativ zu einem metallischen Eisenabsorbenten,QS das Quadrupole-Splitting. Diese Zuordnungen belegen den bevorzugten Einbau von Fe³⁺ in die X-Position. Das Mössbauer-Spektrum von Okhotskit zeigt zwei Doubletten bei Fe _X ³⁺ (IS = 0.37 undQS = 1.13 mm/s) und Fe _Y ³⁺ (IS = 0.42 undQS = 2.18 mm/s). Das Flächenverhältnis zeigt, daß das Verhältnis Fe _X ³⁺ :Fe _Y ³⁺ 94:6 ist. Auf Grund der chemischen und der Mössbauer-Analysen wird das Mn _X ³⁺ :Mn _Y ³⁺ Verhältnis mit 19:81 angegeben. Mn³⁺ zeigt somit eine stärkere Präferenz für die Y-Position als Fe³⁺, ein Resultat, das mit der Jahn-Teller-Theorie konsistent ist. Der bevorzugte Einbau in die Y-Position ist, in dieser Reihenfolge, Al>Mn³⁺>Fe³⁺.

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With 4 Figures     

Late cretaceous to early paleogene paleomagnetic results from Sikhote Alin, far eastern Russia: implications for deformation of East Asia

Welded tuffs in the Bogopol and Sijanov groups were sampled at 27 sites from 12 caldera formations in the Sikhote Alin mountain range around Kavalerovo (44.3°N, 135.0°E) for chronological and paleomagnetic studies. KAr age dates show that the welded tuffs erupted between 66 Ma and 46 Ma. All sites yield reliable paleomagnetic directions, with unblocking temperatures higher than 560°C. The high-temperature component at 12 sites and the medium-temperature component at 3 sites in the Bogopol Group show reversed polarity (D = 193.7°, I = −57.6°,₉₅ = 8.1°). The high-temperature component at 11 sites in the Sijanov Group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north (D = −2.9°, I = 59.6°,₉₅ = 11.2°). The combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250.5°E, 84.1°N (A₉₅ = 8.8°), which falls near Cretaceous paleomagnetic poles from Outer Mongolia, Inner Mongolia, the North China Block and the South China Block. The Sikhote Alin area appears not to have been subjected to detectable motion with respect to East Asia since about 50 Ma. This implies that the Sikhote Alin area behaved as an integral part of East Asia during the opening of the Japan Sea at about 15 Ma. However, significant separation between the paleomagnetic poles of East Asia and Europe during the Jurassic-Paleogene implies a major relative movement between these two blocks since the Paleogene.     

A model of ocean-crust accretion for the Superior province, Canada

One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting.

In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment.

The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences.     

A review of kentriodontine dolphins (Cetacea; Deiphinoidea; Kentriodontidae): Systematics and biogeography

Abstract Fossil dolphins belonging to the extinct family Kentriodontidae are small to medium-sized toothed cetaceans, which probably include the ancestors of some living species. Kentriodontids are known from rocks of Late Oligocene to Late Miocene age in various parts of the world. Among kentriodontids, species in the subfamily Kentriodontinae (e.g. species of Kentriodon Kellogg, 1927) are the most ubiquitous and generalized; these are now known from latest Oligocene to earliest Miocene strata in New Zealand and Patagonia, and Middle Miocene deposits in Maryland, Virginia, California and Japan. The diversity, morphologies and distributions of Miocene species of Kentriodontinae seem to parallel those of the living species of mostly pelagic delphinids in the subfamily Delphininae, and the fossil group may have been an ecological or behavioral/functional counterpart of the latter. Kentriodontines are inferred to have been wide-ranging neritic to pelagic animals that ate small fish and other nectonic organisms; they were probably active echolocators, and might have formed large schools. They are relatively common as fossils and, therefore, are potentially useful for intercontinental correlations of marine deposits.     

Meander of the Kuroshio front and current variability in the East China Sea

To investigate the fluctuation of the Kuroshio front, moored current meters were deployed near the shelf break and on the continental slope in the East China Sea, northwest of Okinawa Island, during a period from 25 June to 22 July 1984. Two mooring arrays were deployed on the slope of about 800 m water depth (under the Kuroshio), about 30 km apart along the path of the Kuroshio. Another two arrays were set near the shelf break of about 300 m water depth. The fluctuation of current on the slope is found to have a predominant period of 11–14 days and a were length of 300–350 km, propagating toward the downstream direction of the Kuroshio with a phase velocity of about 30 cm sec^?1. When the Kuroshio front approaches the shelf break and the crest of the meander covers the mooring site, the current direction moves toward the downstream direction of the Kuroshio and the water temperature increases. On the other hand, when the trough of the meander covers the mooring site, the current direction changes off-shoreward across the Kuroshio or in the upstream direction of the Kuroshio, and the water temperature decreases. Three-dimensional distributions of water temperature and salinity around the mooring site were observed with a CTD twice at 5.5 days intervals, which indicate the meanders of the front is about 180° out of phase. This coincides with a period of 11–14 days obtained with the moored current meters. Wave lengths of the dominant meander of the front in the satellite thermal images were about 350 km and 100–200 km, which also coincides with results obtained with the moored current meters.     

A quadratic element method for evaluating groundwater flow by the inversion of surface tilt with application to the Tono Area, Japan

By modifying a previous method with constant elements, we developed a quadratic element method for more accurately estimating groundwater flow by the inversion of tilt data. In this method: (1) a region of groundwater flow is divided into quadratic elements in which the change in groundwater volume per unit volume of rock (Δv) and the Skempton coefficient (B) vary in a quadratic manner with the coordinates, (2) the values of Δv are set to zero at the boundaries of the region of groundwater flow and (3) the sum of the squared second derivatives of Δv is adopted as a constraining condition that is weighted and added to the sum of the squared errors in tilt. First, analyses were performed for a flow model to determine the accuracy of this method for estimating groundwater flow and also to clarify the effect of the assumed size of a region of groundwater flow. These analyses showed that the quadratic element method proposed in this study gives a much better estimation of Δv than the constant element method and that a large region of groundwater flow should be assumed, rather than a small region, since the values of Δv at points outside of the actual region of groundwater flow are estimated to be nearly zero when a large region is assumed while these values are greatly overestimated when an excessively small region is assumed. Finally, the quadratic element method was applied to the site of the Mizunami Underground Research Laboratory in the Tono area, Japan. Inverse analyses were performed for tilt data measured by four tiltmeters with a resolution of 10⁻⁹ radians during the excavation of two shafts under the assumption that the rock mass is an isotropic and homogeneous half- space. The results showed that the method proposed in this study reproduced the tilt data very accurately. Thus, the distribution of Δv was estimated without sacrificing the reproducibility of the tilt data. The contour maps of B(1 + ν)Δv (ν: Poisson’s ratio) showed that the heterogeneous flow of groundwater occurred at the site and that groundwater volume decreased mainly in the area surrounded by two faults. The latter result is consistent with the finding obtained by previous investigations that these faults have low permeability in the direction perpendicular to the strike and may act as a flow barrier.     

Nonlinear relationship between preservation of microstructure and geochemical composition of Pleistocene brachiopod shells from the Ryukyu Islands,southwestern Japan

Stable carbon and oxygen isotope composition of fossilized brachiopod shells serves as an important source to delineate Earth's paleoenvironmental evolution in the Phanerozoic. However, the original isotopic composition is potentially modified by various kinds of diagenesis. To evaluate the extent to which the original isotopic composition of fossilized brachiopod shells is modified by meteoric diagenesis, microstructure, cathodoluminescence (CL) images, and carbon and oxygen isotope composition of fossilized Kikaithyris hanzawai (rhynchonellate brachiopod) shells were examined. The shells were collected from Pleistocene shallow marine carbonates exposed on the Ryukyu Islands, southwestern Japan. The extent of diagenetic alteration is quantitatively evaluated here as both the preservation state of the original shell microstructure and the luminescence/non‐luminescence of shells. Although altered fibers were commonly observed in the brachiopod shells, the original isotopic composition was almost retained. There are no significant differences in the isotopic composition between the luminescent and non‐luminescent shells. There is no direct relationship between the preservation state of the original shell microstructure and the luminescence/non‐luminescence of shells at three of four horizons, indicating that CL images are not necessarily useful for the detection of diagenetic alteration of shells or shell portions. Applying multiple criteria to assessing diagenetic alteration and cross‐checking them are required to distinguish between diagenetically altered and unaltered brachiopod shells.     

Recent Dasycladales (Chlorophyta) in Okinawa Jima in the Central Ryukyus,southwestern Japan: Paleontological implications

Recent dasycladalean algae (Dasycladales, Chlorophyta) are classified into two families, 11 genera, and 44 species, and inhabit the tropical to warm‐temperate waters of the world. Twelve species of dasycladalean algae occur in Okinawa Jima, Ryukyu Islands. In this paper, the present‐day dasycladalean species and their distributions in Okinawa Jima are reported and their paleontological implications are discussed. Dasycladalean algae grow mainly on coral rubble and rocks, often forming large meadows in calm shallow lagoons. However, Dasycladus vermicularis occurs in deeper waters, at depths of 15–89 m, around Okinawa Jima. Neomeris annulata is found at water depths of 0–30 m. These data throw doubt on the interpretation that the occurrence of dasycladalean algae is linked to warm shallow habitats.     

Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) software

Results from computational morphodynamics modeling of coupled flow–bed–sediment systems are described for 10 applications as a review of recent advances in the field. Each of these applications is drawn from solvers included in the public-domain International River Interface Cooperative (iRIC) software package. For mesoscale river features such as bars, predictions of alternate and higher mode river bars are shown for flows with equilibrium sediment supply and for a single case of oversupplied sediment. For microscale bed features such as bedforms, computational results are shown for the development and evolution of two-dimensional bedforms using a simple closure-based two-dimensional model, for two- and three-dimensional ripples and dunes using a three-dimensional large-eddy simulation flow model coupled to a physics-based particle transport model, and for the development of bed streaks using a three-dimensional unsteady Reynolds-averaged Navier–Stokes solver with a simple sediment-transport treatment. Finally, macroscale or channel evolution treatments are used to examine the temporal development of meandering channels, a failure model for cantilevered banks, the effect of bank vegetation on channel width, the development of channel networks in tidal systems, and the evolution of bedrock channels. In all examples, computational morphodynamics results from iRIC solvers compare well to observations of natural bed morphology. For each of the three scales investigated here, brief suggestions for future work and potential research directions are offered. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd