Mineral chemistry and petrogenesis of chromitites from the Khoy ophiolite complex,Northwestern Iran: Implications for aggregation of two ophiolites          下载免费PDF全文

Fatemeh Zaeimnia  Ali Kananian  Shoji Arai  Mirsaleh Mirmohammadi  Ali Imamalipour  Mohamed Zaki Khedr  Makoto Miura  Khadidja Abbou‐Kebir 《Island Arc》2017,26(6)

The Khoy ophiolitic complex in Northwestern Iran is a part of the Tethyan ophiolite belt, and is divided into two sections: the Eastern ophiolite in Qeshlaq and Kalavanes (Jurassic–Cretaceous) and the Western ophiolite in Barajouk, Chuchak and Hessar (Late Cretaceous). Our chromitites can be clearly classified into two groups: high‐Al chromitites (Cr# = 0.38–0.44) from the Eastern ophiolite, and high‐Cr chromitites (Cr# = 0.54–0.72) from the Western ophiolite. The chromian spinels in high‐Al chromitite include primary mineral inclusions mainly as Na‐bearing diopside and pargasite with subordinate rutile and their formation was probably related to reaction between a MORB (mid‐ocean‐ridge basalt)‐like melt with depleted harzburgite, possibly in a back‐arc setting. Their host harzburgites contain clinopyroxene with higher contents of Al₂O₃, Na₂O, Cr₂O₃, and TiO₂ relative to Western harzburgites and are possibly residue after moderate partial melting (~15 %) whereas the Western harzburgite is residue after high partial melting (~25 %). The chromian spinel in the Western Khoy chromitites contains inclusions such as clinopyroxene, olivine and platinum group mineral‐bearing sulfides. These Western chromitites were possibly formed at two stages during arc growth and are divided into the moderately high‐Cr# chromitites (Barajouk and Hessar) and the high‐Cr# chromitites (Chuchak A and C). The former crystallized from island‐arc‐tholeiite (IAT) melts during reaction with the host depleted harzburgites, whereas the latter crystallized from boninitic melts (second stage melt) during reaction with highly depleted harzburgite in a supra‐subduction‐zone environment. Based on the mineral chemistry of chromian spinels, pyroxenes, and mineral inclusions, the chromitites and the host peridotites from the Eastern and Western Khoy ophiolites were formed in a back‐arc basin and arc‐related setting, respectively. The Khoy ophiolitic complex is a tectonic aggregate of the two different ophiolites formed in two different tectonic settings at different ages.  相似文献   

Geochemistry of spinel-hosted amphibole inclusions in abyssal peridotite: insight into secondary melt formation in melt–peridotite reaction     

Akihiro?TamuraEmail author  Tomoaki?Morishita  Satoko?Ishimaru  Shoji?Arai 《Contributions to Mineralogy and Petrology》2014,167(3):974

Spinel-hosted hydrous silicate mineral inclusions are often observed in dunite and troctolite as well as chromitite. Their origin has been expected as products associated with melt–peridotite reaction, based on the host rock origin. However, the systematics in mineralogical and geochemical features are not yet investigated totally. In this study, we report geochemical variations of the spinel-hosted pargasite inclusions in reacted harzburgite and olivine-rich troctolite collected from Atlantis Massif, an oceanic core complex, in the Mid-Atlantic Ridge. The studied samples are a good example to examine geochemical variations in the inclusions because the origin and geological background of the host rocks have been well constrained, such as the reaction between MORB melt and depleted residual harzburgite beneath the mid-ocean ridge spreading center. The trace-element compositions of the pargasite inclusions are characterized by not only high abundance of incompatible elements but also the LREE and HFSE enrichments. Distinctive trace-element partitioning between the pargasite inclusion and the host-rock clinopyroxene supports that the secondary melt instantaneously formed by the reaction is trapped in spinel and produces inclusion minerals. While the pargasite geochemical features can be interpreted by modal change reaction of residual harzburgite, such as combination of orthopyroxene decomposition and olivine precipitation, degree of the LREE enrichment as well as variation of HREE abundance is controlled by melt/rock ratio in the reaction. The spinel-hosted hydrous inclusion could be embedded evidence indicating melt–peridotite reaction even if reaction signatures in the host rock were hidden by other consequent reactions.  相似文献   

Future climate of the Caribbean from a super-high-resolution atmospheric general circulation model     

Trevor C. Hall  Andrea M. Sealy  Tannecia S. Stephenson  Shoji Kusunoki  Michael A. Taylor  A. Anthony Chen  Akio Kitoh 《Theoretical and Applied Climatology》2013,113(1-2):271-287

Present-day (1979–2003) and future (2075–2099) simulations of mean and extreme rainfall and temperature are examined using data from the Meteorological Research Institute super-high-resolution atmospheric general circulation model. Analyses are performed over the 20-km model grid for (1) a main Caribbean basin, (2) sub-regional zones, and (3) specific Caribbean islands. Though the model’s topography underestimates heights over the eastern Caribbean, it captures well the present-day spatial and temporal variations of seasonal and annual climates. Temperature underestimations range from 0.1 °C to 2 °C with respect to the Japanese Reanalysis and the Climatic Research Unit datasets. The model also captures fairly well sub-regional scale variations in the rainfall climatology. End-of-century projections under the Intergovernmental Panel on Climate Change SRES A1B scenario indicate declines in rainfall amounts by 10–20 % for most of the Caribbean during the early (May–July) and late (August–October) rainy seasons relative to the 1979–2003 baselines. The early dry season (November–January) is also projected to get wetter in the far north and south Caribbean by approximately 10 %. The model also projects a warming of 2–3 °C over the Caribbean region. Analysis of future climate extremes indicate a 5–10 % decrease in the simple daily precipitation intensity but no significant change in the number of consecutive dry days for Cuba, Jamaica, southern Bahamas, and Haiti. There is also indication that the number of hot days and nights will significantly increase over the main Caribbean basin.  相似文献   

Distribution of moon jellyfish Aurelia aurita in relation to summer hypoxia in Hiroshima Bay,Seto Inland Sea     

Jun Shoji  Takaya Kudoh  Hideyuki Takatsuji  Osamu Kawaguchi  Akihide Kasai 《Estuarine, Coastal and Shelf Science》2010,86(3):485-490

Biological and physical surveys were conducted in order to investigate the relationship between environmental conditions and the distribution of moon jellyfish Aurelia aurita in Hiroshima Bay, western Seto Inland Sea, Japan. Moon jellyfish and ichthyoplankton were collected at 13 stations in Hiroshima Bay during monthly surveys from July to September in 2006 and 2007. Surface temperature in 2006 was significantly lower during the August and September cruises and surface salinity was lower during all cruises than in 2007. Moon jellyfish was the most dominant gelatinous plankton collected, accounting for 89.7% in wet weight. Mean moon jellyfish abundance in 2006 was higher than that in 2007 from July through September, with significant inter-year differences for July and September. Variability in precipitation and nutritional input from the Ohta River, northernmost part of Hiroshima Bay, were suggested as possible factors affecting the inter-annual variability in moon jellyfish abundance in the coastal areas of northern Hiroshima Bay. Moon jellyfish were more abundant in the coastal areas of northern Hiroshima Bay, where the dissolved oxygen (DO) concentration was lower, while low in the central part of the bay. Japanese anchovy Engraulis japonicus eggs were most dominant (58.1% in number) among the ichthyoplankton and were abundant in the central area of Hiroshima Bay. Explanatory analysis was conducted to detect possible effects of environmental conditions on the abundance of moon jellyfish and Japanese anchovy eggs during the summer months in Hiroshima Bay. Of the environmental conditions tested (temperature, salinity and DO of surface and bottom layers at each sampling station), bottom DO had the most significant effect on the moon jellyfish abundance: there was a negative correlation between the bottom DO and the moon jellyfish abundance in Hiroshima Bay during summer.  相似文献   

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin   总被引：1，自引：1，他引：0  

Alexey A. Krylov  Oleg M. Khlystov  Akihiro Hachikubo  Hirotsugu Minami  Yutaka Nunokawa  Hitoshi Shoji  Tamara I. Zemskaya  Lieven Naudts  Tatyana V. Pogodaeva  Masato Kida  Gennady V. Kalmychkov  Jeffrey Poort 《Geo-Marine Letters》2010,30(3-4):427-437

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.  相似文献   

Silica enrichment of Group II xenoliths by evolved alkali basalt from Jeju Island, South Korea: implication for modification of intraplate deep-seated rocks     

Kyounghee Yang  Csaba Szabó  Shoji Arai  Jae-Eun Yu  Hoonyeong Jung 《Mineralogy and Petrology》2012,106(1-2):107-130

Group II xenoliths, corresponding to the lithology of dunite, wehrlite to olivine clinopyroxenite and olivine websterite to websterite, occur in Pleisto-Holocene alkali basalts from Jeju Island, South Korea. The large grain size (up to 5?mm), moderate mg# [=100?×?Mg/(Mg?+?Fe_total) atomic ratio] of olivine (79–82) and pyroxenes (77–83), and absence of metamorphic textural features indicate that they are cumulates of igneous origin. Based on textural features, mineral equilibria and major and trace element variations, it can be inferred that the studied xenoliths were crystallized from basaltic melts enriched in incompatible trace elements and belong to the Jeju Pleisto-Holocene magma system. They appear to have been emplaced near the present Moho, an estimated 5–8?kbars beneath Jeju Island. Consolidation of cumulates was followed by infiltration of silica-enriched metasomatic melt, producing secondary orthopyroxenes at the expense of olivine. The metasomatic agent appears to have been a silica-enriched residual melt evolved from an initially slightly silica-undersaturated alkali basalt to silica-saturated compositions by fractional crystallization under relatively high pressure conditions. The result of this study indicates that relatively young olivine-bearing cumulates could have been metasomatized by a silica-enriched melt within underplates, suggesting that silica enrichment can occur in intraplate Moho-related rocks as well as in the upper mantle of the subarc area.  相似文献   

Behavior of MORB magmas at uppermost mantle beneath a fast-spreading axis: an example from Wadi Fizh of the northern Oman ophiolite     

Norikatsu Akizawa  Shoji Arai  Akihiro Tamura 《Contributions to Mineralogy and Petrology》2012,164(4):601-625

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Origin of wehrlite cumulates in the Moho transition zone of the Neoproterozoic Ras Salatit ophiolite,Central Eastern Desert,Egypt: crustal wehrlites with typical mantle characteristics     

Hisham?A.?GahlanEmail author  Shoji?Arai  Fawzy?F.?Abu El-Ela  Akihiro?Tamura 《Contributions to Mineralogy and Petrology》2012,163(2):225-241

Ultramafic cumulates, mainly crustal true wehrlites, were discovered and described in the mantle–crust transition zone (MTZ) and the extremely lower layered gabbro sequence of the Ras Salatit ophiolite, Central Eastern Desert, Egypt. They form either boudinaged lensoidal tabular bodies or interdigitated layers often concordant with the planolinear fabrics of the Ras Salatit ophiolite rocks. The contact between wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous, without chilled margins or any visible deformations. The Ras Salatit wehrlites are orthopyroxene-free and composed mainly of olivine and clinopyroxene. They are texturally equilibrated and show a characteristic poikilitic texture. Crystallization order of the Ras Salatit wehrlites is olivine/spinel followed by clinopyroxene with the absence of plagioclase. Olivine and clinopyroxene of the Ras Salatit wehrlites are compositionally uniform and conspicuously high in Mg#, mostly around 0.93 and 0.92, respectively. Moreover, the clinopyroxene shows low Ti and Al contents coupled with marked depletion in LILE. The calculated melt in equilibrium with clinopyroxene from the Ras Salatit wehrlites is largely similar to lavas from the Izu-Bonin forearc. Given the above characteristics, the Ras Salatit wehrlites were produced by crystal accumulation from a hydrous depleted basaltic/tholeiitic melt corresponding to temperatures between 1,000 and 1,100°C at the oceanic crustal pressure (~2 kbar). The involved hydrous tholeiitic melt has been probably formed by fluid-assisted partial melting of a refractory mantle source (similar to the underlying harzburgites) in a somewhat shallow sub-arc environment.  相似文献   

Structural changes of synthetic opal by heat treatment   总被引：1，自引：0，他引：1  

Akane Arasuna  Masayuki Okuno  Hiroki Okudera  Tomoyuki Mizukami  Shoji Arai  Shin’ichi Katayama  Mikio Koyano  Nobuaki Ito 《Physics and Chemistry of Minerals》2013,40(9):747-755

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO₄ tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO₄ tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.  相似文献   

Summer monsoon rainfall scenario over Bangladesh using a high-resolution AGCM     

Md. Mizanur Rahman  M. Rafiuddin  Md. Mahbub Alam  Shoji Kusunoki  Akio Kitoh  F. Giorgi 《Natural Hazards》2013,69(1):793-807

Summer monsoon rainfall was simulated by a global 20 km-mesh atmospheric general circulation model (AGCM), focusing on the changes in the summer monsoon rainfall of Bangladesh. Calibration and validation of AGCM were performed over Bangladesh for generating summer monsoon rainfall scenarios. The model-produced summer monsoon rainfall was calibrated with a ground-based observational data in Bangladesh during the period 1979–2003. The TRMM 3B43 V6 data are also used for understanding the model performance. The AGCM output obtained through validation process made it confident to be used for near future and future summer monsoon rainfall projection in Bangladesh. In the present-day (1979–2003) climate simulations, the high-resolution AGCM produces the summer monsoon rainfall better as a spatial distribution over SAARC region in comparison with TRMM but magnitude may be different. Summer monsoon rainfall projection for Bangladesh was experimentally obtained for near future and future during the period 2015–2034 and 2075–2099, respectively. This work reveals that summer monsoon rainfall simulated by a high-resolution AGCM is not directly applicable to application purpose. However, acceptable performance was obtained in estimating summer monsoon rainfall over Bangladesh after calibration and validation. This study predicts that in near future, summer monsoon rainfall on an average may decrease about ?0.5 % during the period 2015–2034 and future summer monsoon rainfall may increase about 0.4 % during the period 2075–2099.  相似文献