The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H2O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle. 相似文献
The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H2O, CO2, N2 as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics. 相似文献
Abstract In Japan and Korea, some Lower Cretaceous terrigenous clastic rocks yield detrital chromian spinels. These chromian spinels are divided into two groups: low-Ti and high-Ti. The Sanchu Group and the Yuno Formation in Japan have both groups, whereas the Nagashiba Formation in Japan and the Jinju Formation in Korea have only the low-Ti spinels. High-Ti spinels are thought to have originated in intraplate-type basalt. Low-Ti spinels (higher than 0.6 Cr#) were probably derived from peridotites, which are highly correlated with an arc setting derivation and possibly with a forearc setting derivation. Low-Ti spinels are seen in the Sanchu Group, the Nagashiba Formation and the Jinju Formation. Low-Ti spinels from the Yuno Formation are characterized by low Cr# (less than 0.6) and these chromian spinels appear to have been derived from oceanic mantle-type peridotite, including backarc. According to maps reconstructing the pre-Sea of Japan configuration of the Japanese Islands and the Korean Peninsula, the Korean Cretaceous basin was comparatively close to the Southwest Japan depositional basins. It is possible that these Lower Cretaceous systems were sediments mainly in the forearc and partly in the backarc regions. The peridotite might have infiltrated along major tectonic zones such as the Kurosegawa Tectonic Zone (= serpentinite melange zone) in which left lateral movement prevailed during the Early Cretaceous. 相似文献
It is demonstrated that nonlinear Rossby modes, such as modons and IG eddies, can be excited in planetary fluids by a sufficiently strong forcing of potential vorticity. When a weak forcing is balanced with a weak dissipation, two (linear and nonlinear) equilibrium states can be produced, depending on the initial condition. When the fluid is inviscid, a sufficiently strong steady forcing may generate a sequence of propagating nonlinear eddies. A weak forcing, by contrast, only generates linear Rossby waves. The criterion which divides the high amplitude nonlinear state and the low amplitude linear state may be interpreted in terms of a ratio of a time necessary to force the eddy to a time for a fluid particle to circulate about the nonlinear eddy once. 相似文献
Peridotite samples recovered from IODP Site U1309 at the Atlantis Massif in the Mid-Atlantic Ridge were examined to understand
magmatic processes for the oceanic core complex formation. Original peridotite was fragmented, and the limited short peridotite
intervals are now surrounded by a huge gabbro body probably formed by late-stage melt injections. Each peridotite interval
has various petrographical and geochemical features. A spinel harzburgite in contact with gabbro shows evidence of limited
melt penetrations causing gradual compositional change, in terms of trace-element compositions of pyroxenes, as well as modal
change near the boundary. Geochemistry of clinopyroxenes with least melt effects indicates that the harzburgite is originally
mantle residue formed by partial melting under polybaric conditions, and that such a depleted peridotite is one of the components
of the oceanic core complex. Some of plagioclase-bearing peridotites, on the other hand, have more complicated origin. Although
their original features were partly overprinted by the injected melt, the original peridotites, both residual and non-residual
materials, were possibly derived from the upper mantle. This suggests that the melt injected around an upper mantle region
or into mantle material fragments. The injected melt was possibly generated at the ridge-segment center and, then, moved and
evolved toward the segment end beneath the oceanic core complex. 相似文献
Hydrothermal circulation beneath the spreading axis plays a significant role in the exchange of energy and mass between the solid Earth and the oceans. Deep-seated hydrothermal circulation down to the crust/mantle boundary in the fast-spreading axis has been introduced by a number of studies regarding geological investigations and numerical models. In order to assess a reaction between hydrothermal fluid and host rock around the crust/mantle boundary, we conducted bulk trace element and Sr isotope analyses with a series of in situ investigations for crustal anorthosite, a reaction product between hydrothermal fluid and gabbro in the lowermost crustal section along Wadi Fizh, northern Oman ophiolite. In addition, we conducted titanite U–Pb isotope analyses to evaluate timing of the crustal anorthosite formation in the framework of the evolutional process of the Oman ophiolite. We estimated the formation age of the crustal anorthosite at 97.5 Ma ± 5.0 Ma, overlapping with the timing of the crust formation in the paleo spreading axis. The crustal anorthosite shows high-Th/U ratio (~2.5) and high-initial 87Sr/86Sr ratio (0.7050) due to seawater-derived hydrothermal fluid ingress into the precursor gabbro. With using analytical technique of micro-excavation at cryo-temperature, we detected Cl from a few micrometer-sized inclusion of aqueous fluid and chromite grains. The solubility of Cr was enhanced by complexation reactions with Cl in the hydrothermal fluid. Regarding reconstructed three-dimensional mass distribution of the inclusion and chromite composition, maximum Cr content of parental fluid was estimated at ~69 000 μg/g. The exceptionally high-Cr content was achieved locally by leaking of fluid and synchronous chromite crystallization during fluid entrapment. Presence of the deep-seated hydrothermal circulation could be assigned to the segment end, where cold seawater penetrates into the lowermost crust and extract heat along widely spaced network-like fluid channel. 相似文献
A classification method which takes into account not only spectral but also spatial features for LANDSAT‐4 and 5 Thematic Mapper (TM) data is proposed. In accordance with improvement of Instantaneous Field of View (IFOV), spatial information such as textural, contextual, etc. is also increased so that some treatments of such information is highly required. One of the simplest spatial features is local spectral variability such as standard deviation, variability constant, variance, etc. in small cells such as 2x2,3x3 pixels. Such information can be used together with conventional spectral features in an unified way, for the traditional classifier such as a pixel‐wise Maximum Likelihood Decision Rule (MLDR). From the experiments, there was a substantial improvement in overall classification accuracy for TM forestry data. The probability of correct classification (PCC) for the new clearcut and the alpine meadow classes increased by 7% to 97% correct. The confusion between alpine meadow and new clearcut was reduced from 9% to 3%. 相似文献
Reductive precipitation of hexavalent chromium (Cr(VI)) with magnetite is a well-known Cr(VI) remediation method to improve water quality. The rapid (<a few hr) reduction of soluble Cr(VI) to insoluble Cr(III) species by Fe(II) in magnetite has been the primary focus of the Cr(VI) removal process in the past. However, the contribution of simultaneous Cr(VI) adsorption processes in aged magnetite has been largely ignored, leaving uncertainties in evaluating the application of in situ Cr remediation technologies for aqueous systems. In this study, effects of common groundwater ions (i.e., nitrate and sulfate) on Cr(VI) sorption to magnetite were investigated using batch geochemical experiments in conjunction with X-ray absorption spectroscopy.
Results
In both nitrate and sulfate electrolytes, batch sorption experiments showed that Cr(VI) sorption decreases with increasing pH from 4 to 8. In this pH range, Cr(VI) sorption decreased with increasing ionic strength of sulfate from 0.01 to 0.1 M whereas nitrate concentrations did not alter the Cr(VI) sorption behavior. This indicates the background electrolyte specific Cr(VI) sorption process in magnetite. Under the same ionic strength, Cr(VI) removal in sulfate containing solutions was greater than that in nitrate solutions. This is because the oxidation of Fe(II) by nitrate is more thermodynamically favorable than by sulfate, leaving less reduction capacity of magnetite to reduce Cr(VI) in the nitrate media. X-ray absorption spectroscopy analysis supports the macroscopic evidence that more than 75 % of total Cr on the magnetite surfaces was adsorbed Cr(VI) species after 48 h.
Conclusion
This experimental geochemical study showed that the adsorption process of Cr(VI) anions was as important as the reductive precipitation of Cr(III) in describing the removal of Cr(VI) by magnetite, and these interfacial adsorption processes could be impacted by common groundwater ions like sulfate and nitrate. The results of this study highlight new information about the large quantity of adsorbed Cr(VI) surface complexes at the magnetite-water interface. It has implications for predicting the long-term stability of Cr at the magnetite-water interface.
In order to examine the ecological risks caused by organotin compounds (OTs) in diadromous fish migrating between sea and freshwaters, tributyltin (TBT) and triphenyltin (TPT) compounds and their breakdown products were determined in the catadromous eel Anguilla japonica, which has sea, estuarine and river life histories, collected in Japanese sea, brackish and freshwaters within the same region. Ontogenic changes in otolith strontium (Sr) and calcium (Ca) concentrations were examined along the life history transect to discriminate the migration type. There were generally three different patterns, which were categorized as ‘sea eels’, ‘estuarine eels’ and ‘river eels’ according to the otolith Sr:Ca ratio. The concentrations of TBT in silver eels (mature eels) were significantly higher than that in yellow eels (immature eels), and the percentages of TBT were also higher in silver eels than in yellow eels. A positive correlation was found between TBT concentration and the gonad-somatic index (GSI). It is thus considered that silver eels have a higher risk of contamination by TBT than yellow eels. TBT and TPT concentrations in sea eels were significantly higher than those in river eels, while no significant differences were observed in TBT and TPT concentrations in estuarine eels compared to sea and river eels. These results suggest that sea eels have a higher ecological risk of OT contamination than river eels during their life history, and the risk of OTs in estuarine eels is considered to be intermediate between that of sea and river eels. Positive linear relationships were found between Sr:Ca ratios and the concentrations of TBT and TPT. Therefore, these results suggest that the ecological risk of OTs increase as the sea residence period in the eel becomes longer. TBT and TPT concentrations in sea eels were significantly higher than those in river eels even at the same growth stage. Thus, it is clear that migratory type is the most important factor for OT accumulation during the life history. 相似文献
Summary Xenoliths of harzburgite, lherzolite, dunite and wehrlite (= Group I rocks) in lamprophyre dikes from Shingu are accompanied by large amounts of ultramafic-mafic xeno liths with Al- and Ti-rich clinopyroxene and/or kaersuite (websterite, clinopyroxenite, kaersutite rock, gabbro and anorthosite) (= Group II rocks). The latter rocks often crosscut the Group I rocks as veinlets, indicating that Group II rocks are younger. Although harzburgites and lherzolite from Shingu have ordinary modal compositions, the constituent minerals have extraordinary chemical characteristics; low Mg and Cr and high Ti, Al and Fe3+. Fo values of olivine range from 91 to 77. Cr/(Cr + Al) atomic ratios of spinel are lower than 0.5 even in harzburgites. Fe3+/(Cr+Al+Fe3+) atomic ratios of spinel are sometimes over 0.1. TiO2 contents of clinopyroxene often exceed 0.5 wt%. These characteristics are revealed when Group I rocks are veined or selvaged by Group 11 rocks; chemical compositions of minerals in peridotites systematically change forwards the latter. This strongly suggests that injections of melts with alkali basaltic affinity which had precipitated Group 11 rocks resulted in diffusion metasomatism on the Group I rocks.It is likely that the metasomatized peridotites are widespread underneath the areas where alkali basalt magmatism had fluorished, such as southwestern Japan. Some of Fe-rich lherzolite and harzburgite xenoliths reported in the literature are possibly metasomatites.
Petrologie von Peridotit-Xenolithen in Lamprophyren von Shingu, Südwest-Japan: Hinweise auf die Herkunft Fe-reicher Mantel-Peridotite
Zusammenfassung In lamprophyrischen Gängen von Shingu kommen Xenolithe von Harzburgit, Lherzolith, Dunit and Wehrlit (= Gesteinsgruppe I) vor. Sie werden von einer Vielzahl von ultramaf-isch-mafischen Xenolithen mit Al- and Ti-reichem Klinopyroxen and/oder Kaersutit (Websterit, Klinopyroxenit, Kaersutit-Gestein, Gabbro and Anorthosit) (=Gesteinsgruppe II) begleitet, die die Xenolithe der Gruppe I häufig gangförmig durchkreuzen, was auf ein jü ngeres Alter der Gesteinsgruppe II hinweist. Obwohl die Harzburgite and Lherzolithe von Shingu übliche modale Mineralbestände aufweisen, sind die Mineralchemismen außergewöhnlich: Niedrige Mg- and Cr- and hohe Ti-, Al- and Fe3+-Gehalte. Die Fo-Gehalte von Olivin reichen von 91 bis 77. Die Cr/(Cr+Al)-Atom-Verhältnisse der Spinelle sind kleiner als 0,5, sogar in den Harzburgiten; die Fe3+/(Cr+Al+Fe3+)-Atom-Verhaltnisse teilweise größer als 0,1. Der TiO2-Gehalt im Klinopyroxen ist meist über 0,5 Gew.%. Diese Charakteristika zeigen sich dort, wo die Gesteinsgruppe II die Gesteinsgruppe I durchschlägt oder kontaktiert. Der Mineralchemismus in den Peridotiten ändert sich dabei systematisch. Es wird vermutet, daß Schmelzinjektionen mit alkali-basaltischer Affinität, von denen die Gesteinsgruppe II herstammt, eine Diffusions-Metasomatose der Gesteinsgruppe I verursacht hat.Es wird angenommen, daß metasomatisierte Peridotite an der Basis von alkali-basaltischem Magmatismus weft verbreitet sind, wie zum Beispiel in Südwest-Japan. Einige in der Literatur aufscheinende Fe-reiche Lherzolith- and Harzburgit-Xenolithe sind möglicherweise metasomatisch entstanden.
