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11.
A radiogenic Os component in the oceanic lithosphere? Constraints from Hawaiian pyroxenite xenoliths
Indra Sekhar Sen Michael Bizimis Shichun Huang 《Geochimica et cosmochimica acta》2011,75(17):4899-4916
Platinum Group Element (PGE) concentrations in garnet pyroxenite xenoliths from Oahu, Hawaii, are significantly lower than those in mantle peridotites and show fractionated patterns (e.g. PdN/OsN = 2-10, PdN/IrN = 4-24; N = chondrite normalized) and very high ReN/OsN ratios (∼9-248). Mass balance calculations show that the bulk rock pyroxenite PGE inventory is controlled by the presence of sulfide phases. The 187Os/188Os ratios of these pyroxenites vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/188Os ratios show good correlations with bulk rock and clinopyroxene major and trace element compositions, and bulk rock PGE and sulfur abundances. These observations suggest that the Os isotope compositions in these pyroxenites largely reflect primary processes in the oceanic mantle and Pacific lithosphere.In contrast, bulk rock 187Os/188Os ratios do not correlate with other lithophile isotopic tracers (e.g. Rb-Sr, Sm-Nd, Lu-Hf) which show limited isotopic variability (Bizimis et al., 2005). This and the lack of 187Os/188Os vs. Re/Os correlations suggest that the range in Os isotope ratios is not likely the result of mixing between long-lived depleted and enriched components or aging of these pyroxenites within the Pacific lithosphere after its formation at a mid-oceanic ridge setting some 80-100 million years ago. We interpret the Os isotopes, PGE and lithophile element systematics as the result of melt-lithosphere interaction at the base of the Pacific lithosphere. The major and trace element systematics of the clinopyroxenes and bulk rock pyroxenites and the relatively constant lithophile element isotope systematics are best explained by fractional crystallization of a rather homogenous parental magma. We suggest that during melt crystallization and percolation within the lithosphere, the parental pyroxenite melt assimilated radiogenic Os from the grain boundaries of the peridotitic lithosphere. This radiogenic Os component may reside in the grain boundary sulfides or other trace phases, and may be due to fluids or melts that had previously percolated through the basal part of the lithosphere during its transit from a mid-oceanic ridge to its present position above the Hawaiian plume. As the solidus of the parental pyroxenite melt is lower than the solidus of the lithospheric peridotite, we envision that the pyroxenite-parent melt selectively assimilated the grain boundary sulfide phases with lower melting temperature as it percolated through the lithosphere, without significantly reacting with the silicate minerals. Thus while the parental melt of these pyroxenites originate within the Hawaiian plume, melt-lithosphere interaction during progressive crystallization may have selectively enriched the resulting melts with radiogenic Os, thereby decoupling Os from the lithophile element isotopes, but retaining a link between Os, PGE and fractional crystallization systematics. In this model, Oahu pyroxenites essentially represent melts from different stages of this melt-mantle reaction process at the base of the lithosphere, and we suggest that this process may also explain the similar Os vs. lithophile element decoupling seen in the rejuvenated volcanism in Oahu and Kauai. We further show that the pyroxenites do not posses the requisite Pt/Re ratios, where upon, recycling and aging would generate the coupled enrichments of 186Os-187Os isotope ratios observed in Hawaiian and other lavas. 相似文献
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Shallow landslides and consequent debris flows are an increasing concern in the Western Ghats of Kerala, India. Their increased frequency has been associated with deforestation and unfavourable land‐use practices in cultivated areas. In order to evaluate the influence of vegetation on shallow slope failures a physically based, dynamic and distributed hydrological model (STARWARS) coupled with a probabilistic slope stability model (PROBSTAB) was applied to the upper Tikovil River basin (55·6 km2). It was tuned with the limited evidence of groundwater conditions during the monsoon season of 2005 and validated against observed landslide activity in the hydrological year 2001–2002. Given the data poor conditions in the region some modifications to the original model were in order, including the estimation of parameters on the basis of generalized information from secondary sources, pedo‐transfer functions, empirical equations and satellite remote sensing data. Despite the poor input, the model captured the general temporal and spatial pattern of instability in the area. Sensitivity analysis proved root cohesion, soil depth and angle of internal friction as the most dominant parameters influencing slope stability. The results indicate the importance of root cohesion in maintaining stability and the critical role of the management of rubber plantations in this. Interception and evapotranspiration showed little influence on the development of failure conditions. The study also highlights the importance of high resolution digital terrain models for the accurate mechanistic prediction of shallow landslide initiation. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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M. Geetha Devi Z. S. Shinoon Al-Hashmi G. Chandra Sekhar 《International Journal of Environmental Science and Technology》2012,9(4):713-718
In this study, a naturally available crab shell chitosan of low molecular weight (20?kDa) has been used as adsorbent to evaluate the pollution load in vegetable oil mill effluent. A series of batch experiment was conducted by varying chitosan dosage (100?C400?mg), pH (2?C9), stirring time (15?C90)?min and agitation speed (25?C150?rpm) to study their effects on adsorption and flocculation processes. The parameters considered for adsorption study are chemical oxygen demand, total suspended solids, electrical conductivity and turbidity. The maximum reduction in chemical oxygen demand, total suspended solids, electrical conductivity and turbidity is 74, 70, 56 and 92?% , respectively. The observed experimental result showed that crab shell chitosan could able to reduce significantly the chemical oxygen demand, turbidity, electrical conductivity and suspended matter. The optimum conditions were estimated as 400?mg/l chitosan, pH 4 and 45?min of mixing time with mixing speed of 50?rpm. Chitosan showed very good pollution removal efficiency and can be used for the effective treatment of vegetable oil mill effluent. 相似文献
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Assessment of chemistry of groundwater infiltrated by pit-toilet leachate and contaminant removal by vadose zone form the focus of this study. The study area is Mulbagal Town in Karnataka State, India. Groundwater level measurements and estimation of unsaturated permeability indicated that the leachate recharged the groundwater inside the town at the rate of 1 m/day. The average nitrate concentration of groundwater inside the town (148 mg/L) was three times larger than the permissible limit (45 mg/L), while the average nitrate concentration of groundwater outside the town (30 mg/L) was below the permissible limit. The groundwater inside the town exhibited E. coli contamination, while groundwater outside the town was free of pathogen contamination. Infiltration of alkalis (Na+, K+) and strong acids (Cl?, SO4 2?) caused the mixed Ca–Mg–Cl type (60 %) and Na–Cl type (28 %) facies to predominate groundwater inside the town, while, Ca–HCO3 (35 %), mixed Ca–Mg–Cl type (35 %) and mixed Ca–Na–HCO3 type (28 %) facies predominated groundwater outside/periphery of town. Reductions in E. coli and nitrate concentrations with vadose zone thickness indicated its participation in contaminant removal. A 4-m thickness of unsaturated sand + soft, disintegrated weathered rock deposit facilitates the removal of 1 log of E. coli pathogen. The anoxic conditions prevailing in the deeper layers of the vadose zone (>19 m thickness) favor denitrification resulting in lower nitrate concentrations (28–96 mg/L) in deeper water tables (located at depths of ?29 to ?39 m). 相似文献
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History of landslide susceptibility and a chorology of landslide-prone areas in the Western Ghats of Kerala,India 总被引：3，自引：0，他引：3
Kerala is the third most densely populated state in India. It is a narrow strip of land, of which 47% is occupied by the most
prominent orographic feature of peninsular India, The Western Ghats mountain chain. The highlands of Kerala experience several
types of landslides, of which debris flows are the most common. They are called “Urul Pottal” in the local vernacular. The
west-facing Western Ghats scarps that runs the entire extent of the mountain system is the most prone physiographic unit for
landslides. The highlands of the region experience an annual average rainfall as high as 500 cm through the South-West, North-East
and Pre-Monsoon showers. A survey of ancient documents and early news papers indicates a reduced rate of slope instability
in the past. The processes leading to landslides were accelerated by anthropogenic disturbances such as deforestation since
the early 18th century, terracing and obstruction of ephemeral streams and cultivation of crops lacking capability to add
root cohesion in steep slopes. The events have become more destructive given the increasing vulnerability of population and
property. Majority of mass movements have occurred in hill slopes >20° along the Western Ghats scarps, the only exception
being the coastal cliffs. Studies conducted in the state indicates that prolonged and intense rainfall or more particularly
a combination of the two and the resultant pore pressure variations are the most important trigger of landslides. The initiation
zone of most of the landslides was typical hollows generally having degraded natural vegetation. A survey of post-landslide
investigation and news paper reports enabled the identification of 29 major landslide events in the state. All except one
of the 14 districts in the state are prone to landslides. Wayanad and Kozhikode districts are prone to deep seated landslides,
while Idukki and Kottayam are prone to shallow landslides. 相似文献
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From the seismic point of view, the territory of Pakistan which lies between latitude 23°–37° N and longitude 61°–75° E is one of the most active zones in the world. The importance of this area lies in terms of movements of the Indian plate with respect to Eurasia on the west. Seismicity, as well as focal mechanism- solutions, throws a considerable light on the nature of forces acting in the area. All the available solutions, along with 12 new ones, have been considered for the present study. Their relationship to major faults in the area is discussed. The majority of the solutions in the central and northern parts show strike-slip faulting with a left-lateral sense of motion, followed by thrust faulting; few show normal faulting. This suggests that the Indian plate is moving with respect to the Eurasian plate along the Chaman fault, Quetta transverse zone, Sulaiman Ranges and the Hazara thrusts region joining the Hazara/Kashmir syntaxis. The orientations of P and T axes have been studied. It is seen that in a large number of cases compressive stress is acting nearly in NNW-SSE to N-S directions. The Hazara thrust region appears to be the most complex. Here, the influence of the Himalayan thrust front is evident to a large extent.The nature of faulting along the Chaman fault and Quetta transverse zone is to some extent similar to that of the San Andreas fault system of California. So far as the energy release is concerned, the maximum energy is being released in the form of strike-slip movements close to the Chaman fault and Quetta transverse ranges. 相似文献
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Probabilistic analysis by Monte Carlo Simulation method (MCSM) is a computationally prohibitive task for a reactive solute transport involving coupled PDEs with nonlinear source/sink terms in 3-D heterogeneous porous media. The perturbation based stochastic finite element method (SFEM) is an attractive alternative method to MCSM as it is computationally efficient and accurate. In the present study SFEM is developed for solving nonlinear reactive solute transport problem in a 3-D heterogeneous medium. Here the solution of the biodegradation problem involving a single solute by a single class of microorganisms coupled with dynamic microbial growth is attempted using this method. The SFEM here produces a second-order accurate solution for the mean and a first-order accurate solution for the standard deviation of concentrations. In this study both the physical parameters (hydraulic conductivity, porosity, dispersivity and diffusion coefficient) and the biological parameters (maximum substrate utilization rate and the coefficient of cell decay) are considered as spatially varying random fields. A comparison between the MCSM and SFEM for the mean and standard deviation of concentration is made for 1-D and 3-D problem. The effects of heterogeneity on the degradation of substrate and growth of biomass concentrations for a range of variances of input parameters are discussed for both 1-D and 3-D problems. 相似文献