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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
12.
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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Hydrogeochemical modeling of organo-metallic colloids in the Nsimi experimental watershed,South Cameroon 总被引：1，自引：0，他引：1
M. Sekhar Jean-Jacques Braun K. V. Hayagreeva Rao Laurent Ruiz Henri Robain Jérôme Viers Jules Rémy Ndam Bernard Dupré 《Environmental Geology》2008,54(4):831-841
This paper presents a hydrogeochemical modeling code HYDROS, which combines the multi-component transport model with equilibrium
speciation module MINTEQA2. The processes of adsorption, aqueous speciation and mineral precipitation/dissolution are represented
in the model. The numerical model uses a sequential iterative approach for solving the solute transport and the equilibrium
geochemistry modules. Further the transport part is solved using an operator split approach wherein a finite volume method
is used for solving the advective equations while a classical finite difference method is employed for solving the dispersive
equations. The model performance is evaluated by comparing it with MINTOX for a literature problem. HYDROS is then applied
to the case study of the transfer of transition metals with organic colloids in the swamp groundwater system of the experimental
Nsimi watershed, representative of the humid tropical ecosystem of the South Cameroon Plateau. Field observations at the site
swamp system suggest that the carbon is mainly transferred as organic colloids (i.e., dissolved organic carbon) produced by
the slow biodegradation of the swamp organic matter. Using HYDROS, the behaviour of Al(III) and Fe(III) elements in the base
flow system is simulated during inter rain events of a short rainy season (May–June 1996). The elemental time-series for Al,
Fe, Cl, pH compare well with the simulation results. The colloids are found to have a strong impact on the mobilization and
transfer of Al(III) and Fe(III), which are considered to have low mobility in weathering environment. 相似文献
15.
K. N. Murali Krishna K. S. N. Reddy Ch. Ravi Sekhar K. Bangaku Naidu P. Ganapathi Rao G. V. R. Reddy 《Journal of the Geological Society of India》2016,88(5):637-647
Thirty one sediment samples from different varieties viz. yellow, reddish brown, brick red and light yellow sands from red sediments of Bhimunipatnam, Andhra Pradesh were studied to understand the heavy mineral assemblage, their fractionwise distribution and concentration. The heavy mineral assemblage in red sediments is ilmenite, magnetite, sillimanite, garnet, zircon, rutile, kyanite, monazite etc. Total heavy minerals (THM) wt. % varies from 16.67 to 23.99% and their concentration is not uniform in all the sedimentary samples. The higher THM wt% in brick red sands (23.99%) followed by reddish brown sands (20.24%), light yellow sands (17.10%) and yellow sands (16.67%). The finer fractions have more concentration of THM wt% than coarse fraction. The vertical distribution of heavy minerals in each sedimentary unit indicates that these units are not formed in single phase of deposition. Less concentration of garnets in yellow and light yellow sedimentary units indicates that the garnets might be chemically altered into iron hydroxide–limonite which gives yellow colour to the sediments under slightly oxidizing environment. Low concentration of garnets in brick red and reddish brown sediments indicates that garnets might have been undergone chemical decomposition under acidic conditions leads to produce iron oxides (Hematite) causes for red colorization of these units. The heavy mineral assemblage in different sand units indicates that they are derived from Eastern Ghat Group of rocks (khondalites and charnockites). 相似文献
16.
The Pedda vagu and Ookachetti vagu watersheds located in the semi-arid regions of Mahabubnagar district are highly dependent on groundwater for irrigation owing to unreliable rainfall and over extraction of groundwater. The present study has been conducted to identify spatio-temporal groundwater droughts and drought-prone zones. Temporal groundwater droughts have been determined using a standardized water-level index along with spatial groundwater droughts using spline interpolation in Geographic Information Systems. The study shows that the groundwater droughts varied among the stations during the observation period, i.e. 1998–2011. However, the spatial assessment shows that the region as such experienced more mild groundwater droughts except during severe meteorological drought years (1998, 2002, 2004 and 2008); this indicates that the region has good scope for groundwater exploitation during dry spells and initial stages of droughts. Therefore, it is critical to have plans for the development of groundwater to cope with drought. 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
19.
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