Isotope–geochemical characterization and geothermometrical modeling of Uttarakhand geothermal field,India

Uttarakhand geothermal area, located in the central belt of the Himalayan geothermal province, is one of the important high temperature geothermal fields in India. In this study, the chemical characteristics of the thermal waters are investigated to identify the main geochemical processes affecting the composition of thermal waters during its ascent toward the surface as well as to determine the subsurface temperature of the feeding reservoir. The thermal waters are mainly Ca–Mg–HCO₃ type with moderate silica and TDS concentrations. Mineral saturation states calculated from PHREEQC geochemical code indicate that thermal waters are supersaturated with respect to calcite, dolomite, aragonite, chalcedony, quartz (SI > 0), and undersaturated with respect to gypsum, anhydrite, and amorphous silica (SI < 0). XRD study of the spring deposit samples fairly corroborates the predicted mineral saturation state of the thermal waters. Stable isotopes (δ¹⁸O, δ²H) data confirm the meteoric origin of the thermal waters with no oxygen-18 shift. The mixing phenomenon between thermal water with shallow ground water is substantiated using tritium (³H) and chemical data. The extent of dilution is quantified using tritium content of thermal springs and non-thermal waters. Classical geothermometers, mixing model, and multicomponent fluid geothermometry modeling (GeoT) have been applied to estimate the subsurface reservoir temperature. Among different classical geothermometers, only quartz geothermometer provide somewhat reliable estimation (96–140 °C) of the reservoir temperature. GeoT modeling results suggest that thermal waters have attained simultaneous equilibrium with respect to minerals like calcite, quartz, chalcedony, brucite, tridymite, cristobalite, talc, at the temperature 130 ± 5 °C which is in good agreement with the result obtained from the mixing model.     

Fine mode aerosol chemistry over a tropical urban atmosphere: characterization of ionic and carbonaceous species

An extensive aerosol sampling program was conducted during January-December 2006 over Kolkata (22o33?? N and 88o20?? E), a mega-city in eastern India in order to understand the sources, distributions and properties of atmospheric fine mode aerosol (PM_2.5). The primary focus of this study is to determine the relative contribution of natural and anthropogenic as well as local and transported components to the total fine mode aerosol loading and their seasonal distributions over the metropolis. The average concentrations of fine mode aerosol was found to be 71.2?±?25.2???gm^-3 varying between 34.5???gm^-3 in monsoon and 112.6???gm^-3 in winter. The formation pathways of major secondary aerosol components like nitrate and sulphate in different seasons are discussed. A long range transport of dust aerosol from arid and semi-arid regions of western India and beyond was observed during pre-monsoon which significantly enriched the total aerosol concentration. Vehicular emissions, biomass burning and transported dust particles were the major sources of PM_2.5 from local and continental regions whereas sea-salt aerosol was the major source of PM_2.5 from marine source regions.     

Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H₂O contents. The experimental results are used to explore the influence of H₂O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H₂O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO₂ contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO₂. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H₂O-rich (>4.5--6 wt% H₂O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H₂O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H₂O saturated conditions producing the distinctive high SiO₂ and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H₂O contents (>10 wt% H₂O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.Editorial responsibility: J. Hoefs     

Viscous fluid in a kaluza-klein metric

Exact solutions are obtained in a five-dimensional space-time with an energy-momentum tensor containing a viscous fluid, assuming either an equation of state or a special form for the viscous term in line with the assumption of Belinskii and Khalatnikov (1977). The solutions are, in fact, generalizations of an earlier work by Grøn for a perfect fluid in the 5D rest-mass varying theory of gravity proposed recently by Wesson. It is found that dimensional reduction of the extra space takes place in some of the cases such that the 5-dimensional universe naturally evolves into an effective 4-dimensional one. A huge amount of entropy can be produced following this shrinkage of extra-dimension which may account for the very large value of entropy per baryon observed in our 4D world. Moreover, the observed constancy of the rest-mass in the present era is also interpreted.     

Erosion study of a part of Majuli River-Island using remote sensing data

Majuli, the world’s largest river island, is situated in mid of river Brahmaputra in Assam. River Brahmaputra flows in highly braided channels most of them are transient in nature, being submerged during high monsoon flows and changing drastically their geometry and location. Majuli island, home of about 1.3 million people is endangered because of the erratic behavior of the river. In this study, an attempt has been made to observe the trends of erosion in a small part of Majuli island, the area near Kaniajan village in south Majuli- a stretch of about 11 km, using satellite data of 1991, 1997 and 1998. Image processing of digital data has been done in ILWIS software. Supervised for delineation of river from land and then change detection analysis has been done to find out changes in river course from 1991 to 1997 and further from 1997 to 1998. Erosion and deposition maps of the area have been prepared and the erosion of island is measured at various sections at 1 km interval. Erosion of 1900 ha has been observed during the period of six years from 1991 to 1997 and 845 ha during the period of one year from 1997 to 1998.     

Petrology, geochemistry and tectonic settings of the mafic dikes and sills associated with the evolution of the Proterozoic Cuddapah Basin of south India

In this article we summarize the petrological, geochemical and tectonic processes involved in the evolution of the Proterozoic intracratonic Cuddapah basin. We use new and available ages of Cuddapah igneous rocks, together with field, stratigraphic, geophysical and other criteria, to arrive at a plausible model for the timing of these processes during basin evolution. We present petrological and geochronological evidence of dike emplacement along preferred lineament directions around the basin in response to stresses, which may have been responsible for the evolution of the basin itself. Basaltic dike intrusion started on the south Indian shield around 2400 Ma and continued throughout the Cuddapah basin evolution and sedimentation. A deep mantle perturbation, currently manifested by a lopolithic cupola-like intrusion under the southwestern part of the basin, may have occurred at the onset of basin evolution and played an important role in its development. Paleomagnetic, gravity and geochronological evidence indicates that it was a constant thermal source responsible for dike and sill emplacement between 1500 and 1200 Ma both inside and out-side the basin. Lineament reactivation in the NW-SE and NE-SW directions, in response to the mantle perturbation, intensified between 1400 and 1200 Ma, leading to the emplacement of several cross cutting dikes. Fe-Mg partition coefficients of olivine and augite and Ca-Na partition coefficient of plagioclase, calculated from the composition of these minerals and bulk composition of their host rocks, indicate that the dikes outside the Cuddapah basin are cumulates. The contemporary dikes may be related by fractional crystallization as indicated by a positive correlation between their plagioclase Ca# (atomic Ca/[Ca+Na]) and augite Mg# (atomic Mg/[Mg+Fe]). A few NW-SE and NE-SW cross cutting dikes of the period between 1400 and 1200 Ma, preserve petrographic evidence of episodic magmatic intrusive activity along preferred directions. Petrological reasoning indicates that a magmatic liquid reacted with a set of cross cutting dikes, intruding into one that was already solidified and altering the composition of the magma that produced the other dike. The Cuddapah basin tholeiites may be related by fractional crystallization at 5 kb and 1019-1154‡ C, which occurred in the lopolithic cupola near the southwestern margin of the basin. Xenolith bearing picrites, which occur near the periphery of the cupola, originated by the accumulation of xenoliths in the tholeiites. This is indicated by the composition of the olivine in the xenoliths (Fo_78.7-81.9), which are closely similar to calculated olivine compositions (Fo_77.8-78.3) in equilibrium with the tholeiites under the sameP-T conditions. It is inferred that fractionation in the cupola resulted in crystals settling on its walls. Hence, the xenolith-bearing sills occur at the periphery of the lopolithic body. The tholeiites both inside and outside the basin are enriched in incompatible elements compared to mid oceanic ridge basalts. The Ba, Rb and K contents of the Cuddapah and other Proterozoic Gondwana tholeiites indicate that a widespread metasomatic enrichment of the mantle source may have occurred between R∼2.9 and R∼2.7Ga. There may be local heterogeneity in the source of the Cuddapah tholeiites as indicated by different Ba/Rb, Ti/Zr, Ti/Y, Zr/Nb and Y/Nb in samples inside and outside the basin. Large-scale differences such as the low P₂O₅-TiO₂ and high P₂O₅-TiO₂ basaltic domains of the Jurassic Gondwana basalts, however, did not exist during the Proterozoic time period under consideration. Although we are beginning to understand the tectono-magmatic processes involved in the evolution of the Cuddapah basin, much work remains to be done to obtain a complete picture. Future research in the Cuddapah basin should focus on obtaining accurate ages of the igneous rocks associated with the evolution of the basin.     

Pollution of limestone aquifer due to urban waste disposal around Raipur,Madhya Pradesh,India

During the rainy season deterioration in the quality of water, supplied through dug wells and tube wells, near an abandoned limestone quarry was reported. The abandoned quarry is now being used as an urban waste disposal site. The problem was further complicated by hospitalization of several inhabitants who were using this water for domestic purposes. Looking into the consequences, chemical analysis of water from the quarry, dug wells and tube wells was carried out. The water was found to be contaminated. The transportation of pollutants from the quarry to the groundwater system was facilitated by karst features. Furthermore, four major sources—domestic waste disposal, water conservation structures, landfills, and water wells—contributing to pollution were identified. This case study is an attempt to provide an understanding of how the karst features facilitate groundwater contamination. It will help us answer a few questions such as why karst hydrogeology deserves special attention in urban expansion and what protective measures should be planned in view of rapid urbanization.     

Thermodynamic properties of Zn(Al,Cr)2O4 spinels at high temperatures and pressures

Three Al-Cr exchange isotherms between Zn(Al, Cr)₂O₄ spinel and (Al, Cr)₂O₃ corundum crystalline solutions have been studied experimentally at 900°, 1100°, and 1300° C, at a total pressure of 25 kbar. Employing data on the equation of state of corundum (Chatterjee et al. 1982), the experimental results were evaluated thermodynamically. It was found that the thermodynamic mixing properties of Zn(Al, Cr)₂O₄ spinels are best described in terms of a symmetric Margules equation. The T- and P- dependence of the Margules Parameter, W _G ^Sp , and of ΔG^* of the exchange reaction, 1/2 ZnAl₂O₄ + 1/2 Cr₂O₃ = 1/2 ZnCr₂O₄+1/2 A1₂O₃, are found to be ΔG ^*=1493?2.869·T+0.0081·P and W _G ^Sp (J/mol)=23456+0.0386·P, with T given in K and P in bar.