Evaluation of sea water ingress into an Indian atoll

Groundwater is the prime source of fresh water in most part of world. The groundwater floats in the form of thin lens which is vulnerable to various stresses such as tide, cyclone, draught, abstraction, etc. The problem of getting this meager resource of fresh groundwater sustained for longer time, becomes more difficult task on tiny atoll with large population depending on it. In order to develop sustainable management scheme and identify vulnerable part of aquifer, systematic assessment of groundwater quality on such island have become imperative. Detailed hydrochemical study has been carried out to identify potential fresh groundwater resources on Andrott Island, UT of Lakshadweep, India. The analysis has given an early signal of deterioration in groundwater quality in some parts of the island during non-monsoon period, whereas the quality becomes slightly better during monsoon period. The study suggests immediate measures for arresting the deterioration in groundwater quality as well as augmentation for restoration of aquifer in some parts of the island.     

Origin of the UG2 chromitite layer, Bushveld Complex

Chromitite layers are common in large mafic layered intrusions.A widely accepted hypothesis holds that the chromitites formedas a consequence of injection and mixing of a chemically relativelyprimitive magma into a chamber occupied by more evolved magma.This forces supersaturation of the mixture in chromite, whichupon crystallization accumulates on the magma chamber floorto form a nearly monomineralic layer. To evaluate this and othergenetic hypotheses to explain the chromitite layers of the BushveldComplex, we have conducted a detailed study of the silicate-richlayers immediately above and below the UG2 chromitite and anotherchromitite layer lower in the stratigraphic section, at thetop of the Lower Critical Zone. The UG2 chromitite is well knownbecause it is enriched in the platinum-group elements and extendsfor nearly the entire 400 km strike length of the eastern andwestern limbs of the Bushveld Complex. Where we have studiedthe sequence in the central sector of the eastern Bushveld,the UG2 chromitite is embedded in a massive, 25 m thick plagioclasepyroxenite consisting of 60–70 vol. % granular (cumulus)orthopyroxene with interstitial plagioclase, clinopyroxene,and accessory phases. Throughout the entire pyroxenite layerorthopyroxene exhibits no stratigraphic variations in majoror minor elements (Mg-number = 79·3–81·1).However, the 6 m of pyroxenite below the chromitite (footwallpyroxenite) is petrographically distinct from the 17 m of hangingwall pyroxenite. Among the differences are (1) phlogopite, K-feldspar,and quartz are ubiquitous and locally abundant in the footwallpyroxenite but generally absent in the hanging wall pyroxenite,and (2) plagioclase in the footwall pyroxenite is distinctlymore sodic and potassic than that in the hanging wall pyroxenite(An_45–60 vs An_70–75). The Lower Critical Zone chromititeis also hosted by orthopyroxenite, but in this case the rocksabove and below the chromitite are texturally and compositionallyidentical. For the UG2, we interpret the interstitial assemblageof the footwall pyroxenite to represent either interstitialmelt that formed in situ by fractional crystallization or chemicallyevolved melt that infiltrated from below. In either case, themelt was trapped in the footwall pyroxenite because the overlyingUG2 chromitite was less permeable. If this interpretation iscorrect, the footwall and hanging wall pyroxenites were essentiallyidentical when they initially formed. However, all the modelsof chromitite formation that call on mixing of magmas of differentcompositions or on other processes that result in changes inthe chemical or physical conditions attendant on the magma predictthat the rocks immediately above and below the chromitite layersshould be different. This leads us to propose that the Bushveldchromitites formed by injection of new batches of magma witha composition similar to the resident magma but carrying a suspendedload of chromite crystals. The model is supported by the commonobservation of phenocrysts, including those of chromite, inlavas and hypabyssal rocks, and by chromite abundances in lavasand peridotite sills associated with the Bushveld Complex indicatingthat geologically reasonable amounts of magma can account foreven the massive, 70 cm thick UG2 chromitite. The model requiressome crystallization to have occurred in a deeper chamber, forwhich there is ample geochemical evidence. KEY WORDS: Bushveld complex; chromite; crystal-laden magma; crustal contamination; magma mixing; UG2 chromitite     

Water Quality Monitoring of Tropical Ponds: Location and Depth Effect in Two Case Studies

The variability in water chemistry of samples taken on a monthly basis (March 1999 to February 2000) from two shallow tropical ponds was studied. The effect of location and pond depth on water chemistry was also examined. The study demonstrated that intraannual variability in nutrient concentration is high. Thus, a high annual sampling frequency is required to provide representative annual mean water quality data. Routine monitoring during the monsoons is important for studies on dissolved oxygen and macrophyte growth. Significant differences were found between the topmost and bottommost points for samples of dissolved oxygen collected from the deepest part of both ponds. For nutrient analysis (nitrogen and phosphorus), sample from any location was found to be representative of the whole pond.     

Petrological and Geochemical Characteristics of Archean Gneisses and Granitoids from Bastar Craton, Central India – Implication for Subduction Related Magmatism

The gneisses and granitoids of Bastar craton (with rock suites up to 3.5 Ga) show calc-alkaline trondhjemitic characteristics. The rocks are enriched in both LILE and HFSE than primordial mantle. They have also relatively higher abundances of LILE and strong depletion at P and Ti in the multielement diagram. The depletion of Ti and P indicates retention of these elements by titanite and/or apatite during partial melting. It is proposed that subduction of an oceanic slab and its consequent melting led to the formation of the protoliths of the gneisses without much interaction with the mantle wedge. The granitoids represent temporally distinct suites formed in response to further melting of slab at greater depth and interaction of magma with the mantle wedge during their transport to the crust.     

Impact of Higher Frequency Content of Input Motion Upon Equivalent Linear Site Response Analysis for the Study Area of Delhi

While travelling through the subsoil layers, earthquake generated bedrock motions get modified significantly due to local soil and should be quantified using ground response analysis. Present study concentrates on equivalent linear method of site response analysis in SHAKE2000 software. It is a frequency based analysis tool having default frequency set to 15 Hz. While due consideration is given to amplitude, no to very limited information about the frequency content of the input motion to be considered in ground response analysis is available. In the present work, the effect of the maximum frequency of ground motion in site response analysis using SHAKE2000 is examined. Two sets of analyses are carried out in this work based on 30 globally recorded input motions. In the first analyses, input motion up to 15 Hz maximum frequency, which is a default value in SHAKE2000 is considered while second analyses are based on considering each of the 30 input motions up to the Nyquist frequency. Comparing the results from the two sets of analyses highlight that selection of maximum frequency in SHAKE2000 has considerable effect in ground motion amplification at different depths. As a result, even the peak ground acceleration which controls the building behavior and damage scenario, is going to change considerably even in case same input motion is used in the analysis.     

Gold - sulphide mineralization in ultramafic-mafic-granite complex of Jashpur,Bastar craton,central India: Evidences from geophysical studies

Time Domain Induced Polarization (TDIP) investigation in conjunction with high resolution electrical resistivity tomography surveys carried out at seven sites along a 5.2 km line in Jashpur and Raigarh districts of Chhattisgarh State yielded promising results leading to the demarcation of mineralized zones. These sites are confined to ultramafic-mafic-felsic intrusive complexes representing gabbro-pyroxenite-granite sequences characterized by gold - sulphide mineralization. Two dimensional (2D) geophysical datasets yielded interesting results and a good correlation is found between 2D inverted resistivity tomography and time domain induced polarization (IP) models. Both high resistivity-high chargeability as well as low resistivity-high chargeability conditions have been inferred from these datasets. This observation is consistent with conductive metallic mineralization. Interpretation of the Pharsabahar-Pandripani site, reveals a chargeability magnitude of 9-14 mV/V, which represents a strong metallic conductor associated with gold-sulphide ore body. The results at Samarkachar site are equally significant, showing a good correlation with the presence of a low resistivity and high chargeability zone. Hence, the present study suggests a new scope for application of resistivity and IP tomography surveys for mineral prospecting in favourable geological settings.     

Petrochemistry of sandstones from Neoproterozoic basins of the Bastar craton, Central Indian Shield: Implications for paleoweathering, provenance and tectonic history

Petrographic analysis and chemical analysis of major and trace elements including rare earth elements of the Neoproterozoic sandstones from the Chandarpur Group and the Tiratgarh Formation have been carried out to determine their provenance, tectonic setting and weathering conditions. All sandstone samples are highly enriched in quartz but very poor in feldspar and lithic fragments. Petrographically and geochemically these sandstones are classified as subarkose, sublitharenite and arenite. The Chemical Index of Alteration (CIA mean 68) and Th/U ratios (mean 4.2) for these sandstones suggest their moderate weathering nature. Generally, all sandstone samples are strongly depleted in major elements (except SiO₂), trace elements (except Zr) and REE in comparison with Post Archean Australian Shale (PAAS) and Upper Continental Crust (UCC). Their mineralogy and mean of elemental ratios suitable for determination of provenance and tectonic setting, e.g. Al₂O₃/SiO₂ (0.02), K₂O/Na₂O (10), Eu/Eu* (0.67), (La/Lu)n (10.4), La/Sc (3), Th/Sc (1.2), La/Co (0.22), Th/Co (0.08), and Cr/Th (7.2), support a felsic source and a passive margin tectonic setting for these sandstones. Also these key elemental ratios do not show much variation over a range of SiO₂. Thus we attest their significance in determining source rock characteristics of quartz rich sandstones. Chondrite‐normalized REE patterns with LREE enrichment and a strong negative Eu anomaly are also attributed to felsic source rock characteristics for these sandstones. The source rocks identified are granite and gneiss of the Bastar craton. Minor amounts may have been derived from older supracrustals of the Bastar craton. However, the major element data of the Paleoproterozoic Sakoli schists when compared with those of the Neoproterozoic sandstones indicate that the schists were derived from a mafic source and deposited in an active continental margin tectonic setting. There is, however, little difference in CIA values between the Paleoproterozoic Sakoli schists and Neoproterozoic sandstones, indicating prevailing of similar (moderate‐intense) weathering conditions throughout the Proterozoic in the Bastar craton. Our study also suggests a change in the provenance and tectonic setting of deposition of sediments from dominantly a mafic source and an active continental margin in the Paleoproterozoic to dominantly granite and gneiss (felsic source) and a passive continental margin in the Neoproterozoic in the Bastar craton.     

Spectral properties of the accretion discs around rotating black holes

We study the radiation properties of an accretion disc around a rotating black hole. We solve the hydrodynamic equations and calculate the transonic solutions of accretion disc in the presence of shocks. Then we use these solutions to generate the radiation spectrum in the presence of radiative heating and cooling processes. We present the effect of spin parameter of the black hole on the emitted radiation spectrum. In addition, attention has also been paid to the variation in energy spectral index with Kerr parameter and accretion rate. We find that spectral index becomes harder as the spin parameter changes from negative (accretion disc is counter-rotating with respect to the black hole spin) to a positive value. Finally, we compute and compare the spectral characteristics due to a free-fall flow and a transonic flow. We notice significant differences in high energy contributions from these two solutions.