New paleomagnetic results on $$\sim $$2367 Ma Dharwar giant dyke swarm,Dharwar craton,southern India: implications for Paleoproterozoic continental reconstruction

Here we report new paleomagnetic results and precise paleopole position of the extensional study on \(\sim \)2367 Ma mafic giant radiating dyke swarm in the Dharwar craton, southern India. We have sampled 29 sites on 12 dykes from NE–SW Karimnagar–Hyderabad dykes and Dhone–Gooty sector dykes, eastern Dharwar craton to provide unambiguous paleomagnetism evidence on the spectacular radiating dyke swarm and thereby strengthening the presence of single magmatic event at \(\sim \)2367 Ma. A total of 158 samples were subjected to detailed alternating field and thermal demagnetization techniques and the results are presented here along with previously reported data on the same dyke swarm. The remanent magnetic directions are showing two components, viz., seven sites representing four dykes show component (A) with mean declination of \(94{{}^{\circ }}\) and mean inclination of \(-\,70{{}^{\circ }}\) (\(\hbox {k}=87\), \(\upalpha _{95}=10{{}^{\circ }}\)) and corresponding paleopole at \(16{{}^{\circ }}\hbox {N}\), \(41{{}^{\circ }}\hbox {E}\) (\(\hbox {dp}=15{{}^{\circ }}\) and \(\hbox {dm}=17{{}^{\circ }}\)) and 22 sites representing 8 dykes yielded a component (B) with mean declination of \(41{{}^{\circ }}\) and mean inclination of \(-\,21{{}^{\circ }}\) (\(\hbox {k}=41\), \(\upalpha _{95}=9{{}^{\circ }}\)) with a paleopole at \(41{{}^{\circ }}\hbox {N}\), \(200{{}^{\circ }}\hbox {E}\) (\(\hbox {dp}=5{{}^{\circ }}\) and \(\hbox {dm}=10{{}^{\circ }}\)). Component (A) results are similar to the previously reported directions from the \(\sim \)2367 Ma dyke swarm, which have been confirmed fairly reliably to be of primary origin. The component (B) directions appear to be strongly overprinted by the 2080 Ma event. The grand mean for the primary component (A) combined with earlier reported studies gives mean declination of \(97{{}^{\circ }}\) and mean inclination of \(-\,79{{}^{\circ }}\) (\(\hbox {k}=55\), \(\upalpha _{95}=3{{}^{\circ }}\)) with a paleopole at \(15{{}^{\circ }}\hbox {N}\), \(57{{}^{\circ }}\hbox {E}\) (\(\hbox {dp}=5{{}^{\circ }}\), \(\hbox {dm}=6{{}^{\circ }}\)). Paleogeographical position for the Dharwar craton at \(\sim \)2367 Ma suggests that there may be a chance to possible spatial link between Dharwar dykes of Dharwar craton (India), Widgemooltha and Erayinia dykes of Yilgarn craton (Australia), Sebanga Poort Dykes of Zimbabwe craton (Africa) and Karelian dykes of Kola-Karelia craton (Baltica Shield).     

Ocean sea-ice modelling in the Southern Ocean around Indian Antarctic stations

An eddy-resolving coupled ocean sea-ice modelling is carried out in the Southern Ocean region (9\(^{\circ }\)–78\(^{\circ }\)E; 51\(^{\circ }\)–71\(^{\circ }\)S) using the MITgcm. The model domain incorporates the Indian Antarctic stations, Maitri (11.7\({^{\circ }}\)E; 70.7\({^{\circ }}\)S) and Bharati (76.1\({^{\circ }}\)E; 69.4\({^{\circ }}\)S). The realistic simulation of the surface variables, namely, sea surface temperature (SST), sea surface salinity (SSS), surface currents, sea ice concentration (SIC) and sea ice thickness (SIT) is presented for the period of 1997–2012. The horizontal resolution of the model varies between 6 and 10 km. The highest vertical resolution of 5 m is taken near the surface, which gradually increases with increasing depths. The seasonal variability of the SST, SSS, SIC and currents is compared with the available observations in the region of study. It is found that the SIC of the model domain is increasing at a rate of 0.09% per month (nearly 1% per year), whereas, the SIC near Maitri and Bharati regions is increasing at a rate of 0.14 and 0.03% per month, respectively. The variability of the drift of the sea-ice is also estimated over the period of simulation. It is also found that the sea ice volume of the region increases at the rate of 0.0004 \(\hbox {km}^{3}\) per month (nearly 0.005 \(\hbox {km}^{3}\) per year). Further, it is revealed that the accumulation of sea ice around Bharati station is more as compared to Maitri station.     

Synthesis of monticellite–forsterite and merwinite–forsterite symplectites in the CaO–MgO–SiO<Subscript>2</Subscript> model system: influence of temperature and water content on microstructure evolution

Homogeneous single crystals of synthetic monticellite with the composition \({\text{Ca}}_{0.88}{\text{Mg}}_{1.12}{\text{SiO}}_4\) (Mtc I) were annealed in a piston-cylinder apparatus at temperatures between 1000 and \(1200\,^{\circ }\hbox {C}\), pressures of 1.0–1.4 GPa, for run durations from 10 min to 24 h and applying bulk water contents ranging from 0.0 to 0.5 wt% of the total charge. At these conditions, Mtc I breaks down to a fine-grained, symplectic intergrowth. Thereby, two types of symplectites are produced: a first symplectite type (Sy I) is represented by an aggregate of rod-shaped forsterite immersed in a matrix of monticellite with end-member composition (Mtc II), and a second symplectite type (Sy II) takes the form of a lamellar merwinite–forsterite intergrowth. Both symplectites may form simultaneously, where the formation of Sy I is favoured by the presence of water. Sy I is metastable with respect to Sy II and is successively replaced by the latter. For both symplectite types, the characteristic spacing of the symplectite phases is independent of run duration and is only weeakly influenced by the water content, but it is strongly temperature dependent. It varies from about 400 nm at \(1000\,^{\circ }\hbox {C}\) to 1200 nm at \(1100\,^{\circ }\hbox {C}\) in Sy I, and from 300 nm at \(1000\,^{\circ }\hbox {C}\) to 700 nm at \(1200\,^{\circ }\hbox {C}\) in Sy II. A thermodynamic analysis reveals that the temperature dependence of the characteristic spacing of the symplectite phases is due to a relatively high activation energy for chemical segregation by diffusion within the reaction front as compared to the activation energy for interface reactions at the reaction front. The temperature dependence of the characteristic lamellar spacing and the temperature-time dependence of overall reaction progress have potential for applications in geo-thermometry and geo-speedometry.     

Effect of solar flare on the equatorial electrojet in eastern Brazil region

The effect of solar flare, sudden commencement of magnetic storm and of the disturbances ring current on the equatorial electrojet in the Eastern Brazil region, where the ground magnetic declination is as large as \(20^{^{\circ }}\hbox {W}\) is studied based on geomagnetic data with one minute resolution from Bacabal during November–December 1990. It is shown that the mean diurnal vector of the horizontal field was aligned along \(2{^{\circ }}\hbox {E}\) of north at Huancayo and \(30{^{\circ }}\hbox {W}\) of north at Bacabal during the month of December 1990. Number of solar flares that occurred on 30 December 1990 indicated the direction of solar flare related \(\Delta H\) vector to be aligned along \(5{^{\circ }}\hbox {E}\) of north at Huancayo and \(28{^{\circ }}\hbox {W}\) of north at Bacabal. This is expected as the solar flare effects are due to the enhanced conductivity in the ionosphere. The SC at 2230 UT on 26 November 1990 produced a positive impulse in \(\Delta X\) and negative impulse in \(\Delta Y\) at Bacabal with \(\Delta H\) vector aligned along \(27{^{\circ }}\hbox {W}\) of north. At Huancayo the \(\Delta H\) vector associated with SC is aligned along \(8{^{\circ }}\hbox {E}\) of north, few degrees east to the alignment of the diurnal vector of H. The magnetic storm that followed the SC had a minimum Dst index of –150 nT. The corresponding storm time disturbance in \(\Delta X\) at Huancayo as well as at Bacabal were about –250 nT but \(\Delta Y\) at Bacabal was about +70 nT and very small at Huancayo, that give the alignment of the H vector due to ring current about \(16{^{\circ }}\hbox {W}\) of north at Bacabal and almost along N–S at Huancayo. Thus alignment of the \(\Delta H\) vector due to ring current at Bacabal is \(14{^{\circ }}\hbox {E}\) of the mean direction of \(\Delta H\) vector during December 1990. This is consistent with the direction of ring current dependent on the dipole declination at the ring current altitude which is about \(5{^{\circ }}\hbox {W}\) of north over Bacabal and the deviation of declination due to the ring current during disturbed period given by the angle (\(\psi \)-D).     

Indian summer monsoon forcing on the deglacial polar cold reversals

The deglacial transition from the last glacial maximum at \(\sim \)20 kiloyears before present (ka) to the Holocene (11.7 ka to Present) was interrupted by millennial-scale cold reversals, viz., Antarctic Cold Reversal (\(\sim \)14.5–12.8 ka) and Greenland Younger Dryas (\(\sim \)12.8–11.8 ka) which had different timings and extent of cooling in each hemisphere. The cause of this synchronously initiated, but different hemispheric cooling during these cold reversals (Antarctic Cold Reversal \(\sim \)3\(^{\circ }\hbox {C}\) and Younger Dryas \(\sim \)10\(^{\circ }\hbox {C}\)) is elusive because \(\hbox {CO}_{2}\), the fundamental forcing for deglaciation, and Atlantic meridional overturning circulation, the driver of antiphased bipolar climate response, both fail to explain this asymmetry. We use centennial-resolution records of the local surface water \(\delta ^{18}\hbox {O}\) of the Eastern Arabian Sea, which constitutes a proxy for the precipitation associated with the Indian Summer Monsoon, and other tropical precipitation records to deduce the role of tropical forcing in the polar cold reversals. We hypothesize a mechanism for tropical forcing, via the Indian Summer Monsoons, of the polar cold reversals by migration of the Inter-Tropical Convergence Zone and the associated cross-equatorial heat transport.     

Assessing climate change impacts on river hydrology – A case study in the Western Ghats of India

The objective of this study is to evaluate the hydrological impacts of climate change on rainfall, temperature and streamflow in a west flowing river originating in the Western Ghats of India. The long-term trend analysis for 110 yr of meteorological variables (rainfall and temperature) was carried out using the modified Mann–Kendall trend test and the magnitude of the trend was quantified using the Sen’s slope estimator. The Regional Climate Model (RCM), COordinated Regional climate Downscaling EXperiment (CORDEX) simulated daily weather data of baseline (1951–2005) and future RCP 4.5 scenarios (2006–2060) were used to run the hydrological model, Soil and Water Assessment Tool (SWAT), in order to evaluate the effect of climate change on rainfall, temperature and streamflow. Significant changes were observed with regard to rainfall, which have shown decreasing trend at the rate of 2.63 mm per year for the historical and 8.85 mm per year for RCP 4.5 future scenarios. The average temperature was found to be increasing at \(0.10\,^{\circ }\hbox {C}\) per decade for both historical and future scenarios. The impact of climate change on the annual streamflow yielded a decreasing trend at the rate of \(1.2\,\hbox {Mm}^{3}\) per year and 2.56 \(\hbox {Mm}^{3},\) respectively for the past and future scenarios. The present work also investigates the capability of SWAT to simulate the groundwater flow. The simulated results are compared with the recession limb of the hydrograph and were found to be reasonably accurate.     

The effect of liquid composition on the partitioning of Ni between olivine and silicate melt

We report the results of experiments designed to separate the effects of temperature and pressure from liquid composition on the partitioning of Ni between olivine and liquid, \(D_{\text{Ni}}^{\text{ol/liq}}\). Experiments were performed from 1300 to 1600 °C and 1 atm to 3.0 GPa, using mid-ocean ridge basalt (MORB) glass surrounded by powdered olivine in graphite–Pt double capsules at high pressure and powdered MORB in crucibles fabricated from single crystals of San Carlos olivine at one atmosphere. In these experiments, pressure and temperature were varied in such a way that we produced a series of liquids, each with an approximately constant composition (~12, ~15, and ~21 wt% MgO). Previously, we used a similar approach to show that \(D_{\text{Ni}}^{\text{ol/liq}}\) for a liquid with ~18 wt% MgO is a strong function of temperature. Combining the new data presented here with our previous results allows us to separate the effects of temperature from composition. We fit our data based on a Ni–Mg exchange reaction, which yields \(\ln \left( {D_{\text{Ni}}^{\text{molar}} } \right) = \frac{{ -\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{RT} + \frac{{\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{R} - \ln \left( {\frac{{X_{\text{MgO}}^{\text{liq}} }}{{X_{{{\text{MgSi}}_{ 0. 5} {\text{O}}_{ 2} }}^{\text{ol}} }}} \right).\) Each subset of constant composition experiments displays roughly the same temperature dependence of \(D_{\text{Ni}}^{\text{ol/liq}}\) (i.e.,\(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\)) as previously reported for liquids with ~18 wt% MgO. Fitting new data presented here (15 experiments) in conjunction with our 13 previously published experiments (those with ~18 wt% MgO in the silicate liquid) to the above expression gives \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 3641 ± 396 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 1.597 ± 0.229. Adding data from the literature yields \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 4505 ± 196 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 2.075 ± 0.120, a set of coefficients that leads to a predictive equation for \(D_{\text{Ni}}^{\text{ol/liq}}\) applicable to a wide range of melt compositions. We use the results of our work to model the melting of peridotite beneath lithosphere of varying thickness and show that: (1) a positive correlation between NiO in magnesian olivine phenocrysts and lithospheric thickness is expected given a temperature-dependent \(D_{\text{Ni}}^{\text{ol/liq}} ,\) and (2) the magnitude of the slope for natural samples is consistent with our experimentally determined temperature dependence. Alternative processes to generate the positive correlation between NiO in magnesian olivines and lithospheric thickness, such as the melting of olivine-free pyroxenite, are possible, but they are not required to explain the observed correlation of NiO concentration in initially crystallizing olivine with lithospheric thickness.     

Application of environmental isotopes and hydrochemistry in the identification of source of seepage and likely connection with lake water in Lesser Himalaya,Uttarakhand, India

Oxygen (\({\updelta }^{18}\hbox {O}\)) and hydrogen (\({\updelta }^{2}\hbox {H}\) and \(^{3}\hbox {H}\)) isotopes of water, along with their hydrochemistry, were used to identify the source of a newly emerged seepage water in the downstream of Lake Nainital, located in the Lesser Himalayan region of Uttarakhand, India. A total of 57 samples of water from 19 different sites, in and around the seepage site, were collected. Samples were analysed for chemical tracers like \(\hbox {Ca}^{++}\), \(\hbox {Mg}^{++}\), \(\hbox {Na}^{+}\), \(\hbox {K}^{+}\), \({\hbox {SO}_{4}}^{--}\) and \(\hbox {Cl}^{-}\) using an Ion Chromatograph (Dionex IC-5000). A Dual Inlet Isotope Ratio Mass Spectrometer (DIIRMS) and an Ultra-Low Level Liquid Scintillation Counter (ULLSC), were used in measurements of stable isotopes (\({\updelta }^{2}\hbox {H}\) and \({\updelta }^{18}\hbox {O}\)) and a radioisotope (\(^{3}\hbox {H}\)), respectively. Results obtained in this study repudiate the possibility of any likely connection between seepage water and the lake water, and indicate that the source of seepage water is mainly due to locally recharged groundwater. The study suggests that environmental isotopes (\({\updelta }^{2}\hbox {H}\), \({\updelta }^{18}\hbox {O}\) and \(^{3}\hbox {H}\)) can effectively be used as ‘tracers’ in the detection of the source of seepage water in conjunction with other hydrochemical tracers, and can help in water resource management and planning.     

Ionic conductivity in gem-quality single-crystal alkali feldspar from the Eifel: temperature,orientation and composition dependence

We measured the ion conductivity of single-crystal alkali feldspar originating from two different locations in the Eifel/Germany, named Volkesfeld and Rockeskyller sanidine and having potassium site fractions \(C_\mathrm{K}\) of 0.83 and 0.71, respectively. The dc conductivities resulting from electrochemical impedance spectroscopy over the temperature range of 300–900 \(^{\circ }\hbox {C}\) show a weak composition dependence but pronounced differences between the b-direction [\(\perp (010)\)] and \(c^{*}\)-direction [\(\perp (001)\)] of the monoclinic feldspar structure. Conductivity activation energies obtained from the observed linear Arrhenius plots are close to 1.2 eV in all cases, which is closely similar to the activation energies of the \(^{22}\mathrm{Na}\) tracer diffusivity in the same crystals. Taking into account literature data on K tracer diffusion and diffusion correlation effects, the present results point to a predominance of the interstitialcy mechanism over the vacancy mechanism in mass and charge transport on the alkali sublattice in potassium-rich alkali feldspar.     

Characteristics of surface ozone in Agra,a sub-urban site in Indo-Gangetic Plain

In the present study, measurements of surface ozone (\(\hbox {O}_{3}\)) and its precursors (NO and \(\hbox {NO}_{2}\)) were carried out at a sub-urban site of Agra (\(27{^{\circ }}10'\hbox {N}\), \(78{^{\circ }}05'\hbox {E}\)), India during May 2012–May 2013. During the study period, average concentrations of \(\hbox {O}_{3}\), NO, and \(\hbox {NO}_{2}\) were \(39.6 \pm 25.3\), \(0.8 \pm 0.8\) and \(9.1 \pm 6.6 \, \hbox {ppb}\), respectively. \(\hbox {O}_{3}\) showed distinct seasonal variation in peak value of diurnal variation: summer \({>}\) post-monsoon \({>}\) winter \({>}\) monsoon. However, \(\hbox {NO}_{2}\) showed highest levels in winter and lowest in monsoon. The average positive rate of change of \(\hbox {O}_{3}\) (08:00–11:00 hr) was highest in April (16.3 ppb/hr) and lowest in August (1.1 ppb/hr), while average negative rate of change of \(\hbox {O}_{3}\) (17:00–19:00 hr) was highest in December (–13.2 ppb/hr) and lowest in July (–1.1 ppb/hr). An attempt was made to identify the \(\hbox {VOC--NO}_{\mathrm{x}}\) sensitivity of the site using \(\hbox {O}_{3}/\hbox {HNO}_{3}\) ratio as photochemical indicator. Most of the days this ratio was above the threshold value (12–16), which suggests \(\hbox {NO}_{\mathrm{x}}\) sensitivity of the site. The episodic event of ozone was characterized through meteorological parameters and precursors concentration. Fine particles (\(\hbox {PM}_{2.5}\)) cause loss of ozone through heterogeneous reactions on their surface and reduction in solar radiation. In the study, statistical analyses were used to estimate the amount of ozone loss.     

Near InfraRed Imaging Spectrograph (NIRIS) for ground-based mesospheric OH(6-2) and $$\hbox {O}_{2}$$(0-1) intensity and temperature measurements

This paper describes the development of a new Near InfraRed Imaging Spectrograph (NIRIS) which is capable of simultaneous measurements of OH(6-2) Meinel and \(\hbox {O}_{2}\)(0-1) atmospheric band nightglow emission intensities. In this spectrographic technique, rotational line ratios are obtained to derive temperatures corresponding to the emission altitudes of 87 and 94 km. NIRIS has been commissioned for continuous operation from optical aeronomy observatory, Gurushikhar, Mount Abu (\(24.6^{\circ }\hbox {N}\), \(72.8^{\circ }\hbox {E}\)) since January 2013. NIRIS uses a diffraction grating of 1200 lines \(\hbox {mm}^{-1}\) and 1024\(\times \)1024 pixels thermoelectrically cooled CCD camera and has a large field-of-view (FOV) of \(80^{\circ }\) along the slit orientation. The data analysis methodology adopted for the derivation of mesospheric temperatures is also described in detail. The observed NIRIS temperatures show good correspondence with satellite (SABER) derived temperatures and exhibit both tidal and gravity waves (GW) like features. From the time taken for phase propagation in the emission intensities between these two altitudes, vertical phase speed of gravity waves, \(c_{z}\), is calculated and along with the coherent GW time period ‘\(\tau \)’, the vertical wavelength, \(\lambda _{z}\), is obtained. Using large FOV observations from NIRIS, the meridional wavelengths, \(\lambda _{y}\), are also calculated. We have used one year of data to study the possible cause(s) for the occurrences of mesospheric temperature inversions (MTIs). From the statistics obtained for 234 nights, it appears that in situ chemical heating is mainly responsible for the observed MTIs than the vertical propagation of the waves. Thus, this paper describes a novel near infrared imaging spectrograph, its working principle, data analysis method for deriving OH and \(\hbox {O}_{2}\) emission intensities and the corresponding rotational temperatures at these altitudes, derivation of gravity wave parameters (\(\tau \), \(c_{z}\), \(\lambda _{z}\), and \(\lambda _{y})\), and results on the statistical study of MTIs that exist in the earth’s mesospheric altitudes.     

Carbon dioxide,water vapour and energy fluxes over a semi-evergreen forest in Assam,Northeast India

The eddy covariance method is a powerful technique for quantification of \(\hbox {CO}_{2},\) \(\hbox {H}_{2}\)O and energy fluxes in natural ecosystems. Leaf area index (LAI) and its changes are significant drivers of \(\hbox {CO}_{2}\) and \(\hbox {H}_{2}\)O exchange in a forest ecosystem due to their role in photosynthesis. The present study reports the seasonal variation of \(\hbox {CO}_{2}\) and energy fluxes and their relationship with other meteorological parameters of a semi-evergreen primary forest of Kaziranga National Park, Assam, India during February 2016–January 2017. The diurnal pattern of half hourly average \(\hbox {CO}_{2 }\) fluxes over the forest was found to be mostly dominated by the incident photosynthetically active radiation. During the period of study, diurnal variations of \(\hbox {CO}_{2}\) flux showed a maximum value of \(-9.97\,\upmu \)mol \(\hbox {m}^{-2}\hbox {s}^{-1}\) in the month of June during summer which is also the beginning of the monsoon season. The monthly averaged diurnal \(\hbox {CO}_{2}\) flux and variation in LAI of the forest canopy closely followed each other. The annual net ecosystem exchange of the forest estimated from the \(\hbox {CO}_{2}\) flux data above the canopy is 84.21 g C \(\hbox {m}^{-2}\,\hbox {yr}^{-1}\). Further studies are in progress to confirm these findings. The estimated average annual evapotranspiration of the semi-evergreen forest is 2.8 ± 0.19 mm \(\hbox {day}^{-1}\). The study of partitioning of energy fluxes showed the dominance of latent heat fluxes over sensible heat fluxes. The energy balance closure was found to increase with an increase in instability and the highest closure of around 83% was noted under neutral conditions.     

The high-pressure stability of chlorite and other hydrates in subduction mélanges: experiments in the system Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O

The solubility of chromium in chlorite as a function of pressure, temperature, and bulk composition was investigated in the system Cr₂O₃–MgO–Al₂O₃–SiO₂–H₂O, and its effect on phase relations evaluated. Three different compositions with X _Cr = Cr/(Cr + Al) = 0.075, 0.25, and 0.5 respectively, were investigated at 1.5–6.5 GPa, 650–900 °C. Cr-chlorite only occurs in the bulk composition with X _Cr = 0.075; otherwise, spinel and garnet are the major aluminous phases. In the experiments, Cr-chlorite coexists with enstatite up to 3.5 GPa, 800–850 °C, and with forsterite, pyrope, and spinel at higher pressure. At P > 5 GPa other hydrates occur: a Cr-bearing phase-HAPY (Mg_2.2Al_1.5Cr_0.1Si_1.1O₆(OH)₂) is stable in assemblage with pyrope, forsterite, and spinel; Mg-sursassite coexists at 6.0 GPa, 650 °C with forsterite and spinel and a new Cr-bearing phase, named 11.5 Å phase (Mg:Al:Si = 6.3:1.2:2.4) after the first diffraction peak observed in high-resolution X-ray diffraction pattern. Cr affects the stability of chlorite by shifting its breakdown reactions toward higher temperature, but Cr solubility at high pressure is reduced compared with the solubility observed in low-pressure occurrences in hydrothermal environments. Chromium partitions generally according to \(X_{\text{Cr}}^{\text{spinel}}\) ? \(X_{\text{Cr}}^{\text{opx}}\) > \(X_{\text{Cr}}^{\text{chlorite}}\) ≥ \(X_{\text{Cr}}^{\text{HAPY}}\) > \(X_{\text{Cr}}^{\text{garnet}}\). At 5 GPa, 750 °C (bulk with X _Cr = 0.075) equilibrium values are \(X_{\text{Cr}}^{\text{spinel}}\) = 0.27, \(X_{\text{Cr}}^{\text{chlorite}}\) = 0.08, \(X_{\text{Cr}}^{\text{garnet}}\) = 0.05; at 5.4 GPa, 720 °C \(X_{\text{Cr}}^{\text{spinel}}\) = 0.33, \(X_{\text{Cr}}^{\text{HAPY}}\) = 0.06, and \(X_{\text{Cr}}^{\text{garnet}}\) = 0.04; and at 3.5 GPa, 850 °C \(X_{\text{Cr}}^{\text{opx}}\) = 0.12 and \(X_{\text{Cr}}^{\text{chlorite}}\) = 0.07. Results on Cr–Al partitioning between spinel and garnet suggest that at low temperature the spinel- to garnet-peridotite transition has a negative slope of 0.5 GPa/100 °C. The formation of phase-HAPY, in assemblage with garnet and spinel, at pressures above chlorite breakdown, provides a viable mechanism to promote H₂O transport in metasomatized ultramafic mélanges of subduction channels.     

Semi-empirical model for retrieval of soil moisture using RISAT-1 C-Band SAR data over a sub-tropical semi-arid area of Rewari district,Haryana (India)

We have estimated soil moisture (SM) by using circular horizontal polarization backscattering coefficient (\(\sigma ^{\mathrm{o}}_{\mathrm{RH}}\)), differences of circular vertical and horizontal \(\sigma ^{\mathrm{o}} \, (\sigma ^{\mathrm{o}}_{\mathrm{RV}} {-} \sigma ^{\mathrm{o}}_{\mathrm{RH}})\) from FRS-1 data of Radar Imaging Satellite (RISAT-1) and surface roughness in terms of RMS height (\({\hbox {RMS}}_{\mathrm{height}}\)). We examined the performance of FRS-1 in retrieving SM under wheat crop at tillering stage. Results revealed that it is possible to develop a good semi-empirical model (SEM) to estimate SM of the upper soil layer using RISAT-1 SAR data rather than using existing empirical model based on only single parameter, i.e., \(\sigma ^{\mathrm{o}}\). Near surface SM measurements were related to \(\sigma ^{\mathrm{o}}_{\mathrm{RH}}\), \(\sigma ^{\mathrm{o}}_{\mathrm{RV}} {-} \sigma ^{\mathrm{o}}_{\mathrm{RH}}\) derived using 5.35 GHz (C-band) image of RISAT-1 and \({\hbox {RMS}}_{\mathrm{height}}\). The roughness component derived in terms of \({\hbox {RMS}}_{\mathrm{height}}\) showed a good positive correlation with \(\sigma ^{\mathrm{o}}_{\mathrm{RV}} {-} \sigma ^{\mathrm{o}}_{\mathrm{RH}} \, (R^{2} = 0.65)\). By considering all the major influencing factors (\(\sigma ^{\mathrm{o}}_{\mathrm{RH}}\), \(\sigma ^{\mathrm{o}}_{\mathrm{RV}} {-} \sigma ^{\mathrm{o}}_{\mathrm{RH}}\), and \({\hbox {RMS}}_{\mathrm{height}}\)), an SEM was developed where SM (volumetric) predicted values depend on \(\sigma ^{\mathrm{o}}_{\mathrm{RH}}\), \(\sigma ^{\mathrm{o}}_{\mathrm{RV}} {-} \sigma ^{\mathrm{o}}_{\mathrm{RH}}\), and \({\hbox {RMS}}_{\mathrm{height}}\). This SEM showed \(R^{2}\) of 0.87 and adjusted \(R^{2}\) of 0.85, multiple R=0.94 and with standard error of 0.05 at 95% confidence level. Validation of the SM derived from semi-empirical model with observed measurement (\({\hbox {SM}}_{\mathrm{Observed}}\)) showed root mean square error (RMSE) = 0.06, relative-RMSE (R-RMSE) = 0.18, mean absolute error (MAE) = 0.04, normalized RMSE (NRMSE) = 0.17, Nash–Sutcliffe efficiency (NSE) = 0.91 (\({\approx } 1\)), index of agreement (d) = 1, coefficient of determination \((R^{2}) = 0.87\), mean bias error (MBE) = 0.04, standard error of estimate (SEE) = 0.10, volume error (VE) = 0.15, variance of the distribution of differences \(({\hbox {S}}_{\mathrm{d}}^{2}) = 0.004\). The developed SEM showed better performance in estimating SM than Topp empirical model which is based only on \(\sigma ^{\mathrm{o}}\). By using the developed SEM, top soil SM can be estimated with low mean absolute percent error (MAPE) = 1.39 and can be used for operational applications.     

Fluvial geochemistry of Subarnarekha River basin,India

The fluvial geochemistry of the Subarnarekha River and its major tributaries has been studied on a seasonal basis in order to assess the geochemical processes that explain the water composition and estimate solute fluxes. The analytical results show the mildly acidic to alkaline nature of the Subarnarekha River water and the dominance of \(\hbox {Ca}^{2+}\) and \(\hbox {Na}^{+}\) in cationic and \(\hbox {HCO}_{3}^{-}\) and \({\hbox {Cl}}^{-}\) in anionic composition. Minimum ionic concentration during the monsoon and maximum concentration in the pre-monsoon seasons reflect concentrating effects due to decrease in the river discharge and increase in the base flow contribution during the pre-monsoon and dilution effects of atmospheric precipitation in the monsoon season. The solute acquisition processes are mainly controlled by weathering of rocks, with minor contribution from marine and anthropogenic sources. Higher contribution of alkaline earth \((\hbox {Ca}^{2+}{+}\,\hbox {Mg}^{2+})\) to the total cations \((\hbox {TZ}^{+})\) and high \((\hbox {Na}^{+}+\hbox {K}^{+})/\hbox {Cl}^{-}\), \((\hbox {Na}^{+}+\hbox {K}^{+})/\hbox {TZ}^{+}\), \(\hbox {HCO}_{3}^{-}/(\hbox {SO}_{4}^{2-}+\hbox {Cl}^{-})\) and low \((\hbox {Ca}^{2+}+\hbox {Mg}^{2+})/(\hbox {Na}^{+}+\hbox {K}^{+})\) equivalent ratios suggest that the Subarnarekha River water is under the combined influence of carbonate and silicate weathering. The river water is undersaturated with respect to dolomite and calcite during the post-monsoon and monsoon seasons and oversaturated in the pre-monsoon season. The pH–log \(\hbox {H}_{4}\hbox {SiO}_{4}\) stability diagram demonstrates that the water chemistry is in equilibrium with the kaolinite. The Subarnarekha River annually delivered \(1.477\times 10^{6}\) ton of dissolved loads to the Bay of Bengal, with an estimated chemical denudation rate of \(77\hbox { ton km}^{-2}\hbox { yr}^{-1}\). Sodium adsorption ratio, residual sodium carbonate and per cent sodium values placed the studied river water in the ‘excellent to good quality’ category and it can be safely used for irrigation.     

Audio-magnetotelluric investigation of sulfide mineralization in Proterozoic–Archean greenstone belts of Eastern Indian Craton

Greenstone belts are well known for gold occurrences at different regions of the world. The Dhanjori basin in the eastern Singhbhum region shows major characteristics of a rifted greenstone belt. Initially, we conducted 14 audio-magnetotelluric (AMT) measurements for a profile of \(\sim \)20 km in the frequency range of 1 kHz to 10 Hz over this rather complex geologic environment covering Dhanjori Volcanics (DhV) and Kolhan Group (KG). Subsequently, gravity and magnetic surveys were also conducted over this AMT profile. The purpose of the survey was to identify and map conductive features and to relate them to metallogeny of the area along with the mapping of the basement of Dhanjori basin. The strike analysis showed \(\hbox {N30}^{\circ }\hbox {W}\) strike for DhV for all the frequencies and for sites over KG domain in the frequency range of 100–10 Hz, but for KG domain, the obtained strike in 1 kHz to 100 Hz is \(\hbox {N45}^{\circ }\hbox {E}\). As the combination of transverse electric (TE), transverse magnetic (TM) and tipper (Tzy) can recover the electrical signature in complex geological environment, we discuss the conductivity model obtained from TE+TM+Tzy only. The inversion was carried for the regional profile with 14 sites and for 7 sites over KG domain. Conductivity model shows two well resolved conductors, one each in KG and Quartz Pebble Conglomerate Dhanjori (QPCD) domains respectively showing common linked concordant features between these regional and KG profiles. The conductors are interpreted as sulfide mineralization linked with QPCD group of rocks which may host gold. These conductors are also horizontally disposed due to the intrusive younger Mayurbhanj Granite. These intrusives correlate well with the gravity modeling as well. The thickness of the Dhanjori basin at the central is about 3.0 km, similar to that from gravity modeling. The conductivity model also indicates the presence of shallow conductors, but could not be resolved due to lack of high frequency data. However, the results from the close-by drill site indicate the presence of shallow sulfide mineralization hosting gold. The deep level conductors delineated from AMT studies are associated with gravity high and low magnetic. ICP-AES results of Dhanjori samples show significant concentration of gold \(\sim \)5.0 g/t, which is of economic consideration. Thus, it can be inferred that the conductors have evidences of sulfide mineralization which host gold.     

Crystallochemical studies on davidite from Bichun,Jaipur District,Rajasthan, India

Crystallochemical data on metamict davidite from albitites and albitised rocks from the Bichun area (Jaipur district, Rajasthan, India) of Banded Gneissic Complex (BGC) are provided. Davidite occurs as euhedral, subhedral to anhedral crystals in the form of disseminated grains and also as fracture filled veins. The crystals of davidite are up to 8 cm in length and 6 cm in width. The powder X-ray diffraction (XRD) pattern of the heat-treated davidite (at \(900{^{\circ }}\hbox {C}\)) reveals well-defined reflections of crystallographic planes. The calculated unit-cell parameters of the heat treated davidite are: \(\hbox {a}_{0} = \hbox {b}_{0} = 10.3556 \, \text {\AA }\) and \(\hbox {c}_{0} = 20.9067 \, \text {\AA }\), with unit-cell volume \(\hbox {(V)} = 1941.6385 \, \text {\AA }^{3}\); and \({\upalpha }={\upbeta }= 90^{\circ }\) and \({\upgamma }= 120^{\circ }\), which are in agreement with the values of davidite standard. Geochemical data reveals that the investigated davidite contains 51.5–52.6% \(\hbox {TiO}_{2}\), 14.8–15.1% \(\hbox {Fe}_{2} \hbox {O}_{3}\), 9.8–10.2% FeO, 6.97–7.12% \(\hbox {U}_{3} \hbox {O}_{8}\), 6.72–6.92% \(\hbox {RE}_{2} \hbox {O}_{3}\), 3.85–3.61% \(\hbox {K}_{2}\hbox {O}\), 0.9–1.4% \(\hbox {Al}_{2} \hbox {O}_{3}\), and 0.8–1.2% \(\hbox {SiO}_{2}\). The calculated structural formulae of the two davidite crystals are: D-1: \(\hbox {K}_{0.0044/0.004} \hbox {Ba}_{0.0044/0.005} \hbox {Ca}_{0.20/0.20} \hbox {Na}_{0.012/0.012} \hbox {Mn}_{0.053/0.053} \hbox {Mg}_{0.14/0.14} \hbox {Pb}_{0.0076/0.008} \hbox {Fe}_{2.675/2.675} \hbox {Fe}_{1.59/1.59} \hbox {Y}_{0.1175/0.118} \hbox {P}_{0.053/0.053} \hbox {Nb}_{0.008/0.008} \hbox {Sn}_{0.001/0.001} \hbox {Zr}_{0.033/0.033} \hbox {U}_{0.468/0.468} \hbox {Th}_{0.009/0.009} \,\,\hbox {REE}_{0.6829/0.683})_{6.05/6.05} (\hbox {Ti}_{12.15/12.15}\,\, \hbox {Fe}_{1.9022/1.903} \hbox {Si}_{0.372/0.372}\,\, \hbox {Al}_{0.517/0.517}\,\, \hbox {Cr}_{0.018/0.018} \hbox {Co}_{0.009/0.009} \hbox {Ni}_{0.027/0.027})_{15/15} \hbox {O}_{36/36} (\hbox {OH}_{0.319/0.319[]1.681/1.681})_{2/2}\) and D-2: \((\hbox {K}_{0.004/0.004} \hbox {Ba}_{0.005/0.005} \hbox {Ca}_{0.20/0.20} \hbox {Na}_{0.012/0.012} \hbox {Mn}_{0.05/0.05} \hbox {Mg}_{0.094/0.094} \hbox {Pb}_{0.007/0.007} \hbox {Fe}_{2.58/2.58} \hbox {Fe}_{1.71/1.71} \hbox {Y}_{0.112/0.112} \hbox {P}_{0.106/0.106} \hbox {Nb}_{0.006/0.006} \hbox {Sn}_{0.001/0.001} \hbox {Zr}_{0.03/0.03} \hbox {U}_{0.48/0.48} \hbox {Th}_{0.009/0.009} \hbox {REE}_{0.665/0.665})_{6.088/6.088} (\hbox {Ti}_{12.48/12.48} \hbox {Fe}_{1.87/1.87} \hbox {Si}_{0.249/0.249} \hbox {Al}_{0.334/0.334} \hbox {Cr}_{0.019/0.019} \hbox {Co}_{0.008/0.008} \hbox {Ni}_{0.04/0.04})_{15/15} \hbox {O}_{36/36} (\hbox {OH}_{0.098/0.098[]1.90/1.90})_{2/2}\). The calculated structural formulae are not fully stoichiometric, which could be due to metamict nature of davidite. The characteristic feature of distribution pattern of REE in davidite is unusually high concentration of LREE and HREE and substantially low content of MREE. It may be due to the occupation of REEs in two distinct crystallographic sites in davidite structure, i.e., M(1) and M(O) sites. Chondrite-normalised plot of davidite reveals a pronounced negative Eu-anomaly (\(\hbox {Eu}/\hbox {Eu}^{*} = 0.30{-}0.39\)), which suggests extremely fractionated nature of the metasomatising fluids from which davidite had crystallized. Metamict davidite-bearing U ores not only from Rajasthan, but also from other parts of India are likely to yield very high U leachability, thereby making them attractive sources of U, which otherwise are ignored by mineral engineers as uneconomic U ores.     

Appraisal of groundwater quality in a crystalline aquifer: a chemometric approach

The purpose of this study is to assess the groundwater quality and identify the processes that control the groundwater chemistry in a crystalline aquifer. A total of 72 groundwater samples were collected during pre- and post-monsoon seasons in the year 2014 in a semi-arid region of Gooty Mandal, Anantapur district, Andhra Pradesh, India. The study utilized chemometric analysis like basic statistics, Pearson’s correlation coefficient (r), principal component analysis (PCA), Gibbs ratio, and index of base exchange to understand the mechanism of controlling the groundwater chemistry in the study area. The results reveal that groundwater in the study area is neutral to slightly alkaline in nature. The order of dominance of cations is Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ while for anions, it is \( {\mathrm{HCO}}_3^{-}>{\mathrm{Cl}}^{-} \)>\( {\mathrm{NO}}_3^{-} \)>\( {\mathrm{SO}}_4^{2-} \)>\( {\mathrm{CO}}_3^{2-}>{\mathrm{F}}^{-} \) in both seasons. Based on the Piper classification, most of the groundwater samples are identified as of sodium bicarbonate (\( {\mathrm{Na}}^{+}-{\mathrm{HCO}}_3^{-}\Big) \) type. According to the results of the principal component analysis (PCA), three factors and two factors were identified pre and post monsoon, respectively. The present study indicates that the groundwater chemistry is mostly controlled by geogenic processes (weathering, dissolution, and ion exchange) and some extent of anthropogenic activities.     

Structural and luminescent characterisation of uraniferous fluorapatite and haematite associated with phosphatic rocks of the Bijawar group in Sagar District,Madhya Pradesh (India)

The structural and spectroscopic characteristics of phosphatic ferruginous shale samples from the Bijawar Group rocks from Sagar District of Madhya Pradesh (India) have been probed for identification of uranium species. Fluorapatite (\(\hbox {Ca}_{5}\hbox {(PO}_{4})_{3}\hbox {F}\), FAP) and haematite (\(\upalpha \)-\(\hbox {Fe}_{2}\hbox {O}_{3}\)) were identified as the main phases in the separated mineral concentrates. The photoluminescence (PL) and X-ray absorption near edge spectroscopy (XANES) studies pointed to a strong experimental evidence of both U(IV) and U(VI) oxidation states in the mineral concentrate portion obtained from the same parent host rock. The PL spectrum has confirmed the charge transfer (f–d) transition bands in UV and near-UV regions with emission peaks at ca. 290, 313, 336, 399 and 416 nm, which has been attributed to the substitution of \(\hbox {Ca}^{2+}\) ions by U(IV) in FAP and broad structureless emission due to stabilisation of U(VI) as \(\hbox {UO}_{6}^{6-}\) in haematite. Time-resolved spectroscopy studies have revealed biexponential decay components lasting 2–5 ns for U(IV) species and \(10\,\upmu \hbox {s}\) for U(VI) species. These characterisations revealed the fundamental information about the oxidation state and form of uranium in this region. Remediation measures for the Bijawar region are also suggested.     

Petrography and geochemistry of the Proterozoic sandstones of Somanpalli Group from Pomburna area,Eastern Belt of Pranhita–Godavari Valley,central India: Implications for provenance,weathering and tectonic setting

In this paper, we, for the first time, report geochemistry of sandstone from Somanpalli Group from Pomburna area in the Eastern Belt of Pranhita–Godavari (PG) Valley, central India and studied to infer their provenance, intensity of paleo-weathering and depositional tectonic setting. Petrographic study of sandstones show QFL modal composition of arenite. Chemical results show high \(\hbox {SiO}_{2}\) and CIA but lower \(\hbox {Al}_{2}\hbox {O}_{3}, \hbox {TiO}_{2}\), Rb, Sr, \(\hbox {K}_{2}\hbox {O}\) indicating mixed sources. Major elements chemistry parameters such as, \(\hbox {K}_{2}\hbox {O/Al}_{2}\hbox {O}_{3}\) ratio and positive correlation of Rb with \(\hbox {K}_{2}\hbox {O}\), reflects a warm and humid climate for study area. The tectonic discrimination plots (\(\hbox {SiO}_{2}/20\)–\(\hbox {K}_{2}\hbox {O} + \hbox {Na}_{2}\hbox {O}\)–\(\hbox {TiO}_{2} + \hbox {Fe}_{2}\hbox {O}_{3} + \hbox {MgO};\,\hbox {K}_{2}\hbox {O}/\hbox {Na}_{2}\hbox {O}\) vs. \(\hbox {SiO}_{2}\); Th–Sc–Zr/20) indicate dominantly passive margin and slight active tectonic setting. Concentrations of Zr, Nb, Y, and Th are higher compared to the UCC values and the trends in Th/Cr, Th/Co, La/Sc and Cr/Zr ratios support a felsic and mafic source for these sandstones and deposition in passive margin basin. Chondrite normalized REE pattern reflects LREE depletion, negative Eu anomaly and flat HREE similar to UCC, felsic components. ICV value (0.95) also supports tectonically quiescent passive margin settings. CIA values (74) indicate high degree of chemical weathering and warm and humid paleoclimatic condition.