Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite(NHO) belt in the Indo-Myanmar range(IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene.Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO, northeastern India to address(i) the mantle processes and tectonic regimes involved in their genesis and(ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle.The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Washelo and Lacham areas.The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase.The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber.Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE.These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions.Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone(SSZ) fore-arc regime coherent with the subduction initiation process.The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process:(i) a depleted fore arc basalt(FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle;(ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration.The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.     

Low level of stratospheric ozone near the Jharia coal field in India

The Indian reserve of coking coal is mainly located in the Jharia coal field in Jharkhand. Although air pollution due to oxides and dioxides of carbon, nitrogen and sulphur is reported to have increased in this area due to large-scale opencast mining and coal fires, no significant study on the possible impact of coal fires on the stratospheric ozone concentration has been reported so far. The possible impact of coal fires, which have been burning for more than 90 years on the current stratospheric ozone concentration has been investigated using satellite based data obtained from Upper Atmospheric Research Satellite (UARS MLS), Earth Observing System Microwave Limb Sounder (EOS MLS) and Ozone Monitoring Instrument (OMI) in this paper. The stratospheric ozone values for the years 1992–2007, in the 28–36 km altitude range near Jharia and places to its north are found to be consistently lower than those of places lying to its south (up to a radius of 1000 km around Jharia) by 4.0–20%. This low stratospheric ozone level around Jharia is being observed and reported for the first time. However, due to lack of systematic ground-based measurements of tropospheric ozone and vertical ozone profiles at Jharia and other far off places in different directions, it is difficult to conclude strongly on the existence of a relationship between pollution from coal fires and stratospheric ozone depletion.     

Biogenic hydrogen sulphide emissions and non-sea sulfate aerosols over the Indian Sundarban mangrove forest

Temporal variations in atmospheric hydrogen sulphide concentrations and its biosphere-atmosphere exchanges were studied in the World’s largest mangrove ecosystem, Sundarbans, India. The results were used to understand the possible contribution of H₂S fluxes in the formation of atmospheric aerosol of different size classes (e.g. accumulation, nucleation and coarse mode). The mixing ratio of hydrogen sulphide (H₂S) over the Sundarban mangrove atmosphere was found maximum during the post-monsoon season (October to January) with a mean value of 0.59?±?0.02 ppb and the minimum during pre-monsoon (February to May) with a mean value of 0.26?±?0.01 ppb. This forest acted as a perennial source of H₂S and the sediment-air emission flux ranged between 1213?±?276 μg S m^?2 d^?1(December) and 457?±?114 μg S m^?2 d^?1 (August) with an annual mean of 768?±?240 μg S m^?2d^?1. The total annual emissions of H₂S from the Indian Sundarban were estimated to be 1.2?±?0.6 Tg S. The accumulation mode of aerosols was found to be more enriched with non-sea salt sulfate with an average loading of 5.74 μg m^?3 followed by the coarse mode (5.18 μg m^?3) and nucleation mode (1.18 μg m^?3). However, the relative contribution of Non-sea salt sulfate aerosol to total sulfate aerosol was highest in the nucleation mode (83%) followed by the accumulation (73%) and coarse mode (58%). Significant positive relations between H₂S flux and different modes of NSS indicated the likely link between H₂S, a dominant precursor for the non-sea salt sulfate, and non-sea sulfate aerosol particles. An increase in H₂S emissions from the mangrove could result in an increase in enhanced NSS in aerosol and associated cloud albedo, and a decrease in the amount of incoming solar radiation reaching the Sundarban mangrove forest.     

Multicomponent diffusion in garnets I: general theoretical considerations and experimental data for Fe–Mg systems

We have carried out a combined theoretical and experimental study of multicomponent diffusion in garnets to address some unresolved issues and to better constrain the diffusion behavior of Fe and Mg in almandine–pyrope-rich garnets. We have (1) improved the convolution correction of concentration profiles measured using electron microprobes, (2) studied the effect of thermodynamic non-ideality on diffusion and (3) explored the use of a mathematical error minimization routine (the Nelder-Mead downhill simplex method) compared to the visual fitting of concentration profiles used in earlier studies. We conclude that incorporation of thermodynamic non-ideality alters the shapes of calculated profiles, resulting in better fits to measured shapes, but retrieved diffusion coefficients do not differ from those retrieved using ideal models by more than a factor of 1.2 for most natural garnet compositions. Diffusion coefficients retrieved using the two kinds of models differ only significantly for some unusual Mg–Mn–Ca-rich garnets. We found that when one of the diffusion coefficients becomes much faster or slower than the rest, or when the diffusion couple has a composition that is dominated by one component (>75 %), then profile shapes become insensitive to one or more tracer diffusion coefficients. Visual fitting and numerical fitting using the Nelder-Mead algorithm give identical results for idealized profile shapes, but for data with strong analytical noise or asymmetric profile shapes, visual fitting returns values closer to the known inputs. Finally, we have carried out four additional diffusion couple experiments (25–35 kbar, 1,260–1,400 °C) in a piston-cylinder apparatus using natural pyrope- and almandine-rich garnets. We have combined our results with a reanalysis of the profiles from Ganguly et al. (1998) using the tools developed in this work to obtain the following Arrhenius parameters in D = D ₀ exp{–[Q _1bar + (P–1)ΔV ⁺]/RT} for D _Mg^* and D _Fe^*: Mg: Q _1bar = 228.3 ± 20.3 kJ/mol, D ₀ = 2.72 (±4.52) × 10⁻¹⁰ m²/s, Fe: Q _1bar = 226.9 ± 18.6 kJ/mol, D ₀ = 1.64 (±2.54) × 10⁻¹⁰ m²/s. ΔV ⁺ values were assumed to be the same as those obtained by Chakraborty and Ganguly (1992).     

Metamorphism of Greater and Lesser Himalayan rocks exposed in the Modi Khola valley, central Nepal

Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks. This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust. Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata, juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about 15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades, although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic metamorphism.     

Arc‐nascent back‐arc signature in metabasalts from the Neoarchaean Jonnagiri greenstone terrane,Eastern Dharwar Craton,India

The Neoarchaean Jonnagiri greenstone terrane (JGT) is located at the centre of the arcuate Hutti–Jonnagiri–Kadiri–Kolar composite greenstone belt in the eastern Dharwar Craton. High MgO (MgO = ~14 wt.%; Nb = 0.2 ppm), low Nb (LNB) (MgO = 7.8–12 wt.%; Nb = 0.1–5.1 ppm) and high Nb basalts (HNB) (MgO = 5.6–10.1 wt.%; Nb = 9.0–10.6 ppm) metamorphosed to lower amphibolite facies are identified based on their geochemical compositions. These metabasalts exhibit depleted HFSE (Nb–Ta, Zr–Hf), pronounced LREE and LILE enrichments suggesting contribution from subduction‐related components during their genesis. Th and U enrichment over Nb–Ta indicates influx of fluids dehydrated from subducted oceanic lithosphere. The high MgO basalts with higher Mg# (51) than that of the associated LNB and HNB (Mg# = 34–47) represent early fractionated melts of subduction‐modified mantle peridotite. The LNB were produced by partial melting of mantle wedge metasomatized by slab‐dehydrated fluids, whereas the HNB represents melts of subducted oceanic crust and hybridized mantle wedge. Lower Dy/Yb and variable La/Yb ratios suggest their generation at shallower depth within spinel peridotite stability field. The low Ce–Yb trend of these metabasalts reflects intraoceanic type subduction which straddles the fields of arc and back‐arc basin basalts, resembling the Mariana‐type arc basalts. The Jonnagiri metabasalts were derived in a paired arc‐back‐arc setting marked by nascent back‐arc rift system that developed in the proximity of an intraoceanic arc. Copyright © 2014 John Wiley & Sons, Ltd.     

The energetics of natural garnet solid solution

Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)₃Al₂Si₃O₁₂, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C.     

Reaction texture and Fe-Mg zoning in granulite garnet from SØstrene Island, Antarctica: Modeling and constraint on the time scale of metamorphism during the Pan-African collisional event

Garnets from the S⊍strene island, Antarctica, show reaction textures corresponding to two metamorphic episodes, one at c. 1000 Ma (M1) and the other at c. 500 Ma (M2). The latter is associated with a Pan-African tectono-metamorphic event that has been interpreted to represent a continent-continent collision followed by extensional collapse. Reaction-diffusion modeling of the compositional zoning of garnet associated with the development of reaction texture during M2 yields a time scale of ∼ 5–16 Myr for the duration of the peak of this overprinting metamorphism at ∼ 730 ± 20‡C. The associated velocity of the reaction front is ∼∼ 5.0-1.6Μm/Myr. The inferred duration of peak metamorphism during the Pan-African event seems to be in good agreement with the available U-Pb SHRIMP ages of zircon and monazite that may be interpreted to have formed at the beginning and end stages of crystallization of granite during the metamorphic peak.