Assessment of groundwater contamination by NO<Subscript>3</Subscript><Superscript>−</Superscript> using geographical information system in the Zhangye Basin,Northwest China

Geographical information system (GIS) technology was used to investigate NO₃ ⁻ concentration of groundwater in the Zhangye Basin, Northwest China. Thematic information and NO₃ ⁻ data of groundwater from the Zhangye Basin were analyzed in a GIS environment to study the extent and variation of NO₃ ⁻ concentration and to establish spatial relationships with corresponding land use types. About 38.8% of groundwater samples showed NO₃ ⁻ concentration above the human affected value (13 mg/l NO₃ ⁻), while more than 10.2% exceeded the maximum acceptable level (50 mg/l NO₃ ⁻) according to WHO regulations. The spatial distribution of NO₃ ⁻ of groundwater in the Zhangye Basin showed an obvious character of zonation; the high NO₃ ⁻ concentration was mainly located in the city seat areas, and Zhangye City was the high-value center. The probability of over 90% for NO₃ ⁻ concentration exceeding 13 mg/l was found in urban seat areas. The spatial analyses indicated that groundwater contamination by NO₃ ⁻ was closely related to one specific land use class, the urban. The NO₃ ⁻ concentration of groundwater under urban was significantly higher than that under irrigation land or sand dune. Most of the unacceptable NO₃ ⁻ levels were found in groundwater samples assigned to urban area, but a few were also found allotted to irrigation land class. Therefore, urban source was considered the principal source of NO₃ ⁻ contamination of groundwater in the Zhangye Basin, while irrigation land source was still a possible source of NO₃ ⁻ contamination.     

The effect of chlorite interlayering on <Superscript>40</Superscript>Ar–<Superscript>39</Superscript>Ar biotite dating: an <Superscript>40</Superscript>Ar–<Superscript>39</Superscript>Ar laser-probe and TEM investigations of variably chloritised biotites

Biotite is one of the most common minerals dated by the ⁴⁰Ar–³⁹Ar method. It frequently shows K contents below the expected stoichiometric value, suggesting the presence of low-K impurities. The most common low-K alteration product of biotite is chlorite. Therefore, it is important to understand the effects of chlorite interlayering on ⁴⁰Ar–³⁹Ar ages in order to correctly interpret ⁴⁰Ar–³⁹Ar data. This study examines the outcome of ⁴⁰Ar–³⁹Ar dating analyses on variably chloritised biotites from Ordovician intrusive rocks. The infrared (IR) laser-probe technique and different gas extraction methods were adopted. Incremental laser-heating data on bulk samples yielded hump-shaped age profiles with meaningless young and old age steps. Both the extent of anomalous old age steps and the degree of discordance of the age spectra were much more pronounced in the more chloritised biotite samples. In contrast, in situ data on rock chips and total-fusion ages on single biotite flakes yielded ages concordant with, or younger than, the inferred emplacement ages. Transmission electron microscopy (TEM) was used to texturally characterise biotite samples at the nanometre scale. It was also used to document the complex decomposition-transformation process affecting interlayered biotite–chlorite during in-vacuo IR-laser heating to temperatures ranging from ~600 to >1,000 °C. TEM results suggest that hump-shaped age profiles result from an interplay between ³⁹Ar_K redistribution by recoil during sample irradiation and differential release of argon isotopes hosted in three main reservoirs. These reservoirs are (from least to most retentive): extended defects, chlorite and biotite. The final descending age segment is attributed to the progressive release of argon with increasing temperature from large biotite domains for which ³⁹Ar_K recoil loss was less important. ⁴⁰Ar–³⁹Ar data support previous findings, which suggest that ⁴⁰Ar–³⁹Ar ages when recoil effects are minimised, provide minimum estimates that approach the true biotite age, when the pristine domains are analysed. The most effective approach for obtaining meaningful ⁴⁰Ar–³⁹Ar ages was using individual total-fusion analyses on carefully selected, single flakes previously split along the basal cleavage by wet-grinding and corresponding to a sample mass of a few micrograms.Editorial responsibility: I Parsons     

Single-crystal EPR and DFT study of a <Superscript>VI</Superscript>Al–O<Superscript>−</Superscript>–<Superscript>VI</Superscript>Al center in jeremejevite: electronic structure and <Superscript>27</Superscript>Al hyperfine constants

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al₆B₅O₁₅(F, OH)₃, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an ²⁷Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g ₁ = 2.02899(1), g ₂ = 2.02011(2), g ₃ = 2.00595(1); A ₁/g _e β _e  = −0.881(1) mT, A ₂/g _e β _e  = −0.951(1) mT, and A ₃/g _e β _e  = −0.972(2) mT, with the orientations of the g ₃- and A ₃-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g ₁- and A ₁-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al³⁺ ions, after the removal of the proton (i.e., a ^VIAl–O⁻–^VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental ²⁷Al hyperfine coupling constants and directions, supporting the proposed structural model. The ^VIAl–O⁻–^VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the ^VIAl–O⁻–^VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.     

Origin and evolution of oilfield brines from Tertiary strata in western Qaidam Basin: Constraints from <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and water chemistry

Chemistry of major and minor elements, ⁸⁷Sr/⁸⁶Sr, δD, δ¹⁸O and δ³⁴S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry data show that all oilfield brines are CaCl₂ type. They were enriched in Ca²⁺, B³⁺, Li⁺, Sr²⁺, Br⁻, and were depleted in Mg²⁺, SO₄ ²⁻, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ¹⁸O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line (QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range of metamorphosing water. The ⁸⁷Sr/⁸⁶Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition, all oilfield brines have obviously much positive δ³⁴S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting in positive shift of δ³⁴S value and depleteed SO₄ ²⁻ in oilfield brines. Combined with water chemical data and δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ³⁴S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so on) for sustainable utilization of salt lake resources in the Qaidam Basin.     

Radiation-induced defects in euclase: formation of O<Superscript>−</Superscript> hole and Ti<Superscript>3+</Superscript> electron centers

Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless specimens of rare gemological quality euclase, BeAlSiO₄(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron paramagnetic resonance (EPR). The first one is an O⁻ hole center interacting with one Al neighbor and the second is a Ti³⁺ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results suggest that O⁻ hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH⁻ ions interconnect distorted Al octahedra and Be tetrahedra in O₅ positions. During irradiation, the electrons are captured by titanium ions (Ti⁴⁺ + e⁻), leading to the formation of paramagnetic Ti³⁺ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal annealing experiments show that the O⁻ hole centers and Ti³⁺ electron centers are directly connected through the radiation process.     

Carbonate Rocks from the Riphean–Vendian Boundary Deposits of the Anabar Uplift: Isotope (<Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) Systematics and Chemostratigraphic Consequences

New ⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸О chemostratigraphic data were obtained for carbonate rocks of the Lower Riphean Yusmastakh and the Vendian Starorechenskaya formations. The δ¹³С values in dolomites of the Yusmastakh Formation varies from–0.6 to–0.1‰ and in dolomites and dolomitic limestones of the Starorechenskaya Formation, from–1.2 to–0.4‰ PDB, and δ¹⁸О values, from 24.4 to 26.4‰ and from 25.3 to 27.6‰ SMOW, respectively. The Rb–Sr systematics of carbonate rocks was studied using the refined method of stepwise dissolution of samples in acetic acid, including chemical removal of up to one-third of the ground sample by preliminary acid leaching and subsequent partial dissolution of the rest of the sample. Owing to this procedure, secondary carbonate material is removed, which enables one to improve the quality of the Sr-chemostratigraphic data obtained. The initial ⁸⁷Sr/⁸⁶Sr ratios in carbonate rocks of the Yusmastakh (0.70468–0.70519) and Starorechenskaya (0.70832–0.70883) formations evidence the Riphean–Vendian boundary in the Precambrian sequence of the Anabar Uplift.     

Rb–Sr and <Superscript>147</Superscript>Sm–<Superscript>143</Superscript>Nd systematics of gabbro and dolerites from fragments of ophiolite complexes of the Middle Urals

The Rb–Sr and ¹⁴⁷Sm–¹⁴³Nd age data obtained for sheeted dolerite dykes and rocks of the Platinum Belt of the Urals within the Tagil segment of the paleoceanic spreading structure (Middle Urals) are discussed. The study of the Rb–Sr isotope systematics of gabbro allowed us to reveal errochronous dependencies, which yielded ages of 415 and 345 Ma at (⁸⁷Sr/⁸⁶Sr)₀ = 0.70385 ± 0.00068 and 0.7029 ± 0.0010, correspondingly. The ¹⁴⁷Sm–¹⁴³Nd isotope age data demonstrate a specific coincidence of the chronometric ages of the sheeted dolerite dyke complex (426 ± 54, 426 ± 34, and 424 ± 19 Ma) and gabbro from the Revda gabbro–ultramafic massif (431 ± 27 Ma) and from screens between dolerite dykes in the sheeted dyke complex (427 ± 32 Ma, 429 ± 26 Ma). The proximity of the ¹⁴⁷Sm–¹⁴³Nd ages of gabbro and dolerite can be explained by the thermal effect of the basaltic melt, which is the protolith for the dyke complex, on the hosting gabbro.     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

Isotope (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genetic features of manganese carbonates of the Mazul deposit (Krasnoyarsk region)

New isotope and mineral data on manganese carbonates of the Mazul deposit (Krasnoyarsk region) in combination with morphology of ore bodies suggest that the ores were formed in several stages with the involvement of meteoric solutions through infiltration and, possibly, exfiltration mechanisms. Based on the geological–geochemical data, manganese carbonates of the Mazul deposit may be ascribed to a new genetic subtype of the catagenesis (epigenesis) zone.     

Geochemistry of isotopes (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and depositional environment of the upper Kazanian carbonate sediments in the Volga-Vyatka interfluve

This work reports the results of lithological and isotopic study of carbonate rocks from the Pechishchi stratotype section (Kazan) and three adjacent sections of Kazanian rocks of the Volga-Vyatka region at the eastern Russian Platform. These sections were recovered by the Kremeshki, Popovtsevo, and Chimbulat quarries (near the town of Sovetsk, southeastern Kirov district). Lithological features and wide variations of δ¹³C (from −6.0 to 6.8‰) and δ¹⁸ O (from 22.9 to 33.4 ‰) indicate that the rocks were formed in a shallow-marine basin with rapidly varying conditions of sedimentation which characterize different facies (and/or paleoecological) zones: lagoonal, supralittoral, littoral, shoal. They also suggest processes of postsedimentary alteration (mainly, under supergene conditions). Numerous short-term hiatuses are also recorded.     

Using stable isotopes (δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr) to identify sources of water in abandoned mines in the Fengfeng coal mining district,northern China

With depleted coal resources or deteriorating mining geological conditions, some coal mines have been abandoned in the Fengfeng mining district, China. Water that accumulates in an abandoned underground mine (goaf water) may be a hazard to neighboring mines and impact the groundwater environment. Groundwater samples at three abandoned mines (Yi, Er and Quantou mines) in the Fengfeng mining district and the underlying Ordovician limestone aquifer were collected to characterize their chemical and isotopic compositions and identify the sources of the mine water. The water was HCO₃·SO₄-Ca·Mg type in Er mine and the auxiliary shaft of Yi mine, and HCO₃·SO₄-Na type in the main shaft of Quantou mine. The isotopic compositions (δD and δ¹⁸O) of water in the three abandoned mines were close to that of Ordovician limestone groundwater. Faults in the abandoned mines were developmental, possibly facilitating inflows of groundwater from the underlying Ordovician limestone aquifers into the coal mines. Although the Sr²⁺ concentrations differed considerably, the ratios of Sr²⁺/Ca²⁺ and ⁸⁷Sr/⁸⁶Sr and the ³⁴S content of SO₄^2? were similar for all three mine waters and Ordovician limestone groundwater, indicating that a close hydraulic connection may exist. Geochemical and isotopic indicators suggest that (1) the mine waters may originate mainly from the Ordovician limestone groundwater inflows, and (2) the upward hydraulic gradient in the limestone aquifer may prevent its contamination by the overlying abandoned mine water. The results of this study could be useful for water resources management in this area and other similar mining areas.     

Assessment of NO<Subscript>3</Subscript><Superscript>−</Superscript> contamination in a karstic aquifer,with the use of geochemical data and spatial analysis

Kopaida plain is a cultivated region of Eastern Greece, with specific characteristic related with the paleogeographic evolution and the changes in land use. The present article examines the contamination that derives from nitrates, in terms of contaminant levels, definition of sources and spatial distribution of contaminant plume. For this purpose, 50 water samples were collected from the karstic aquifer and analyzed for 15 parameters including major ions, trace elements, physicochemical parameters, and stable isotopes. The assessment of the above parameter values along with the notes derived by the statistical process revealed the existence of nitrate contamination which has been spatial defined with the aid of spatial interpolation techniques. The correlation of NO₃ ⁻ concentrations with the stable isotope values, defined the infiltration conditions and showed contaminant transport. Nitrate values revealed the potential environmental threat for local people, as 10% of the samples exceeded the parametric value of 50 ppm and 54% of them are above 25 ppm, indicating no optimal quality conditions. The origin of nitrate contamination seems to derive exclusively from the application of N-fertilizers, since the rest of potential sources were not verified by analytical data and field works.     

Residence time distribution in a large unconfined–semiconfined aquifer in the Argentine Pampas using <Superscript>3</Superscript>H/<Superscript>3</Superscript>He and CFC tracers

The Pampa region in Argentina includes vast unconfined–semiconfined aquifers that local economies depend upon, but detailed knowledge of the associated water resources is still lacking. The Pampeano aquifer in the Pampa plain of Argentina covers around 1.5 million km². In order to achieve a better understanding of the hydrogeological system through the estimation of mean residence times (MRT), water samples were taken from 12 monitoring wells, drilled at different depths in four locations, and analyzed for environmental tracers. The concentrations of ³H, tritiogenic ³He and chlorofluorocarbons (CFCs) can be explained by mixtures of young waters adjusted to exponential piston flow models (EPM) or dispersion models (DM), and different proportions of tracer-free waters (dead water). The sampling site located very close to the water divide shows a dominance of young waters: 85 % of water best represented by a DM model with a MRT of 3 years. For the shallow wells at other sites, best-fitting models result in a DM with MRT between 20 and 35 years, and proportions of dead water between 40 and 60 %. These results lead to important updates in the conceptual model of the Pampeano aquifer. Large proportions of dead water at a few meters depth can be the consequence of upward flows in a multilayered aquifer or diffusive retardation in the inter-bedded clay layers.     

Ab initio calculations on the O<Subscript>2</Subscript><Superscript>3−</Superscript>-Y<Superscript>3+</Superscript> center in CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript>: its electronic structure and hyperfine constants

The O₂ ^3?-Y³⁺ center in fluorite-type structures (CaF₂ and SrF₂) has been investigated at the density functional theory (DFT) level using the CRYSTAL06 code. Our calculations were performed by means of the hybrid B3PW method in which the Hartree–Fock exchange is mixed with the DFT exchange functional, using Becke’s three parameter method, combined with the non-local correlation functionals by Perdew and Wang. Our calculations confirm the stability and the molecular character of the O₂ ^3?-Y³⁺ center. The unpaired electron is shown to be almost exclusively localized on and equally distributed between the two oxygen atoms that are separated by a bond distance of 2.47 Å in CaF₂ and 2.57 Å in SrF₂. The calculated ¹⁷O and ¹⁹F hyperfine constants for of the O₂ ^3?-Y³⁺ center are in good agreement with their corresponding experimental values reported by previous electron paramagnetic resonance and electron nuclear double resonance studies, while discrepancies are notable for the ⁸⁹Y hyperfine constants.     

Spatio-temporal variations of δ<Superscript>2</Superscript>H and δ<Superscript>18</Superscript>O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau,China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ²H and δ¹⁸O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km²). In the Caopingxigou Experimental Watershed (0.1 km²), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ²H–δ¹⁸O relation of groundwater was found to be δ²H = 3.22 × δ¹⁸O − 38.1, deviating from the local meteoric water line δ²H = 7.57 × δ¹⁸O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.     

Isotopic Composition (δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) and the Origin of Carbonates from Manganese Deposits of the Southern Urals

Isotopic compositions of carbon (δ¹³C from −51.4 to −10.8 PDB) and oxygen (δ¹⁸O from 14.4 to 21.4 SMOW) were studied in rhodochrosite and calcite from manganese ores in the South Faizuly and Kyzyltash deposits of the southern Urals. The geological, petrographic, and isotopic data indicate that the studied carbonates are diagenetic formations. It is suggested that the main ore element (Mn) was delivered to the marine basin with hydrothermal solutions percolating in the oceanic crust. Manganese precipitated on the oceanic bottom as oxides near solution discharge zones. Manganese carbonates formed in sediments as a result of the oxidation of organic matter by manganese oxides. High biological productivity of the environment was caused by proximity to the hydrothermal vent that provided favorable biogeochemical conditions for the development of biocoenosis. Anomalously low ¹³C values in the South Faizuly deposit testify to the large-scale oxidation of methane in the course of manganese carbonate formation.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 416–429.Original Russian Text Copyright © 2005 by Kuleshov, Brusnitsyn.     

Structural characterization of high-pressure C–Na<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript> by single-crystal diffraction and <Superscript>29</Superscript>Si MAS NMR

Single crystals of C–Na₂Si₂O₅ have been synthesized from the hydrothermal recrystallization of a glass. The title compound is monoclinic, space group P2₁/c with Z= 8 and unit-cell parameters a= 4.8521 (4)Å, b=23.9793(16)Å, c=8.1410(6)Å, β=90.15(1)° and V=947.2(2)ų. The structure has been determined by direct methods and belongs to the group of phyllosilicates. It is based on layers of tetrahedra with elliptically six-membered rings in chair conformation. The sequence of directedness within a single ring is UDUDUD. The sheets are parallel to (010) with linking sodium cations in five- and sixfold coordination. Concerning the shape and the conformation of the rings, C–Na₂Si₂O₅ is closely related to β-Na₂Si₂O₅. However, both structures differ in the stacking sequences of the layers. A possible explanation for the frequently observed polysynthetic twinning of phase C is presented. In the ²⁹Si MAS-NMR spectrum of C–Na₂Si₂O₅ four well-resolved lines of equal intensity are observed at ?86.0, ?86.3, ?87.4, and ?88.2?ppm. The narrow range of isotropic chemical shifts reflects the great similarity of the environments of the different Si sites. This lack of pronounced differences in geometry renders a reliable assignment of the resonance lines to the individual sites on the basis of known empiric correlations and geometrical features impossible.     

Gravity anomalies over the Central Indian Ridge between 3<Superscript>∘</Superscript>S and 11<Superscript>∘</Superscript>S,Indian Ocean: Segmentation and crustal structure

High-resolution shipboard geophysical investigations along the Indian Ocean ridge system are sparse especially over the Carlsberg and Central Indian ridges. In the present study, the shipboard gravity and multibeam bathymetry data acquired over a 750 km long section of the Central Indian Ridge between 3 ^°S and 11 ^°S have been analysed to understand the crustal structure and the ridge segmentation pattern. The mantle Bouguer anomalies (MBA) and the residual mantle Bouguer anomalies (RMBA) computed in the study area have shown significant variations along the ridge segments that are separated by transform and non-transform discontinuities. The MBA lows observed over the linear ridge segments bounded by well-defined transform faults are attributed to the thickening of the crust at the middle portions of the ridge segments. The estimates of crustal thickness from the RMBA shows an average of 5.2 km thick crust in the axial part of the ridge segments. The MBA and relative RMBA highs along the two non-transform discontinuities suggests a thinner crust of up to 4.0 km. The most significant MBA and RMBA highs were observed over the Vema transform fault suggesting thin crust of 4 km in the deepest part of the transform fault where bathymetry is more than 6000 m. The identified megamullion structures have relative MBA highs suggesting thinner crust. Besides MBA lows along the ridge axis, significant off-axis MBA lows have been noticed, suggesting off-axis mantle upwelling zones indicative of thickening of the crust. The rift valley morphology varies from the typical V-shaped valley to the shallow valley floor with undulations on the inner valley floor. Segments with shallow rift valley floor have depicted well-defined circular MBA lows with persistent RMBA low, suggesting modulation of the valley floor morphology due to the variations in crustal thickness and the mantle temperature. These are supported by thicker crust and weaker lithospheric mantle.     

Transmission of <Emphasis Type="Italic">δ</Emphasis><Superscript>13</Superscript>C signals and its paleoclimatic implications in Liangfeng Cave system of Guizhou Province,SW China

A series of samples, including vegetation, soil organic matter, soil waters, spring, bedrock, pool water, drip waters (upper-drip waters and ground-drip waters) and their corresponding speleothems were collected at Liangfeng Cave (LFC) system of Guizhou Province, southwest of China, respectively, from 2003 to 2004 year, then their stable carbon isotopes were measured and analyzed. Results reveal that vegetation is C₃ type in LFC system; cave overlying δ ¹³C signals, including values and variations, could be transmitted to drip water (speleothem); speleothem δ ¹³C mainly shows a biogenic δ ¹³C value character (soil CO₂ from plant respiration and decay); and there are remarkable seasonal variations of δ ¹³C values for drip water TDIC (speleothem), which are lighter at least 2.0‰ in the rainy seasons than in the dry ones. So, it could be feasible to reconstruct high-resolution changes of paleoecology and paleoclimate by using speleothem δ ¹³C values.     

The origin of high δ<Superscript>18</Superscript>O zircons: marbles,megacrysts, and metamorphism

The oxygen isotope ratios (δ¹⁸O) of most igneous zircons range from 5 to 8‰, with 99% of published values from 1345 rocks below 10‰. Metamorphic zircons from quartzite, metapelite, metabasite, and eclogite record δ¹⁸O values from 5 to 17‰, with 99% below 15‰. However, zircons with anomalously high δ¹⁸O, up to 23‰, have been reported in detrital suites; source rocks for these unusual zircons have not been identified. We report data for zircons from Sri Lanka and Myanmar that constrain a metamorphic petrogenesis for anomalously high δ¹⁸O in zircon. A suite of 28 large detrital zircon megacrysts from Mogok (Myanmar) analyzed by laser fluorination yields δ¹⁸O from 9.4 to 25.5‰. The U–Pb standard, CZ3, a large detrital zircon megacryst from Sri Lanka, yields δ¹⁸O = 15.4 ± 0.1‰ (2 SE) by ion microprobe. A euhedral unzoned zircon in a thin section of Sri Lanka granulite facies calcite marble yields δ¹⁸O = 19.4‰ by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble. Small oxygen isotope fractionations between zircon and most minerals require a high δ¹⁸O source for the high δ¹⁸O zircons. Predicted equilibrium values of Δ¹⁸O(calcite-zircon) = 2–3‰ from 800 to 600°C show that metamorphic zircon crystallizing in a high δ¹⁸O marble will have high δ¹⁸O. The high δ¹⁸O zircons (>15‰) from both Sri Lanka and Mogok overlap the values of primary marine carbonates, and marbles are known detrital gemstone sources in both localities. The high δ¹⁸O zircons are thus metamorphic; the 15–25‰ zircon values are consistent with a marble origin in a rock-dominated system (i.e., low fluid_(external)/rock); the lower δ¹⁸O zircon values (9–15‰) are consistent with an origin in an external fluid-dominated system, such as skarn derived from marble, although many non-metasomatized marbles also fall in this range of δ¹⁸O. High δ¹⁸O (>15‰) and the absence of zoning can thus be used as a tracer to identify a marble source for high δ¹⁸O detrital zircons; this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks. Metamorphic zircon megacrysts have not been reported previously and appear to be associated with high-grade marble. Identification of high δ¹⁸O zircons can also aid geochronology studies that seek to date high-grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble.