Preferential solution of234U from recoil tracks and234U/238U radioactive disequilibrium in natural waters

A mathematical model to calculate the²³⁴U/²³⁸U activity ratio (AR) in an aqueous phase in contact with rock/soil is presented. The model relies on the supply of²³⁸U by dissolution and that of²³⁴U by dissolution and preferential release from radiation damaged regions (recoil tracks). The model predicts that values of²³⁴U/²³⁸U AR>1 in the aqueous phase can be obtained only from weathering “virgin” surfaces. Thus, to account for the observed steady-state supply of²³⁴U excess to the oceans by the preferential leaching model, ‘virgin’ rock/soil surfaces would have to be continually exposed and weathered. The²³⁸U concentration and²³⁴U/²³⁸U AR in continental waters allow us to estimate the exposure rates of “virgin” rock/soil surfaces.     

GEOSECS AtlanticSi profiles

Measurements of five cosmogenic³²Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO₂ extracted range from 2 to 54 g; the amounts of water from which SiO₂ was extracted range between 540 kg and 270, 000 kg. In additon, SiO₂ recovered from four surface particulate composites (64°N to 61°S) were also analyzed for³²Si.³²Si measurements were made by milking and counting the daughter activity, ³²P. The net³²P activities range from 0.7 to 6.8 cph; typical errors in measurements of the³²P activities are 20–30%.The³²Si concentrations vary from 0.6 dpm/10⁶ kg of water in the North Atlantic surface waters to 235 dpm/10⁶ kg at 400 m depth in the circumpolar waters. The vertical profiles of³²Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and³²Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of³²Si (³²Si/SiO₂ ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2–4 times, than the bottom water values. These data are consistent with generation of the highest specific activity³²Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic³²Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high³²Si specific activities.The³²Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1–1.4) × 10⁻² dpm³²Si/cm², which is consistent with the expected fallout of cosmogenic³²Si. However, the³²Si column inventories south of 40°S are higher by a factor of 5–7, whereas the corresponding Si inventories increase by only a factor of 3. This excess³²Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of³²Si dissolved from sinking particulates.     

The Archean Craton of Southern India: Metamorphic evolution and P-T conditions

The Archaean craton of southern India is characterized by a highly complicated and not yet fully understood geological history comprizing several cycles of sedimentation and volcanism, deformation and metamorphism in the span between about 3400 and 2500 m. y. The large scale regional variation in metamorphic grade observed today is essentially related to a metamorphic event at about 2600 m. y. ago which affected an older migmatite, gneiss-greenstone terrain (2900–3400 m. y.).The southern area is characterized by granulite facies (700–750° C/8–10 kb). An extensive charnockite-khondalite belt has been generated by atectonic transformation of the migmatite-gneiss terrain through the influx of a CO₂-rich fluid. Towards the north of the terrain the metamorphic grade decreases to amphibolite facies (600° C/6–8 kb) and to greenschist facies (400° C) which is restricted to the Dharwar greenstone belts. Metamorphism related to younger shear zones in the southern part of the craton led to retrogression of the charnockite-khondalite series under conditions of amphibolite to greenschist facies.The P-T conditions of metamorphism have been evaluated applying mineral stability data and methods of geothermometry and geobarometry in an area between Shimoga-Chitradurga (North) and Coimbatore-Karur (South).

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Zusammenfassung Der archaische Kraton Südindiens ist durch eine komplexe, noch nicht vollständig aufgeklärte geologische Entwicklungsgeschichte gekennzeichnet. Sie umfaßt mehrere Zyklen von Sedimentation und Vulkanismus, Deformation und Metamorphose in der Zeitspanne von etwa 3400 bis 2500 Ma. Die großregionale metamorphe Zonierung wird im wesentlichen als das Ergebnis eines Metamorphose-Ereignisses vor ca. 2600 Ma angesehen, das den gesamten archaischen Komplex aus granitoiden Gneisen, Migmatiten und Greenstone-Serien erfaßte.Das südliche Gebiet ist durch Bedingungen der Granulitfazies (700–750° C/8–10 kb) charakterisiert. Hier entstand eine ausgedehnte Charnockit-Khondalit-Zone durch post-tektonische Umwandlung der Migmatit-Gneisserien infolge Zufuhr einer CO₂-reichen fluiden Phase. Nach Norden zu nimmt der Metamorphosegrad bis zur niedrigtemperierten Amphibolitfazies (600° C/6–8 kb) und in den Dharwar-Greenstone Belts sogar bis zur Grünschieferfazies (400° C) ab.Eine jüngere Metamorphose von Amphibolit-bis Grünschieferfazies ist auf proterozoische Scherzonen beschränkt. Im südlichen Gebiet führte sie zu einer örtlich unterschiedlich intensiven retrograden Überprägung der Charnockit-Khondalit-Serie.Die P-, T-Bedingungen der Regionalmetamorphose wurden anhand von Mineralstabilitätsdaten und mit Methoden der Geothermometrie und Geobarometrie für das Gebiet zwischen Shimoga-Chitradurga (Norden) und Coimbatore-Karur (Süden) abgeleitet.

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Résumé Le craton archéen de l'Inde méridionale est caractérisé par une évolution géologique complexe et mal connue. Elle comprend plusieurs cycles de sédimentation et de volcanisme, de déformation et métamorphisme d'une durée d'environ 3400 jusqu'à 2500 Ma.La variation du degré de métamorphisme observée aujourd'hui est considérée comme le résultat d'un épisode de métamorphisme datant d'environ 2600 Ma, qui a affecté l'ensemble du domaine archéen composé de granitoïdes et de ceintures de roches vertes.La région méridionale de craton est caractérisée par le faciès granulite (700–750 °C/ 8–10 kb). Une zone étendue de charnockites et khondalites est le produit d'une transformation postdéformative résultant de l'apport d'une phase fluide riche en CO₂. Vers le nord le degré de métamorphisme décroit jusqu'au faciès amphibolites (600 °C/6–8 kb) et même jusqu'au faciès schistes verts (400 °C) dans les ceintures dharwariennes de roches vertes.Un épisode tardif de métamorphisme de faciès amphibolites et schistes verts est limité aux zones à décollement intense d'âge protérozoïque. Dans la région méridionale il s'est produit un rétromorphisme des roches granulitiques dans la zone des charnockites et khondalites.Les conditions de pression et température ont été déduites de l'application de données sur la stabilité des minéraux et de méthodes de géothermométrie et de géobarométrie pour la région entre Shimoga-Chitradurga (nord) et Coimbatore-Karur (sud).

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, . , , 3400 2500 . , , 2600 , , , . (700–750 °, 8–10 ). / - , ₂. (600 °, 6–8 ) Dharwar (400 °). . / , . , Shimoga-Chitradurga () Coimbatore-Karur ().

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The present study is part of a joint project of Benaras Hindu University at Varanasi and University of Kiel. Financial Support by the Deustche Forschungsgemeinschaft is gratefully acknowledged.

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We especially thank M. N. Viswanatha, Geological Survey of India, for his cooperation during the field work.     

2-D Crustal thermal structure along Thuadara-Sindad DSS profile across Narmada-Son lineament,central India

Central India is traversed by a WSW-ENE trending Narmada-Son lineament (NSL) which is characterized by the presence of numerous hot springs, feeder dykes for Deccan Traps and seismicity all along its length. It is divided in two parts by the Barwani-Sukta Fault (BSF). To the west of this fault a graben exists, whereas to the east the basement is uplifted between Narmada North Fault (NNF) and Narmada South Fault (NSF). The present work deals with the 2-D thermal modeling to delineate the crustal thermal structure of the western part of NSL region along the Thuadara-Sindad Deep Seismic Sounding (DSS) profile which runs almost in the N-S direction across the NSL. Numerical results of the model reveal that the conductive surface heat flow value in the region under consideration varies between 45 and 47mW/m². Out of which 23mW/m² is the contribution from the mantle heat flow and the remaining from within the crust. The Curie depth is found to vary between 46 and 47 km and is in close agreement with the earlier reported Curie depth estimated from the analysis of MAGSAT data. The Moho temperature varies between 470 and 500°C. This study suggests that this western part of central Indian region is characterized by low mantle heat flow which in turn makes the lower crust brittle and amenable to the occurrence of deep focused earthquakes such as Satpura (1938) earthquake.     

Love waves in an inhomogeneous interstratum

Summary The propagation of Love waves in an inhomogeneous interstratum, whose rigidity and density follow generalized power law variation, lying between two homogeneous half-spaces has been considered. The characteristic frequency equations have been obtained. The computational results for some special cases are presented in the form of dispersion curves showing the variation of phase and group velocity of Love waves with wave number.     

Actinometric determination of turbidity parameters in India

Summary A study of the wavelength exponent of aerosol scattering in the Ångström relation for extinction by aerosol has been made from the ground-based measurments of direct solar radiation using Ångström pyrheliometer with and without Schott filters. It has been observed that in India, mainly for the middle part of the year this exponent is zero or even negative which means that the aerosol scattering is nearly neutral which is in marked contrast with the condition prevailing in middle latitudes. It is evident from the -values that the aerosol size distribution in India is far different from that prevailing in middle latitudes. At four representative stations in India, the values of the wavelength exponent and the atmospheric turbidity coefficient have been determined using the method introduced byÅngström [1,2]) and are discussed here.     

Teleseismic delay-time tomography of the upper mantle beneath southeastern India: imprint of Indo–Antarctica rifting

A 3-D P -velocity map of the crust and upper mantle beneath the southeastern part of India has been reconstructed through the inversion of teleseismic traveltimes. Salient geological features in the study region include the Archean Dharwar Craton and Eastern Ghat metamorphic belt (EGMB), and the Proterozoic Cuddapah and Godavari basins. The Krishna–Godavari basin, on the eastern coastal margin, evolved in response to the Indo–Antarctica breakup. A 24-station temporary network provided 1161 traveltimes, which were used to model 3-D P -velocity variation. The velocity model accounts of 80 per cent of the observed data variance. The velocity picture to a depth of 120 km shows two patterns: a high velocity beneath the interior domain (Dharwar craton and Cuddapah basin), and a lower velocity beneath the eastern margin region (EGMB and coastal basin). Across the array velocity variations of 7–10 per cent in the crust (0–40 km) and 3–5 per cent in the uppermost mantle (40–120 km) are observed. At deeper levels (120–210 km) the upper-mantle velocity differences are insignificant among different geological units. The presence of such a low velocity along the eastern margin suggests significantly thin lithosphere (<100 km) beneath it compared to a thick lithosphere (>200 km) beneath the eastern Dharwar craton. Such lithospheric thinning could be a consequence of Indo–Antarctica break-up.     

Influence of an americium solid phase on americium concentrations in solutions

Americium-241 concentrations in solutions contacting contaminated sediments for up to 2 yr were measured as a function of pH. Steady-state concentrations were reached within a few days. The solubility-limited Am concentration was found to decrease approximately 10-fold with one unit increase in pH. The log equilibrium constant for the solubility of Am_(soil) solid $\text{[Am}{}_{\mathrm{(soil)}}\text{+ H}{}^{\mathrm{+}}\text{? Am}{}_{\mathrm{(aq\; complex)}}{}^{\mathrm{+}}\text{]}$ was found to be ?4.12. The predictions based upon thermodynamic data suggest that Am_{(aq complex)}⁺ is likely to be Am(OH)₂⁺. Although the chemical formula of Am_(soil) was not determined, it does not appear to be Am(OH)₃(a).Published data on sorption coefficients of Am by different rocks, soils, and minerals were critically evaluated. Final Am solution concentrations calculated from the sorption coefficients of a variety of earth materials with several solutions agreed well with the concentrations predicted from the solubility of Am_(soil) solid, indicating that the sorption coefficient data are controlled by Am precipitation.     

Algal Dynamics in Relation to Physico-Chemical Factors in a High Altitude Pond of Shillong,India

A one year phycological study of a high altitude pond has been made with a view to studying the algal dynamics as affected by physico-chemical characteristics of the water. This has revealed the presence of 34 genera of algae belonging to Chlorophyta, Bacillariophyta, Cyanophyta, and Euglenophyta. Chlorophytes were the most dominant and diatoms the most frequent representatives of algae. The community diversity along with water chemistry are indicative of the oligotrophic nature of this pond.     

Annealing studies of fission tracks in allanite

The annealing characteristics of fission tracks in allanite have been studied. All the tracks in the mineral fade at 720° C for one hour heating. The activation energy of allanite ranges from 1.4 to 3.4 eV.