Use of Sr isotopes as a tool to decipher the soil weathering processes in a tropical river catchment, southwestern India

River water composition (major ion and Sr-87/Sr-86 ratio) was monitored on a monthly basis over a period of three years from a mountainous river (Nethravati River) of southwestern India. The total dissolved solid (TDS) concentration is relatively low (46 mg L-1) with silica being the dominant contributor. The basin is characterised by lower dissolved Sr concentration (avg. 150 nmol L-1), with radiogenic Sr-87/Sr-86 isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and Sr-87/Sr-86 and their correlation with silicate derived cations in the river basin reveal that their dominant source is from the radiogenic silicate rock minerals. Their composition in the stream is controlled by a combination of physical and chemical weathering occurring in the basin. The molar ratio of SiO2/Ca and Sr-87/Sr-86 isotopic ratio show strong seasonal variation in the river water, i.e., low SiO2/Ca ratio with radiogenic isotopes during non-monsoon and higher SiO2/Ca with less radiogenic isotopes during monsoon season. Whereas, the seasonal variation of Rb/Sr ratio in the stream water is not significant suggesting that change in the mineral phase being involved in the weathering reaction could be unlikely for the observed molar SiO2/Ca and Sr-87/Sr-86 isotope variation in river water. Therefore, the shift in the stream water chemical composition could be attributed to contribution of ground water which is in contact with the bedrock (weathering front) during non-monsoon and weathering of secondary soil minerals in the regolith layer during monsoon. The secondary soil mineral weathering leads to limited silicate cation and enhanced silica fluxes in the Nethravati river basin. (C) 2015 Elsevier Ltd. All rights reserved.

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U-Th series and REEs

The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500-1200 mm. In the semi-arid zone (MAR: 500-900 mm), carbonates occur (i) as rhick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900-1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size. Chemical and isotopic (Sr-87/Sr-86) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the Sr-87/Sr-86 signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations. The Sr, U and Mg contents and the (U-234/U-238) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (U-234/U-238) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U-Th method range from 1.33 +/- 0.84 kyr to 7.5 +/- 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils. (C) 2010 Elsevier Ltd. All rights reserved.

川西北马脑壳金矿床流体包裹体Rb-Sr同位素组成：对矿床成因的制约

位于四川西北部的马脑壳金矿床为陕甘川金三角区内一重要矿床，许多学者对其进行过大量的研究，对其地质地球化学特征有了较为全面的认识，同时对矿床的成因也做了多方面的探讨。为了确定矿床的成矿时代，从层状矿体中采集了富含流体包裹体的石英矿物。通过测定包裹体的Rb-Sr同位素组成，应用ISOPLOT程序计算获得(210±35)Ma的成矿年龄，该年龄与容矿地层的形成时代相近。流体包裹体的锶同位素初始比值与三叠纪时期海水的锶同位素比值相一致。综合矿体产出特征、矿石组构及区域构造演化历史，认为作为矿床主体的层状矿体是同生沉积的产物，脉状网脉状矿体则是由后期造山运动体制下所产生的成矿热流体沿裂隙构造交代充填所形成。

威宁宣威组底部硅质页岩Rb-Sr古混合线年龄及其地质意义

在滇黔相邻地区，二叠纪宣威组下部发育一层硅质页岩和碳质页岩。页岩层位稳定、分布广泛，与下伏 峨眉山玄武岩组上部凝灰岩层呈假整合接触关系。用迭代法恢复硅质页岩的Rb-Sr同位索古混合线，得到硅 质页岩的年龄为(255 4-12)Ma。这个年龄不仅支持地层古地磁研究结果，而且符合地层年代学研究结果，表明 Rb-Sr同位索古混合线年代学方法在中生代地层能够适用。这个年龄与普遍认为的峨眉山玄武岩主喷发期258 Ma从时代关系上相符，而且有效地约束了峨眉山玄武岩喷发的上限。

An 40Ar/39Ar, Rb/Sr, and stable isotope study of micas in low-grade fold-and-thrust belt: An example from the Swiss Helvetic Alps

White micas in carbonate-rich tectonites and a few other rock types of large thrusts in the Swiss Helvetic fold-and-thrust belt have been analyzed by Ar-40/Ar-39 and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental Ar-40/Ar-39 heating experiments of 25 weakly metamorphosed (anchizone to low greenschist) samples yield plateau and staircase spectra. We interpret most of the staircase release spectra result from variable mixtures of syntectonic (neoformed) and detrital micas. The range in dates obtained within individual spectra depends primarily on the duration of mica nucleation and growth, and relative proportions of neoformed and detrital mica. Rb/Sr analyses of 12 samples yield dates of ca. 10-39 Ma (excluding one anomalously young sample). These dates are slightly younger than the Ar-40/Ar-39 total gas dates obtained for the same samples. The Rb/ Sr dates were calculated using initial Sr-87/Sr-86 ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher Sr-87/Sr-86 ratios (and lower O-18/O-16 ratios). Model dates calculated using Sr-87/Sr-86 values typical of Mesozoic marine carbonates more closely approximate the Ar-40/Ar-39 total gas dates for most of the samples. The similarities of Rb/Sr and Ar-40/Ar-39 total gas dates are consistent with limited amounts of detrital mica in the samples. The delta(18)O values range from 24-15%. (VSMOW) for 2-6 mum micas and 27-16parts per thousand for the carbonate host rocks. The carbonate values are significantly lower than their protolith values due to localized fluid-rock interaction and fluid flow along most thrust surfaces. Although most calcite-mica pairs are not in oxygen isotope equilibrium at temperatures of ca. 200-400 degreesC, their isotopic fractionations are indicative of either 1) partial exchange between the minerals and a common external fluid, or 2) growth or isotopic exchange of the mica with the carbonate after the carbonate had isotopically exchanged with an external fluid. The geological significance of these results is not easily or uniquely determined, and exemplifies the difficulties inherent in dating very fine-grained micas of highly deformed tectonites in low-grade metamorphic terranes. Two generalizations can be made regarding the dates obtained from the Helvetic thrusts: 1) samples from the two highest thrusts (Mt. Gond and Sublage) have all of their Ar-40/Ar-39 steps above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of Ar-39 release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the foreland-imbricating system, and some thrusts remained active or were reactivated down to 15 Ma.

Aplicação de métodos radiométricos (Rb-Sr e Sm-Nd) na análise de bacias sedimentares o exemplo da Bacia do Paraná

Os métodos radiométricos (Rb-Sr e Sm-Nd) têm sido aplicados, com sucesso, em rochas sedimentares visando a obtenção de idades deposicionais e informações sobre proveniência. Em muitos casos, apesar dos resultados geologicamente significativos, ainda persistem dúvidas em relação a extensão e interpretação dos dados obtidos. Isto ocorre porque as rochas sedimentares resultam da mistura de fragmentos detríticos de diversas proveniências com diferentes sistemas isotópicos, conseqüentemente, já contendo um registro isotópico da rocha fonte. No caso do método Rb-Sr, aplicado em rochas sedimentares de granulometria fina, pode-se obter idades absolutas para o evento deposicional. Isto é viável desde que este registro isotópico proveniente das rochas fonte seja apagado no momento da deposição, ou seja, deve ocorrer a homogeneização isotópica do Sr no ambiente sedimentar. Para tanto, devem ser observados alguns pré-requisitos em relação a amostragem, granulometria, ambiente deposicional e composição mineralógica das amostras, entre outros. No entanto, ainda existem questionamentos em relação a esta metodologia especialmente quanto à ocorrência e à extensão do processo de homogeneização isotópica do Sr em ambiente sedimentar.(Continua0 O método Sm-Nd, apesar de ser uma técnica relativamente nova quando aplicada a rochas sedimentares, tem se tornado uma ferramenta fundamental para auxílio na determinação de proveniência de rochas sedimentares. As maiores dificuldades estão na correta interpretação dos valores obtidos e sua associação com um ambiente sedimentar, ou seja, de baixa temperatura. Neste trabalho, foram aplicados os métodos radiométricos Rb-Sr e Sm-Nd em amostras de rochas sedimentares coletadas em diferentes contextos geológicos da Bacia do Paraná. Foram tentativamente testados os diferentes parâmetros que atuam no sentido da homogeneização isotópica do Sr e do comportamento do Nd. Os resultados obtidos permitiram aprimorar a metodologia radiométrica Rb-Sr e Sm-Nd quando aplicada em rochas sedimentares, bem como obter resultados sobre a idade deposicional de diferentes unidades sedimentares e, por vezes, sua proveniência.