Strontium Isotopic Signatures of the Streams and Lakes of Taylor Valley, Southern Victoria Land, Antarctica: Chemical Weathering in a Polar Climate

We have collected and analyzed a series of water samples from three closed-basin lakes (Lakes Bonney, Fryxell, and Hoare) in Taylor Valley, Antarctica, and the streams that flow into them. In all three lakes, the hypolimnetic waters have different ⁸⁷Sr/⁸⁶Sr ratios than the surface waters, with the deep water of Lakes Fryxell and Hoare being less radiogenic than the surface waters. The opposite occurs in Lake Bonney. The Lake Fryxell isotopic ratios are lower than modern-day ocean water and most of the whole-rock ratios of the surrounding geologic materials. A conceivable source of Sr to the system could be either the Cenozoic volcanic rocks that make up a small portion of the till deposited in the valley during the Last Glacial Maximum or from marble derived from the local basement rocks. The more radiogenic ratios from Lake Bonney originate from ancient salt deposits that flow into the lake from Taylor Glacier and the weathering of minerals with more radiogenic Sr isotopic ratios within the tills. The Sr isotopic data from the streams and lakes of Taylor Valley strongly support the notion documented by previous investigators that chemical weathering has been, and is currently, a major process in determining the overall aquatic chemistry of these lakes in this polar desert environment.     

The isotopic composition of strontium in non-marine carbonate rocks: the Flagstaff Formation of Utah*

The isotopic composition of strontium in surface water in continental basins is determined primarily by the geology of the basin and to a lesser extent by climatic conditions. Consequently, the ⁸⁷Sr/⁸⁶Sr ratios of brines in such basins can change only as a result of changes in the geology or climate. This principle of isotope geology was studied by analysis of a suite of non-marine carbonate rocks from the Flagstaff Formation (Palaeocene-Eocene) of Utah. The samples were collected from a section in Fairview Canyon of Sanpete County. They include both limestone and dolomite and were selected to have low non-carbonate residues. The concentrations of strontium in calcites averages 383 ± 128 p.p.m., while those of dolomites increase from 354 ± 74 p.p.m. in the lower 43 m of section to a maximum of 2259 p.p.m. higher up. The increase in the strontium content of dolomite is interpreted as evidence for a change from steady-state to progressively more evaporitic conditions. Two dolomites have isotopic compositions of oxygen expressed as δ¹⁸O = -2.75‰ (relative to the PDB standard) and are enriched in ¹⁸O relative to two calcites whose average δ¹⁸O value is -9.9‰. The ⁸⁷Sr/⁸⁶Sr ratios of the carbonate minerals range from 0.70890 to 0.71260. These values are clearly greater than the ⁸⁷Sr/⁸⁶Sr ratio of marine carbonates of Early Eocene age which is 0.70744. The variation of the ⁸⁷Sr/⁸⁶Sr ratio in this section of the Flagstaff Formation is real and reflects the occurrence of geological events which changed the isotopic composition of Sr entering Lake Flagstaff. The non-carbonate fractions of six carbonate rocks and one sandstone fit a straight line on the strontium mixing diagram in co-ordinates of initial ⁸⁷Sr/⁸⁶Sr and 1/Sr concentration. These results suggest that the isotopic composition of strontium in Lake Flagstaff may have been modulated by periodic input of volcanogenic detritus of felsic composition.     

Some aspects of the Ca and Sr weathering cycle in the Lake Kinneret (Lake Tiberias) drainage basin

Sr/Ca ratios in the clay (< 2 μ) weathering products of basalt rocks from the Lake Kinneret drainage basin were found to be very similar to the ratios in the rocks, indicating similar depletion intensities for the two elements. In the clays from the scoriae and some lapilli tuffs the ratios were found to be somewhat higher than in the corresponding rocks, indicating preferential retention of strontium in the clays. The ratio in drainage water from these areas was much lower than that obtained by mass balance calculations.The Sr/Ca ratio in Jordan river water draining primarily limestone areas is 2.3 · 10^?3. Very high Sr/Ca ratios in Lake Kinneret water were attributed to the influence of saline sublacustrine springs. The carbonate fraction from recent lake sediments was shown to have the Sr/Ca ratios characteristic for calcite precipitated biogenically in marine environments. The relative enrichment in strontium of the lake water was explained by the intensive precipitation of carbonates with low strontium distribution coefficients.     

Strontium isotope composition of runoff from a glaciated carbonate terrain

The relationship between subglacial chemical weathering processes and the Sr isotope composition of runoff from Robertson Glacier, Alberta, Canada, is investigated. This glacier rests on predominantly carbonate bedrock of Upper Devonian age, but silicate minerals are also present. The provenance of solute in meltwaters is found to vary systematically with solute concentration and, by inference, subglacial water residence time. In dilute waters, the principal process of solute acquisition is calcite dissolution fueled by protons derived from the dissolution of CO₂ and subsequent dissociation of carbonic acid. At higher solute concentrations, dolomite dissolution coupled to sulfide oxidation is more important. Sr concentration is found to increase with total solute concentration in two separate meltwater streams draining from the glacier, but ⁸⁷Sr/⁸⁶Sr only increases in the eastern melt stream. Carbonate and K-feldspar sources are shown to dominate the Sr content of the western stream, irrespective of concentration. They also dominate the Sr content of the eastern stream at low and intermediate concentrations, but at higher concentrations, muscovite (with high ⁸⁷Sr/⁸⁶Sr) is also an important Sr source. This reflects the outcrop of muscovite-bearing lithologies in the catchment of the eastern stream and an increase in the rate of weathering of K-silicates relative to that of carbonates as more concentrated solutions approach saturation with respect to carbonates. Nonstoichiometric release of ⁸⁷Sr/⁸⁶Sr and preferential release of Sr over K from freshly ground K-silicate surfaces may also occur. This may help to explain the radiogenic nature of runoff from distributed subglacial drainage systems, which are characterized by long water:rock contact times and water flow through environments in which crushing and grinding of bedrock are active processes.Although the exchangeable Sr in tills has higher ⁸⁷Sr/⁸⁶Sr than local carbonate bedrock, only the more concentrated meltwaters from the eastern stream display similarly high values. The most dilute waters, which probably transport the bulk of the dissolved Sr flux from the glacier, have ⁸⁷Sr/⁸⁶Sr characteristic of local carbonate bedrock. Thus, the results suggest that although enhanced weathering of silicate minerals containing radiogenic Sr (such as muscovite) does occur in glaciated carbonate terrains, it is unlikely to contribute to any enhanced flux of radiogenic Sr from glaciated continental surfaces to the oceans.     

Strontium isotope systematics of base flow in Piedmont Province watersheds,Georgia (USA)

⁸⁷Sr/⁸⁶Sr ratio variations were analyzed in rainfall, shallow ground water and base flow collected from 4 Piedmont streams within the Middle Oconee River basin in northeastern Georgia during the period between March, 2003 and March, 2004. They Sr isotope ratio analyses were accompanied by measurements of stream discharge, rainfall, stable O isotope ratios and major ion and ³H concentrations. The average Sr ion concentration and ⁸⁷Sr/⁸⁶Sr ratio for the terminal stream basin (the Middle Oconee River) were 23.6 μg/L and 0.7172, respectively. The average ⁸⁷Sr/⁸⁶Sr ratios of the rainwater and shallow ground water were below 0.7125, indicating that most of the Sr in this stream water is input by weathering reactions in deeper ground water, rather than by ion exchange in shallow soil horizons. This is consistent with the higher alkalinity concentrations (∼23–47 mg/L) and specific conductance values (60–113 μS/cm) that characterize stream base flow. Piedmont streams are characterized by lower concentrations of Sr and higher ⁸⁷Sr/⁸⁶Sr ratios than average global stream flow.Base flow rates decreased by a factor of 2–3 during the summer months and this is accompanied by increased alkalinity concentrations. ⁸⁷Sr/⁸⁶Sr ratios, however, were temporally invariant for a given stream basin and were independent of season, antecedent rainfall, and discharge. ⁸⁷Sr/⁸⁶Sr ratios were unique for each of the 4 basins and a general trend toward higher ratios with increasing basin area was apparent. The inferred contribution from minerals with high Rb contents such as K feldspar and muscovite may have resulted from the greater integration of flow from mineralogically diverse pathways afforded by a larger basin area. The basin specificity and temporal or seasonal invariability make ⁸⁷Sr/⁸⁶Sr ratios an invaluable hydrological tracer that can be readily employed in mass balance studies of stream flow within the Piedmont Province.     

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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr 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.     

Rare earth element and strontium isotope geochemistry in Dujiali Lake,central Qinghai-Tibet Plateau,China: Implications for the origin of hydromagnesite deposits

Rare earth element (REE) and strontium isotope data (⁸⁷Sr/⁸⁶Sr) are presented for hydromagnesite and surface waters that were collected from Dujiali Lake in central Qinghai-Tibet Plateau (QTP), China. The goal of this study is to constrain the solute sources of hydromagnesite deposits in Dujiali Lake. All lake waters from the area exhibit a slight LREE enrichment (average [La/Sm]_PAAS = 1.36), clear Eu anomalies (average [Eu/Eu*]_PAAS = 1.31), and nearly no Ce anomalies. The recharge waters show a flat pattern (average [La/Sm]_PAAS = 1.007), clear Eu anomalies (average [Eu/Eu*] _PAAS = 1.83), and nearly no Ce anomalies (average [Ce/Ce*]_PAAS = 1.016). The REE+Y data of the surface waters indicate the dissolution of ultramafic rock at depth and change in the hydrogeochemical characteristics through fluid-rock interaction. These data also indicate a significant contribution of paleo-groundwater to the formation of hydromagnesite, which most likely acquired REE and Sr signatures from the interaction with ultramafic rocks. The ⁸⁷Sr/⁸⁶Sr data provide additional insight into the geochemical evolution of waters of the Dujiali Lake indicating that the source of Sr in the hydromagnesite does not directly derive from surface water and may have been influenced by both Mg-rich hydrothermal fluids and meteoric water. Additionally, speciation modeling predicts that carbonate complexes are the most abundant dissolved REE species in surface water. This study provides new insights into the origins of hydromagnesite deposits in Dujiali Lake, and contributes to the understanding of hydromagnesite formation in similar modern and ancient environments on Earth.     

Constraints on water chemistry by chemical weathering in the Lake Qinghai catchment, northeastern Tibetan Plateau (China): clues from Sr and its isotopic geochemistry

Lake water, river water, and groundwater from the Lake Qinghai catchment in the northeastern Tibetan Plateau, China have been analyzed and the results demonstrate that the chemical components and ⁸⁷Sr/⁸⁶Sr ratios of the waters are strictly constrained by the age and rock types of the tributaries, especially for groundwater. Dissolved ions in the Lake Qinghai catchment are derived from carbonate weathering and part from silicate sources. The chemistry of Buha River water, the largest tributary within the catchment, underlain by the late Paleozoic marine limestone and sandstones, constrains carbonate-dominated compositions of the lake water, being buffered by the waters from the other tributaries and probably by groundwater. The variation of ⁸⁷Sr/⁸⁶Sr ratios with cation concentrations places constraint on the Sr-isotopic compositions of the main subcatchments surrounding Lake Qinghai. The relative significance of river-water sources from different tributaries (possibly groundwater as well) in controlling the Sr distribution in Lake Qinghai provides the potential to link the influence of hydrological processes to past biological and physical parameters in the lake. The potential role of groundwater input in the water budget and chemistry of the lake emphasizes the need to further understand hydrogeological processes within the Lake Qinghai system.     

Hydrologic controls on radiogenic Sr in meltwater from an alpine glacier system: Athabasca Glacier,Canada

Filtered subglacial meltwater samples were collected daily during the onset of melt (May) and peak melt (July) over the 2011 melt season at the Athabasca Glacier (Alberta, Canada) and analyzed for strontium-87/strontium-86 (⁸⁷Sr/⁸⁶Sr) isotopic composition to infer the evolution of subglacial weathering processes. Both the underlying bedrock composition and subglacial water–rock interaction time are the primary influences on meltwater ⁸⁷Sr/⁸⁶Sr. The Athabasca Glacier is situated atop Middle Cambrian carbonate bedrock that also contains silicate minerals. The length of time that subglacial meltwater interacts with the underlying bedrock and substrate is a predominant determining factor in solute concentration. Over the course of the melt season, increasing trends in Ca/K and Ca/Mg correspond to overall decreasing trends in ⁸⁷Sr/⁸⁶Sr, which indicate a shift in weathering processes from the presence of silicate weathering to primarily carbonate weathering.Early in the melt season, rates of carbonate dissolution slow as meltwater approaches saturation with respect to calcite and dolomite, corresponding to an increase in silicate weathering that includes Sr-rich silicate minerals, and an increase in meltwater ⁸⁷Sr/⁸⁶Sr. However, carbonate minerals are preferentially weathered in unsaturated waters. During the warmest part of a melt season the discharged meltwater is under saturated, causing an increase in carbonate weathering and a decrease in the radiogenic Sr signal. Likewise, larger fraction contributions of meltwater from glacial ice corresponds to lower ⁸⁷Sr/⁸⁶Sr values, as the meltwater has lower water–rock interaction times in the subglacial system. These results indicate that although weathering of Sr-containing silicate minerals occurs in carbonate dominated glaciated terrains, the continual contribution of new meltwater permits the carbonate weathering signal to dominate.     

川西坳陷新场气田须二气藏地层水成因研究——水化学和锶同位素证据

地层水是影响气田产能开发的重要因素之一,气井遇水会降低储层气相渗透率,从而导致气产量迅速下降,严重制约单井产能的提高。目前已有的研究对须二气藏地层水的物质来源认识仍然存在一定争议。本文从主微量元素、水岩相互作用以及锶同位素组成特征方面对川西坳陷新场气田须二气藏11口生产井的地层水进行了系统研究,结果显示须二气藏地层水具有低矿化度(55 166.00~122 547.41 mg/L)、高Br^-和Sr²⁺含量(分别为642.00~1 711.00 mg/L和670.00~1 780.00 mg/L)及⁸⁷Sr/⁸⁶Sr值(0.715 27~0.721 77)的特征,表明研究区地层水Sr同位素来源区别于海相碳酸盐岩,更符合砂岩风化特征。地层水中Ca的富集主要与砂岩储层成岩自生矿物的溶解有关,在深部水岩相互作用下,Sr以类质同象的方式代替Ca并且获得了有机质中的Br,同时使得Br更加富集。以上研究结果为今后该气藏赋水层位的预测提供了理论依据。     

Halogen geochemistry of the McMurdo dry valleys lakes, Antarctica: Clues to the origin of solutes and lake evolution

We have determined the halogen and boron concentrations in the ice-covered lakes of Taylor Valley, Antarctica, to better establish the sources of salts and evolutionary histories of these unusual water bodies. In addition, we report on a series of ¹²⁹I measurements that were compared with previous ³⁶Cl data that also help constrain the source of solutes and histories of the lakes. The new data, when put into context of previous work on these systems over the past forty years, allow us to make the following conclusions. The primary source of solutes to Lake Hoare, the youngest of the lakes, is the dissolution of marine aerosols and aeolian salts and the chemical weathering of dust on the glaciers. The geochemistry of Lake Fryxell, the brackish water lake, is primarily dominated by the diffusion from a halite-saturated brine at the sediment-water interface and the recent infilling of the lake by glacier meltwater. These waters have chemical weathering and marine aerosols components. Lake Bonney has two distinct lobes whose hypersaline hypolimnia have different chemistries. Both of the lobes are remnants of ancient marine waters that have been modified by the input of weathering products. This lake has also been modified by periods of cryogenic concentration when solutes have been lost via mineral precipitation. Thus the geochemistry of Lake Bonney owes its unusual geochemistry, in part, to variations in the climate in the Taylor Valley over at least the past 300kyr. The ¹²⁹I data from the Taylor Valley are similar to those from fracture fluids in crystalline rocks from the Northern Hemisphere.     

湘南泥盆系碳酸盐岩中锶的分布特点及其环境意义

湘南泥盆系碳酸盐沉积环境,可分为台地和台盆两大类和六种相类型.250块岩样的分析结果表明,Sr和Sr/Ca×1000的含量比值变化与沉积环境密切相关,即由浅水台地相到深水台盆相,其比值从0.27增至2.37.其原因是由于不同沉积环境中碳酸盐矿物组份不同.影响了锶的分布.因此锶的含量可作为相分析的工具.     

Strontium Isotope Stratigraphy: Principles and State of the Art

The methodical basis, development, and current state of a new method of chronostratigraphic studies, i.e., strontium isotope stratigraphy (SIS), are considered. This method makes it possible to date and correlate geographically distant sedimentary sequences without involving the biostratigraphic and isotope geochronological data. SIS is based on secular variations in ⁸⁷Sr/⁸⁶Sr in the paleocean, resulting from the redistribution of the roles of two global strontium flows formed in the mantle and continental reservoirs of the Earth. Isotopic homogeneity of Sr in the paleoceans and in the linked seas leads to the fact that the ⁸⁷Sr/⁸⁶Sr ratio in the sea basins is individual for each geological time point and is inherited in marine chemogenic sediments under deposition of dissolved Sr as an isomorphic impurity. Low-Mg calcite and also fragments of fossilized paleontological remains buried in situ are the best minerals that are capable of retaining the Sr isotopic signature of the sedimentation environment. SIS is carried out with geochemical diagnostics of secondary alteration of the studied material and selective dissolution of the samples to produce a carbonate material that adequately reflects isotopic signature of the sedimentary basin. Interregional correlations of the Proterozoic and Cenozoic sea sediments and their relation to the SIS-based stratigraphic scale are given as an example.     

Magnesium isotope systematics of the lithologically varied Moselle river basin, France

Magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the Moselle basin, northeastern France. This small basin is remarkable for its well-constrained and varied lithology on a small distance scale, and this is reflected in river water Sr isotope compositions. Upstream, where the Moselle River drains silicate rocks of the Vosges mountains, waters are characterized by relatively high ⁸⁷Sr/⁸⁶Sr ratios (0.7128-0.7174). In contrast, downstream of the city of Epinal where the Moselle River flows through carbonates and evaporites of the Lorraine plateau, ⁸⁷Sr/⁸⁶Sr ratios are lower, down to 0.70824.Magnesium in river waters draining silicates is systematically depleted in heavy isotopes (δ²⁶Mg values range from −1.2 to −0.7‰) relative to the value presently estimated for the continental crust and a local diorite (−0.5‰). In comparison, δ²⁶Mg values measured in soil samples are higher (∼0.0‰). This suggests that Mg isotope fractionation occurs during mineral leaching and/or formation of secondary clay minerals. On the Lorraine plateau, tributaries draining marls, carbonates and evaporites are characterized by low Ca/Mg (1.5-3.2) and low Ca/Sr (80-400) when compared to local carbonate rocks (Ca/Mg = 29-59; Ca/Sr = 370-2200), similar to other rivers draining carbonates. The most likely cause of the Mg and Sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer. δ²⁶Mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year (from −1.4‰ to −1.6‰ and from −1.2‰ to −1.4‰, respectively), within the range defined for the underlying rocks. Downstream of Epinal, the compositions of the Moselle River samples in a δ²⁶Mg vs. ⁸⁷Sr/⁸⁶Sr diagram can be explained by mixing curves between silicate, carbonate and evaporite waters, with a significant contribution from the Vosgian silicate lithologies (>70%). Temporal co-variation between δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr for the Moselle River throughout year is also observed, and is consistent with a higher contribution from the Vosges mountains in winter, in terms of runoff and dissolved element flux. Overall, this study shows that Mg isotopes measured in waters, rocks and soils, coupled with other tracers such as Sr isotopes, could be used to better constrain riverine Mg sources, particularly if analytical uncertainties in Mg isotope measurements can be improved in order to perform more precise quantifications.     

Isotopic constraints on the genesis and age of autochthonous glaucony in the Oligo-Miocene Torquay Group, south-eastern Australia

The Oligo‐Miocene Torquay Group at Bird Rock in south‐eastern Australia comprises a sequence of fine‐grained skeletal carbonates and argillaceous and glauconitic sandstones, deposited in a cool‐water, mid‐shelf environment. The Bird Rock glaucony is autochthonous and consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. The results of Rb–Sr and oxygen isotopic analysis of samples taken from a single glauconitic horizon (the BW horizon) indicate that the glaucony developed through a series of simultaneous dissolution–crystallization reactions, which occurred during very early diagenesis in a closed or isochemical system, isolated from the ambient marine environment. The constituent ions of the glaucony were derived primarily from terrigenous clay minerals, but considerable potassium may have been sourced indirectly from sea water, through potassium enrichment of clay precursors. The pore fluids associated with glauconitization were marine derived, but progressively modified by the dissolution–crystallization of detrital clay minerals and autochthonous glaucony. Rb–Sr data for the BW horizon indicate that dating glauconies may be somewhat problematic, as co‐genetic glauconitic minerals can show a range of initial strontium compositions, which reflect the incorporation of strontium derived from mineralogical precursors and/or contemporaneous sea water. Rb–Sr isochrons indicate that the glaucony of the BW horizon formed at 23 ± 3 Ma. This age is in good agreement with both the established biostratigraphy and a ⁸⁷Sr/⁸⁶Sr age for the horizon (23 ± 1 Ma), but could only be determined using the independent age constraint and the estimate of the ⁸⁷Sr/⁸⁶Sr ratio of contemporaneous sea water provided by analysis of associated biogenic carbonate.     

Rubidium,strontium and the isotopic composition of strontium in ultramafic nodule minerals and host basalts

The concentrations of rubidium and strontium and the isotopic composition of strontium have been determined in minerals separated from ultramaflc nodules occurring in late Tertiary and Quaternary basalts of wide geographic distribution. Clinopyroxene, orthopyroxene and olivine from each of three Iherzolite nodules show a relatively wide range of ⁸⁷Sr/⁸⁶Sr disequilibrium and none of the minerals is in isotopic equilibrium with its host basalt. In two cases there is a correlation between ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr ratios of the nodule minerals, indicating apparent isochron relationships which may represent relict mantle events. Clinopyroxene and olivine from each of two wehrlite nodules are not in isotopic equilibrium, although the magnitude of the disequilibrium is smaller than that observed in the Iherzolite nodules. None of these ultramafic nodules can be a crystal cumulate from its host basalt, and it is doubtful that any type of genetic relationship exists. The strontium isotopic disequilibrium between nodule minerals seems to be a primary feature inherited from past mantle histories.     

Strontium isotope composition of marine carbonates of Middle Triassic to Early Jurassic age,Lombardic Alps,Italy*

The ⁸⁷Sr/⁸⁶Sr ratios and strontium concentrations for thirty-three samples of marine carbonate rocks of Middle Triassic to Early Jurassic age have been determined. The samples were collected from four measured sections in the areas of Val Camonica in northern Italy. The strontium concentrations vary from 40 to 7000 ppm. Most of the samples are calcitic limestones containing less than 10% of non-carbonate residues. Dolomitic samples and those containing appreciable non-carbonate residues have significantly diminished strontium concentrations. ⁸⁷Sr/⁸⁶Sr ratios of the carbonate phases of these rocks appear to be unaffected by dolomitization and by the presence of non-carbonate minerals. The average ⁸⁷Sr/⁸⁶Sr ratios of the formations vary systematically in a stratigraphic sense. The ratio increased from Early Anisian to Early-Middle Ladinian, declined during Late Ladinian and Carnian, rose again during the Norian and then declined throughout the Late Norian (Rhaetian), Hettangian, Sinemurian and Pliens-bachian ages. The average ⁸⁷Sr/⁸⁶Sr ratios, relative to 0.7080 for the Eimer and Amend standard, are: Anisian: 0.70805 ± 00019; Early Ladinian: 0.7085 ± 0.00038; Late Ladinian: 0.70791 ± 0.00013; Carnian: 0.70776 ± 0.00015; Norian and Rhaetian: 0.70791 ± 0.00014; Hettangian: 0.70762 ± 0.00021; Sinemurian: 0.7070 ± 0.00038; Pliensbachian: 0.7070 ± 0.00015. These variations reflect changes in the isotopic composition of Sr entering the oceans in early Mesozoic time due to varying rates of weathering and erosion of young volcanic rocks (low ⁸⁷Sr/⁸⁶Sr) and old granitic rocks (high ⁸⁷Sr/⁸⁶Sr). The data presented in this report contribute to a growing body of information regarding the changes that have occurred in the ⁸⁷Sr/⁸⁶Sr ratio of the oceans in Phanerozoic time.     

塔中地区寒武系-奥陶系碳酸盐岩Sr元素和Sr同位素特征

使用VG354固体同位素质谱仪对中1、中4井的25个碳酸盐岩样品做了Sr同位素测试, 并利用电感耦合等离子质谱仪(ICP-MS) 对塔中地区4口井共109个碳酸盐岩样品测试了Sr、Mn元素的含量.通过对Sr、Mn元素含量分析, 及中1、中4井碳酸盐岩的Sr同位素组成分析, 对比全球奥陶系海相碳酸盐的Sr同位素分析结果及演化趋势, 得出了如下认识: (1) 塔中地区奥陶纪87Sr/86Sr比值与全球海水Sr同位素演化趋势基本一致, 具有随时间下降的总体趋势, 这与广阔陆表海和有关的沉积物对放射性成因锶的封存作用有关, 说明海平面变化和白云岩化作用仍然是该区海相碳酸盐岩锶同位素组成与演化的主要控制因素; (2) 塔中地区早奥陶世的87Sr/86Sr比值与全球海水Sr同位素比值相当, 说明该区早奥陶世碳酸盐岩成岩环境为正常海水, 且早奥陶世87Sr/86Sr比值有单调降低的规律, 说明与海平面变化有关; (3) 塔中地区晚奥陶世87Sr/86Sr比值比全球海水高, 其原因是白云岩化作用和晚奥陶世盆地抬升近地表水带来高87Sr/86Sr比值, 且晚奥陶世87Sr/86Sr比值为整体升高的趋势; (4) 塔中地区奥陶纪碳酸盐岩中Mn元素含量变化不大, 反映了塔中地区奥陶纪成岩环境主要为浅水相, 但也有深水相, 白云岩化对其影响不大; (5) 塔中地区奥陶纪碳酸盐岩中Sr元素含量变化较大, 反映该时期该区碳酸盐岩成岩流体主要为海水, 但也有混合水, 白云岩主要为Ⅲ类白云岩和Ⅰ类白云岩.      

Strontium distribution and origins in a natural clayey formation (Callovian-Oxfordian, Paris Basin, France): A new sequential extraction procedure

Strontium is a good monitor of geochemical processes in natural clayey formations. In the Callovian-Oxfordian formation of Bure in France, strontium is sorbed on clay minerals and carried by carbonates, detrital minerals and accessory celestite. In order to determine the strontium distribution among these different phases, four-step sequential extractions (1. cobalt hexamine trichloride, 2. acetic acid, 3. EDTA and 4. tri-acid) were performed on samples from different levels of the clayey formation. The leachates were also analyzed for strontium isotopes, in order to determine the strontium origins. This sequential procedure is well suited to determining strontium distribution in claystones, although it is less efficient in clay-rich limestones and in celestite-rich samples. The carbonates (38-47% of the total strontium) show ⁸⁷Sr/⁸⁶Sr ratios (0.7070-0.7071) that have recorded the isotopic composition of the Callovian-Oxfordian seawater. Diagenetic carbonates (dolomite, ankerite and siderite) have almost not incorporated any strontium, which has been trapped by celestite during the late diagenesis. The major part of the celestite shows ⁸⁷Sr/⁸⁶Sr ratios (0.7069-0.7070) quite close to the primary carbonates. However, a second generation of celestite (0.7074) shows a slight ⁸⁷Sr-enrichment and is isotopically in equilibrium with the exchangeable strontium (27-48% of the total strontium with a mean ⁸⁷Sr/⁸⁶Sr value of 0.70745) and the present-day porewater (0.7074). This very low ⁸⁷Sr-enrichment could be explained by the partial destabilisation of detrital minerals (feldspars, micas, clays) which exhibit ⁸⁷Sr/⁸⁶Sr ratios consistent with their pristine Hercynian origin (0.7229-0.7350). Diffusion of strontium from the subjacent Dogger aquifers (0.7076-0.7082) could also be invoked to explain the slight ⁸⁷Sr-enrichment.     

川东北普光气田下三叠统飞仙关组白云岩成因——来自岩石结构与Sr同位素和Sr含量的证据

研究了川东北普光气田下三叠统飞仙关组白云岩储层的岩石结构Sr的含量和Sr同位素组成,讨论了它的成因,飞仙关组优质储层为成岩期埋藏交代白云化作用的产物,来自岩石结构和Sr同位素和Sr含量的证据包括如下几个方面:(1)与准同生白云岩比较,埋藏白云岩的岩石结构和Sr同位素和Sr含量地球化学特征与前者有显著差别;(2)飞仙关组所有各类碳酸盐岩(或矿物)具有早三叠世海水Sr同位素组成特征,~(87)Sr/~(86)Sr比值变化范围为0.706588～0.708187,覆盖了全球早三叠世海水Sr同位素的变化范围(0.7076～0.7078),平均值0.707656与全球早三叠世平均值(0.707743)基本一致;(3)埋藏白云岩~(87)Sr/86Sr比值变化范围为0.707122～0.707419,平均值0.707421,都略低于全球早三叠世海水Sr同位素变化范围和平均值,但与已报道的川东北早三叠世飞仙关期海水Sr同位素变化范围(0.707330～0.707383)和平均值(0.707350)都非常接近,说明白云石化流体具有强烈的川东北地区早三叠世飞仙关期海水Sr同位素组成特征;(4)综合岩石结构、Sr同位素和Sr含量地球化学特征,证明飞仙关组白云岩储层为成岩期埋藏交代作用产物,白云石化流体来自地层中高Sr和高盐度的海源地层水.