Evidence of hydrological control of Sr behavior in stream water (Strengbach catchment,Vosges mountains,France)

Strontium and particularly ⁸⁷Sr/⁸⁶Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and ⁸⁷Sr/⁸⁶Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and ⁸⁷Sr/⁸⁶Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, ⁸⁷Sr/⁸⁶Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, ⁸⁷Sr/⁸⁶Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members.     

The strontium isotopic composition and origin of burial cements in the Lincolnshire Limestone (Bajocian) of central Lincolnshire, England

Strontium isotopic composition (⁸⁷Sr/⁸⁶Sr) of two petrographically, chemically and isotopically (δ¹⁸O and δ¹³C) distinct phases of burial calcites from the Lincolnshire Limestone are indistinguishable (0.70820± 26). The mean ⁸⁷Sr/⁸⁶Sr ratio of these phases is considerably more radiogenic than ⁸⁷Sr/⁸⁶Sr ratios of Bajocian marine waters (～0.70725). Neither Bajocian marine waters nor meteoric waters buffered by host marine carbonate in the Limestone could have precipitated the burial spars. Radiogenic strontium may have been contributed from K-feldspar dissolution and/or clay recrystallization, either within clastic portions of the Limestone itself, or from major clastic units adjacent to the Limestone. Alternatively, Palaeozoic marine waters or remobilized Palaeozoic marine carbonate and/or sulphate could have supplied the necessary radiogenic strontium.     

A strontium isotopic study of formation waters from the Illinois basin, U.S.A.

The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated ⁸⁷Sr/⁸⁶Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution.     

87Sr/86Sr isotope composition of bottled British mineral waters for environmental and forensic purposes

Mineral waters in Britain show a wide range of ⁸⁷Sr/⁸⁶Sr isotope compositions ranging between ⁸⁷Sr/⁸⁶Sr = 0.7059 from Carboniferous volcanic rock sources in Dunbartonshire, Scotland to ⁸⁷Sr/⁸⁶Sr = 0.7207 in the Dalradian aquifer of Aberdeenshire, Scotland. The ⁸⁷Sr/⁸⁶Sr composition of the waters shows a general correlation with the aquifer rocks, resulting in the waters from older rocks having a more radiogenic signature than those from younger rocks. This wide range of values means that the Sr isotope composition of mineral water has applications in a number of types of studies. In the modern commercial context, it provides a way of fingerprinting the various mineral waters and hence provides a method for recognising and reducing fraud. From an environmental perspective, it provides the first spatial distribution of bio-available ⁸⁷Sr/⁸⁶Sr in Britain that can be used in modern, historical and archaeological studies.     

Strontium isotope geochemistry of groundwater affected by human activities in Nandong underground river system, China

The Nandong Underground River System (NURS) is located in a typical karst area dominated by agriculture in SE Yunnan Province, China. Groundwater plays an important role in the social and economical development in the area. The effects of human activities (agriculture and sewage effluents) on the Sr isotope geochemistry were investigated in the NURS. Seventy-two representative groundwater samples, which were collected from different aquifers (calcite and dolomite), under varying land-use types, both in summer and winter, showed significant spatial differences and slight seasonal variations in Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios. Agricultural fertilizers and sewage effluents significantly modified the natural ⁸⁷Sr/⁸⁶Sr ratios signature of groundwater that was otherwise dominated by water-rock interaction. Three major sources of Sr could be distinguished by ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in karst groundwater. Two sources of Sr are the Triassic calcite and dolomite aquifers, where waters have low Sr concentrations (0.1-0.2 mg/L) and low ⁸⁷Sr/⁸⁶Sr ratios (0.7075-0.7080 and 0.7080-0.7100, respectively); the third source is anthropogenic Sr from agricultural fertilizers and sewage effluents with waters affected having radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7080-0.8352 for agricultural fertilizers and 0.7080-0.7200 for sewage effluents, respectively), with higher Sr concentrations (0.24-0.51 mg/L). Due to the overlapping ⁸⁷Sr/⁸⁶Sr ratios, it is difficult to distinguish the sources of Sr in groundwater samples contaminated by agricultural fertilizers or sewage effluents based only on their ⁸⁷Sr/⁸⁶Sr ratios. However, ⁸⁷Sr/⁸⁶Sr ratios do provide key information for natural and anthropogenic sources in karst groundwater.     

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.     

87Sr/86Sr in waters from the Lincolnshire Limestone aquifer,England, and the potential of natural strontium isotopes as a tracer for a secondary recovery seawater injection process in oilfields

The Sr isotope composition of formation waters is a sensitive indicator of diagenetic processes in the host sediments, mixing processes between different bodies of water, and the connectivity of hydrological systems. The⁸⁷Sr/⁸⁶Sr ratio of present seawater is constant worldwife, while formation waters in hydrocarbon reservoirs have various values, depending on the aforementioned effects, in most cases different from modern seawater. This forms the basis of anatural tracer technique for seawater injection projects, involving characterization of the⁸⁷Sr/⁸⁶Sr ratios and Sr contents of formation waters in the reservoir before injection commences, followed by monitoring of these parameters in the produced water as injection proceeds. This method is best suited to reservoirs in which the formation waters have low Sr concentrations and⁸⁷Sr/⁸⁶Sr ratios much higher or lower than seawater. Available data for reservoir formation waters suggest that breakthrough recognition could be expected at <10% seawater in many sandstone reservoirs, while the method would be less sensitive in carbonate reservoir or situations where the formation waters had interacted with evaporites, as the associated waters tend to have high Sr contents. In heterogeneous but well-mapped reservoirs, it may be possible to obtain information about flow paths/mechanismsbefore breakthrough. Combination with other chemical and isotopic tracers creates a very powerful tool, the Sr method acting as a safeguard should the batch of water containing the conventional tracers be overtaken by subsequently injected seawater. The Sr method could also be used for injection projects that were begun without the addition of tracers. A natural analogue of a water injection process is found in the Jurassic Lincolnshire Limestone aquifer in England, where rapidly moving fresh meteoric water mixes progressively with an older saline formation water. The⁸⁷Sr/⁸⁶Sr data enable quantitative modelling of this mixing process. The infiltrating fresh water becomes progressively modified by dissolution of detrital carbonate and calcite cement in the limestone, with depth becoming increasingly dominated by Sr derived from the more soluble detrital components. The saline formation water contains water molecules of meteoric origin and an⁸⁷Sr/⁸⁶Sr ratio much higher than Jurassic seawater or marine carbonate; the solute content has been influence by interaction of the water with non-carbonate phases.     

Hydrogeochemistry and strontium isotopes of spring and mineral waters from Monte Vulture volcano,Italy

This paper describes the results of a study that was conducted to determine the relationship between hydrogeochemical composition and ⁸⁷Sr/⁸⁶Sr isotope ratios of the Mt. Vulture spring waters. Forty samples of spring waters were collected from local outcrops of Quaternary volcanites. Physico-chemical parameters were measured in the field and analyses completed for major and minor elements and ⁸⁷Sr/⁸⁶Sr isotopic ratios. A range of water types was distinguished varying from alkaline-earth bicarbonate waters, reflecting less intense water–rock interaction processes to alkali bicarbonate waters, probably representing interaction with volcanic rocks of Mt. Vulture and marine evaporites. The average ⁸⁷Sr/⁸⁶Sr isotope ratios suggest at least 3 different sources. However, some samples have average Sr isotope ratios (0.70704–0.70778) well above those of the volcanites. These ratios imply interaction with other rocks having higher ⁸⁷Sr/⁸⁶Sr ratios, probably Triassic evaporites, which is substantiated by their higher content of Na, SO₄ and Cl. The Sr isotope ratios for some samples (e.g. Toka and Traficante) are intermediate between the value for the Vulture volcanites and that for the local Mesozoic rocks. The salt content of these samples also lies between the value for waters interacting solely with the volcanites and the value measured in the more saline samples. These waters are thus assumed to result from the mixing of waters circulating in volcanic rocks with waters presumably interacting with the sedimentary bedrock (marine evaporites).     

Input of Sr/Sr ratios and Sr geochemical signatures to update knowledge on thermal and mineral waters flow paths in fractured rocks (N-Portugal)

Strontium isotopes and other geochemical signatures are used to determine the relationships between CO₂-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have ⁸⁷Sr/⁸⁶Sr isotopic ratios between 0.716713 and 0.728035. ⁸⁷Sr/⁸⁶Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO₂-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have ⁸⁷Sr/⁸⁶Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO₂-rich thermal and mineral waters and the granitic rocks. The mean ⁸⁷Sr/⁸⁶Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.     

Source and origin of active and fossil thermal spring systems,northern Upper Rhine Graben,Germany

Thermal water samples and related young and fossil mineralization from a geothermal system at the northern margin of the Upper Rhine Graben have been investigated by combining hydrochemistry with stable and Sr isotope geochemistry. Actively discharging thermal springs and mineralization are present in a structural zone that extends over at least 60 km along strike, with two of the main centers of hydrothermal activity being Wiesbaden and Bad Nauheim. This setting provides the rare opportunity to link the chemistry and isotopic signatures of modern thermal waters directly with fossil mineralization dating back to at least 500–800 ka. The fossil thermal spring mineralization can be classified into two major types: barite-(pyrite) fracture filling associated with laterally-extensive silicification; and barite, goethite and silica impregnation mineralization in Tertiary sediments. Additionally, carbonatic sinters occur around active springs. Strontium isotope and trace element data suggest that mixing of a hot (>100 °C), deep-sourced thermal water with cooler groundwater from shallow aquifers is responsible for present-day thermal spring discharge and fossil mineralization. The correlation between both Sr and S isotope ratios and the elevation of the barite mineralization relative to the present-day water table in Wiesbaden is explained by mixing of deep-sourced thermal water having high ⁸⁷Sr/⁸⁶Sr and low δ³⁴S with shallow groundwater of lower ⁸⁷Sr/⁸⁶Sr and higher δ³⁴S. The Sr isotope data demonstrate that the hot thermal waters originate from an aquifer in the Variscan crystalline basement at depths of 3–5 km. The S isotope data show that impregnation-type mineralization is strongly influenced by mixing with SO₄ that has high δ³⁴S values. The fracture style mineralization formed by cooling of the thermal waters, whereas impregnation-type mineralization precipitated by mixing with SO₄-rich groundwater percolating through the sediments.     

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.     

Strontium isotopic composition of oil-field and gas-field waters,Japan

The⁸⁷Sr/⁸⁶Sr ratios of 10 formation waters in petroleum and natural gas reservoirs along the western coast of northeastern Honshu, Japan, ranged from 0.7052 to 0.7084, and are distinctly lower than the ratio in seawater. This forms a marked contrast to oil-field brines from the U.S.A. which have higher⁸⁷Sr/⁸⁶Sr ratios. Taking into consideration that petroleum and natural gas in Japan accumulate in volcanic rocks derived from intense submarine volcanism in the Middle Miocene, the⁸⁷Sr/⁸⁶Sr ratios of the water samples are explained in terms of the isotope exchange between waters containing seawater Sr and reservoir rocks with lower ratios.     

Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A.

Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported ⁸⁷Sr/⁸⁶Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of ⁸⁷Sr/⁸⁶Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of ⁸⁷Sr/⁸⁶Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have ⁸⁷Sr/⁸⁶Sr and δ³⁴S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have ⁸⁷Sr/⁸⁶Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous ⁸⁷Sr/⁸⁶Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic ⁸⁷Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic ⁸⁷Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous ⁸⁷Sr/⁸⁶Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin.     

Comparison of the Sr isotopic signatures in brines of the Canadian and Fennoscandian shields

A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fennoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1–75 g L⁻¹ below 1500 m depth in the Fennoscandian Shield and from 10 up to 300 g L⁻¹ below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water–rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fennoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water–rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of ⁸⁷Sr/⁸⁶Sr ratios for the deep brines in different selected sites. All sites show a specific ⁸⁷Sr/⁸⁶Sr signature and the occurrence of large ⁸⁷Sr/⁸⁶Sr variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the ⁸⁷Sr/⁸⁶Sr ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fennoscandian, and the well-constrained ⁸⁷Sr/⁸⁶Sr signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water–rock interaction.     

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.     

Determination of Sr/Sr mass-dependent isotopic fractionation and radiogenic isotope variation of Sr/Sr in the Neoproterozoic Doushantuo Formation

We measured both mass-dependent isotope fractionation of δ⁸⁸Sr (⁸⁸Sr/⁸⁶Sr) and radiogenic isotopic variation of Sr (⁸⁷Sr/⁸⁶Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The δ⁸⁸Sr and ⁸⁷Sr/⁸⁶Sr compositions showed three remarkable characteristics: (1) high radiogenic ⁸⁷Sr/⁸⁶Sr values and gradual decrease in the ⁸⁷Sr/⁸⁶Sr ratios, (2) anomalously low δ⁸⁸Sr values at the lower part cap carbonate, and (3) a clear correlation between ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO₂ content caused sudden precipitation of cap carbonate from the surface seawater with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal δ⁸⁸Sr ratios, probably caused gradual decrease of the ⁸⁷Sr/⁸⁶Sr ratios of the seawater and deposition of carbonate with normal δ⁸⁸Sr ratios. The combination of ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr isotope systematics gives us new insights on the surface evolution after the Snowball Earth.     

Geochemistry of carbonate cements in the Sag River and Shublik Formations (Triassic/Jurassic), North Slope, Alaska: implications for the geochemical evolution of formation waters

Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have ⁸⁷Sr/⁸⁶Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe-rich (up to 5·0 mol%) and are characterized by increasing ⁸⁷Sr/⁸⁶Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age). These geochemical variations appear principally to result from changes in pore-water chemistry during diagenesis. The increase in ⁸⁷Sr/⁸⁶ Sr and Sr content of the cements is most likely due to interaction between pore waters and ⁸⁷ Sr-rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock ⁸⁷Sr/⁸⁶ Sr signatures for the mudrocks are > 0·716). Pore-water Fe²⁺ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak. The low δ¹⁸ O value of some calcite cements (-11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid-Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid-Neocomian rifting.     

Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California

High-K mafic alkalic lavas (5.4 to 3.2 wt% K₂O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; ⁸⁷Sr/⁸⁶Sr ratios decrease and ɛ_Nd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7121 to 0.7105 and ɛ_Nd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, ²⁰⁶Pb/²⁰⁴Pb ∼17.2, ²⁰⁷Pb/²⁰⁴Pb ∼15.5, and ²⁰⁸Pb/²⁰⁴Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛ_Nd values and unradiogenic ²⁰⁶Pb/²⁰⁴Pb ratios have relatively low ⁸⁷Sr/⁸⁶Sr ratios (the EM I array), whereas basalts with low ɛ_Nd values and high ⁸⁷Sr/⁸⁶Sr ratios have radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios and low ɛ_Nd values and ²⁰⁶Pb/²⁰⁴Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998     

Mineral dissolution in the Cape Cod aquifer, Massachusetts, USA: I . Reaction stoichiometry and impact of accessory feldspar and glauconite on strontium isotopes, solute concentrations, and REY distribution

To compare relative reaction rates of mineral dissolution in a mineralogically simple groundwater aquifer, we studied the controls on solute concentrations, Sr isotopes, and rare earth element and yttrium (REY) systematics in the Cape Cod aquifer. This aquifer comprises mostly carbonate-free Pleistocene sediments that are about 90% quartz with minor K-feldspar, plagioclase, glauconite, and Fe-oxides. Silica concentrations and pH in the groundwater increase systematically with increasing depth, while Sr isotopic ratios decrease. No clear relationship between ⁸⁷Sr/⁸⁶Sr and Sr concentration is observed. At all depths, the ⁸⁷Sr/⁸⁶Sr ratio of the groundwater is considerably lower than the Sr isotopic ratio of the bulk sediment or its K-feldspar component, but similar to that of a plagioclase-rich accessory separate obtained from the sediment. The Si-⁸⁷Sr/⁸⁶Sr-depth relationships are consistent with dissolution of accessory plagioclase. In addition, solutes such as Sr, Ca, and particularly K show concentration spikes superimposed on their respective general trends. The K-Sr-⁸⁷Sr/⁸⁶Sr systematics suggests that accessory glauconite is another major solute source to Cape Cod groundwater. Although the authigenic glauconite in the Cape Cod sediment is rich in Rb, it is low in in-grown radiogenic ⁸⁷Sr because of its young Pleistocene age. The low ⁸⁷Sr/⁸⁶Sr ratios are consistent with equilibration of glauconite with seawater. The impact of glauconite is inferred to vary due to its variable abundance in the sediments. In the Cape Cod groundwater, the variation of REY concentrations with sampling depth resembles that of K and Rb, but differs from that of Ca and Sr. Shale-normalized REY patterns are light REY depleted, show negative Ce anomalies and super-chondritic Y/Ho ratios, but no Eu anomalies. REY input from feldspar, therefore, is insignificant compared to input from a K-Rb-bearing phase, inferred to be glauconite. These results emphasize that interpretation of groundwater chemistry, even in relatively simple aquifers, may be complicated by solute contributions from “exotic” accessory minerals such as glauconite. To detect such peculiarities, groundwater studies should combine the study of elemental concentration and isotopic composition of several solutes that show different geochemical behavior.     

Strontium isotope geochemistry of groundwaters and streams affected by agriculture,Locust Grove,MD

The effects of agriculture on the isotope geochemistry of Sr were investigated in two small watersheds in the Atlantic coastal plain of Maryland. Stratified shallow oxic groundwaters in both watersheds contained a retrievable record of increasing recharge rates of chemicals including NO₃⁻, Cl, Mg, Ca and Sr that were correlated with increasing fertilizer use between about 1940 and 1990. The component of Sr associated with recent agricultural recharge was relatively radiogenic (⁸⁷Sr/⁸⁶Sr=0.715) and it was overwhelming with respect to Sr acquired naturally by water–rock interactions in the oxidized, non-calcareous portion of the saturated zone. Agricultural groundwaters that penetrated relatively unoxidized calcareous glauconitic sediments at depth acquired an additional component of Sr from dissolution of early Tertiary marine CaCO₃ (⁸⁷Sr/⁸⁶Sr=0.708) while undergoing O₂ reduction and denitrification. Ground-water discharge contained mixtures of waters of various ages and redox states. Two streams draining the area are considered to have higher ⁸⁷Sr/⁸⁶Sr ratios and NO₃⁻ concentrations than they would in the absence of agriculture; however, the streams have consistently different ⁸⁷Sr/⁸⁶Sr ratios and NO₃⁻ concentrations because the average depth to calcareous reducing (denitrifying) sediments in the local groundwater flow system was different in the two watersheds. The results of this study indicate that agriculture can alter significantly the isotope geochemistry of Sr in aquifers and streams and that the effects could vary depending on the types, sources and amounts of fertilizers added, the history of fertilizer use and groundwater residence times.