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1.
Processes controlling hydrogeochemistry in the Yuncheng Basin, China, were characterised using major-ion chemistry, 87Sr/86Sr ratios and ??13C values. Evapotranspiration during recharge increased solute concentrations by factors of ??5?C50 in deep palaeowaters, while higher degrees of evapotranspiration have occurred in shallow, modern groundwater. Aquifer sediments (loess) contain approximately 15 weight% calcite; trends in groundwater HCO3 concentrations and ??13C values (ranging from ?16.4 to ?8.2??) indicate that carbonate weathering is a significant source of DIC. Groundwater 87Sr/86Sr ratios (0.7110?C0.7162, median of 0.7116) are similar to those in both loess carbonate (0.7109?C0.7116) and local rainfall (0.7112), and are significantly lower than Sr in aquifer silicates (0.7184?C0.7251). Despite evidence for substantial carbonate dissolution, groundwater is generally Ca-poor (<?10% of total cations) and Na-rich, due to cation exchange. Saturation with respect to carbonate minerals occurs during or soon after recharge (all calcite and dolomite saturation indices are positive). Subsequent carbonate dissolution in the deep aquifer must occur as a second-stage process, in response to Ca loss (by ion exchange) and/or via incongruent dissolution of dolomite and impure calcite. The latter is consistent with positive correlations between ??13C values and Mg/Ca and Sr/Ca ratios (r 2?=?0.32 and 0.34).  相似文献   

2.
Carbonates in a 30 cm wide zoned kimberlite dyke from the De Beers Mine, Kimberley, S. Africa were studied by cathodoluminescence and electron microprobe techniques and their 87Sr/86Sr ratios were measured using an AEI-IM20 ion microprobe. Primary carbonates (including calcite dendrites, rhombohedral calcites in segregation vesicles and mosaic dolomite) have high Sr (0.69–1.35 wt.% SrO) and Ba (0.24–0.44% BaO) and 87Sr/86Sr ratios in the range 0.7046 to 0.7056. Secondary sparry calcite in amygdales and veins is characterised by low Ba (<0.05% BaO) and 87Sr/86Sr near 0.72. Rhombohedral calcite 0.5 cm from a contact with 2,900 my. old biotite-gneiss has minor element chemistry like that of primary carbonate, but an elevated 87Sr/86Sr ratio of 0.7103, possibly indicating crustal contamination in a boundary layer of the kimberlite magma. Amygdale-like segregations of carbonate and/or serpentine originated as gas-cavities and were not formed by liquid immiscibility. They are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid. However, dendritic calcite and primary dolomite and calcite with high Sr, Ba and low 87Sr/86Sr demonstrate shared chemical characteristics between these carbonates and carbonatite. The primary kimberlite magma had initial 87Sr/86Sr close to 0.7046.  相似文献   

3.
The Cretaceous-Paleocene (K-T) transition has been recorded in sedimentary carbonate rocks in northwestern Argentina and southern Chile. In the Yacoraite Basin, Argentina, this transition has been preserved in a 2 m thick marly layer, at the base of the Tunal Formation, which overlies lacustrine/marine carbonates of the Yacoraite Formation (Cabra Corral dam). The K-T transition is also preserved at Maimara, where Tertiary sandstones overlie a 50 m thick limestone bed of the Yacoraite Formation. In the Magellan Basin, Chile, glauconitic sandstones with calcitic cement and limestone concretions of the Maastrichtian Punta Rocallosa Formation are overlain by sandstones, claystones, and limestones of the Chorillo Chico Formation. The K-T transition is preserved in the lower portion of the Chorillo Chico Formation.

Carbonates of the Yacoraite Formation display bulk-rock δ13C values from +1 to +2‰ PDB, with a negative incursion (?4‰ PDB) at the K-T transition. δ13C values in the Tunal Formation marls vary from ?3 to ?1‰ PDB. At Rocallosa Point, δ13C values in limestone strata, calcite cement, and limestone concretions vary from ?4 to ?33 ‰ PDB, and the lowest value in the Chorillo Chico Formation apparently marks the K-T transition. The δ18O fluctuations in the Yacoraite and Magellan carbonate rocks suggest a temperature drop at the K-T transition, followed by a temperature rise.

High 87Sr/86Sr ratios (0.7140-0.7156) characterize the studied profiles of the Yacoraite Formation, documenting an important 87Sr-enriched source of Sr to the water from which these carbonates precipitated. At the Magellan basin, 87Sr/86Sr ratios are closer to the expected values for the global Late Cretaceous-Paleocene ocean.  相似文献   

4.
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.  相似文献   

5.
Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO2 consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO2 consumption to silicate weathering than methods utilizing Ca/Sr and 87Sr/86Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and 87Sr/86Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (PCO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca2+-Mg2+-HCO3 waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca2+ and Sr2+, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and PCO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO2. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg2+, Ca2+, and HCO3 mass balances support interpretations of preferential Ca2+ removal by calcite precipitation. On the basis of patterns of saturation state and PCO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca2+ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca2+ during riverine transport brings the Ca/Sr and 87Sr/86Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr2+ budget and suggest that the conventional application of two-component Ca/Sr and 87Sr/86Sr mixing equations has overestimated silicate-derived Sr2+ and HCO3 fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.  相似文献   

6.
Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater 87Sr/86Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated 87Sr/86Sr ratios and there is disagreement as to how much of the 87Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when 87Sr/86Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the 87Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated.  相似文献   

7.
Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The δ18O values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56–68°C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of δ13C and 87Sr/86Sr values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative δ13C, high 87Sr/86Sr, and low Sr content in the uppermost 50–150 m of the Jurassic profile (upper Oxfordian). The 87Sr/86Sr of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.  相似文献   

8.
We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr ≈ 0.20 μmol/nmol, 87Sr/86Sr ≈ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr ≈ 0.98 to 5.3 μmol/nmol, 87Sr/86Sr ≈ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr ≈ 1.0 to 2.8 μmol/nmol, 87Sr/86Sr ≈ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ∼0.5 to ∼55 kyr revealed that carbonate dissolution provides more than ∼90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ∼1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87Sr/86Sr ratios released by carbonate weathering decrease from ∼3.6 to ∼0.20 μmol/nmol and ∼0.84 to ∼0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.  相似文献   

9.
Calcite is frequently cited as a source of excess Ca, Sr and alkalinity in solutes discharging from silicate terrains yet, no previous effort has been made to assess systematically the overall abundance, composition and petrogenesis of accessory calcite in granitoid rocks. This study addresses this issue by analyzing a worldwide distribution of more than 100 granitoid rocks. Calcite is found to be universally present in a concentration range between 0.028 to 18.8 g kg−1 (mean = 2.52 g kg−1). Calcite occurrences include small to large isolated anhedral grains, fracture and cavity infillings, and sericitized cores of plagioclase. No correlation exists between the amount of calcite present and major rock oxide compositions, including CaO. Ion microprobe analyses of in situ calcite grains indicate relatively low Sr (120 to 660 ppm), negligible Rb and 87Sr/86Sr ratios equal to or higher than those of coexisting plagioclase. Solutes, including Ca and alkalinity produced by batch leaching of the granitoid rocks (5% CO2 in DI water for 75 d at 25°C), are dominated by the dissolution of calcite relative to silicate minerals. The correlation of these parameters with higher calcite concentrations decreases as leachates approach thermodynamic saturation. In longer term column experiments (1.5 yr), reactive calcite becomes exhausted, solute Ca and Sr become controlled by feldspar dissolution and 87Sr/86Sr by biotite oxidation. Some accessory calcite in granitoid rocks is related to intrusion into carbonate wall rock or produced by later hydrothermal alteration. However, the ubiquitous occurrence of calcite also suggests formation during late stage (subsolidus) magmatic processes. This conclusion is supported by petrographic observations and 87Sr/86Sr analyses. A review of thermodynamic data indicates that at moderate pressures and reasonable CO2 fugacities, calcite is a stable phase at temperatures of 400 to 700°C.  相似文献   

10.
The Early Devonian Gumeshevo deposit is one of the largest ore objects pertaining to the dioritic model of the porphyry copper system paragenetically related to the low-K quartz diorite island-arc complex. The (87Sr/86Sr)t and (ɛNd)t of quartz diorite calculated for t = 390 Ma are 0.7038–0.7045 and 5.0–5.1, respectively, testifying to a large contribution of the mantle component to the composition of this rock. The contents of typomorphic trace elements (ppm) are as follows: 30–48 REE sum, 5–10 Rb, 9–15 Y, and 1–2 Nb. The REE pattern is devoid of Eu anomaly. Endoskarn of low-temperature and highly oxidized amphibole-epidote-garnet facies is surrounded by the outer epidosite zone. Widespread retrograde metasomatism is expressed in replacement of exoskarn and marble with silicate (chlorite, talc, tremolite)-magnetite-quartz-carbonate mineral assemblage. The 87Sr/86Sr ratios of epidote in endoskarn and carbonate in retrograde metasomatic rocks (0.7054–0.7058 and 0.7053–0.7065, respectively) are intermediate between the Sr isotope ratios of quartz dioritic rocks and marble (87Sr/86Sr = 0.70784 ± 2). Isotopic parameters of the fluid equilibrated with silicates of skarn and retrograde metasomatic rocks replacing exoskarn at 400°C are δ18O = +7.4 to +8.5‰ and δD = −49 to −61‰ (relative to SMOW). The δ13C and δ18O of carbonates in retrograde metasomatic rocks after marble are −5.3 to +0.6 (relative to PDB) and +13.0 to +20.2% (relative to SMOW), respectively. Sulfidation completes metasomatism, nonuniformly superimposed on all metasomatic rocks and marbles with formation of orebodies, including massive sulfide ore. The δ34S of sulfides is 0 to 2‰ (relative to CDT);87Sr/86Sr of calcite from the late calcite-pyrite assemblage replacing marble is 0.704134 ± 6. The δ13C and 87Sr/86Sr of postore veined carbonates correlate positively (r = 0.98; n = 6). The regression line extends to the marble field. Its opposite end corresponds to magmatic (in terms of Bowman, 1998b) calcite with minimal δ13C, δ18O, and 87Sr/86Sr values (−6.9 ‰, +6.7‰, and 0.70378 ± 4, respectively). The aforementioned isotopic data show that magmatic fluid was supplied during all stages of mineral formation and interacted with marble and other rocks, changing its Sr, C, and O isotopic compositions. This confirms the earlier established redistribution of major elements and REE in the process of metasomatism. A contribution of meteoric and metamorphic water is often established in quartz from postore veins.  相似文献   

11.
The fluorite deposits of the Valle de Tena, Central Pyrenees, include stratabound (Portalet) and vein (Lanuza and Tebarray) deposits the formation of which are linked to a Namurian-Westfalian emersion episode and to post-Hercynian hydrothermal systems similar to those occurring elsewhere in Hercynian Europe. In this study, strontium isotopes were used to determine the source(s) of strontium, and by inference calcium, of the fluorite mineralizations, as well as the nature of the ore-forming fluids. Fluorite and calcite from each deposit have similar 87Sr/86Sr ratios (Portalet 0.7085–0.7108; Lanuza 0.7086–0.7104 and Tebarray 0.7086–0.7101). In all deposits, the Sr isotope composition of most of the Ca-minerals is more radiogenic than that of the host limestones. This indicates that the Ca-minerals contain a mixture of Sr derived locally from the host limestones and 87 Sr-enriched Sr leached from silicate minerals in the siliciclastic portion of the basement sequence and in granites from the study area. Volcanic rocks are ruled out as a significant Sr source for the fluorite deposits. The observed trend in 87Sr/86Sr versus 1/Sr support a fluid-rock interaction model which satisfactorily reproduces the marked 87Sr-enrichment in the fluorites and calcites from the deposits. Received: 19 February 1997 / Accepted: 22 July 1997  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
《Applied Geochemistry》1988,3(6):591-600
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.  相似文献   

15.
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.  相似文献   

16.
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.  相似文献   

17.
The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77% (Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40% and in most cases it exceeds the carbonate Sr. The observations that (i) the87Sr/86Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between87Sr/86Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low87Sr/86Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and87Sr/86Sr in the headwaters on a basin-wide scale, however, the high87Sr/86Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of87Sr/86Sr and Ca/Sr data of the source waters show that they diverge from a low87Sr/86Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO2 consumption rates and their global significance.  相似文献   

18.
The mineral composition and U-Pb and Rb-Sr systematics of phosphorites from the Satka Formation of Lower Riphean carbonates, the Burzyan Group of Southern Urals, are studied. Phosphorites occurring as small lenses between stromatolite layers are composed largely of fluorapatite with admixture of detrital quartz, feldspars, illite, and chlorite. Phosphorite samples have been subjected to stepwise dissolution in 1 N (fraction L-1) and 2 N (fraction L-2) HCl. As is established, the maximum apatite content is characteristic of fraction L-1, while fraction L-2 is enriched in products of dolomite and sulfide dissolution and in elements leached from siliciclastic components. The Sr content in the Satka apatites (280–560 ppm) is substantially lower as compared with that in unaltered marine apatite. The 87Sr/86Sr “initial ratio in the phosphorites studied (0.71705–0.72484) and host dolomites from the lower part of the Satka Formation is significantly higher than in the Early Riphean seawater that indicates a reset of the Rb-Sr original systems in sediments. The Pb-Pb age of 1340 ± 30 Ma (MSWD = 6.4) estimated based on 7 data points characterizing fractions L-1 and L-2 is younger than the formation time of overlying Burzyan sediments, being consistent, within the error range, with date of the Mashak rifting event recorded at the Early-Middle Riphean boundary. The comparative U-Pb characteristics of two soluble fractions (L-1 and L-2) and silicate residue of phosphorites show that epigenetic redistribution of Pb and U was characteristic of the phosphorite horizon only. The initial Pb isotope composition and μ (238U/204Pb) estimated according to model by Stacey and Kramers for the early diagenetic fluids in carbonate and phosphate sediments of the Satka Formation suggest that they were in isotopic equilibrium with erosion products of the Taratash crystalline complex.  相似文献   

19.
锶同位素已经成为全球海平面变化、造山运动、古气候和古环境等全球地质事件研究与对比的有效工具之一。本文以四川盆地东部地区早中三叠世蒸发岩的野外剖面和钻孔岩心为主要研究对象,测试了碳酸盐岩、硫酸盐岩和石盐岩的锶同位素组成,并建立了相应的锶同位素演化曲线。研究结果显示,碳酸盐岩的87Sr/86Sr值平均为0.707 895,硬石膏岩的87Sr/86Sr值平均为0.708 174,石盐岩的87Sr/86Sr值平均为0.708 177,同时碳酸盐岩的87Sr/86Sr值从早三叠世的0.707 413快速增加到中三叠世早期的0.708 515,而后呈现下降趋势。从总体上看,这些87Sr/86Sr值与全球早中三叠世同期的87Sr/86Sr值数据接近,说明四川盆地东部地区早中三叠世钾盐的物质来源大部分为海水,并沉积于海相沉积环境,同时由于火山...  相似文献   

20.
Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca2+ (50%) > Si (22%) > Na+ (17%) > Mg2+ (6%) > K+ (5%). For major anions, the relative molar abundance is HCO3 (89%) > SO42− (7%) > Cl (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca2+ and HCO3 mass-balance. Stream water Ca/Sr and 87Sr/86Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; 87Sr/86Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; 87Sr/86Sr = 0.7076 to 0.7097).Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca2+ and ∼30% more Sr2+ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr2+ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.  相似文献   

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