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1.
《Geochimica et cosmochimica acta》1999,63(13-14):1905-1925
Himalayan rivers have very unusual Sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic. We present Sr, O, and C isotopic data from river and rain water, bedload, and bedrock samples for the western and central Nepal Himalaya and Bangladesh, including the monsoon season. Central Himalayan rivers receive Sr from several sources: carbonate and clastic Tethyan sediments, High Himalayan Crystalline (HHC) gneisses and granitoids with minor marbles, carbonates and metasediments of the Lesser Himalaya (LH), and Miocene-Recent foreland basin sediment from the Siwaliks group and the modern flood plain. In the Tethyan Himalaya rivers have dissolved [Sr] ≈ 6 μmol/l and 87Sr/86Sr ≈ 0.717, with a large contribution from moderately radiogenic carbonate. Rivers draining HHC gneisses are very dilute with [Sr] ≈ 0.2 μmol/l and 87Sr/86Sr ≈ 0.74. Lesser Himalayan streams also have low [Sr] ≈ 0.4 μmol/l and are highly radiogenic (87Sr/86Sr ≥ 0.78). Highly radiogenic carbonates of the LH do not contribute significantly to the Sr budget because they are sparse and have very low [Sr]. In large rivers exiting the Himalaya, Sr systematics can be modeled as a mixture between Tethyan rivers, where slightly radiogenic carbonates (mean 87Sr/86Sr ≈ 0.715) are the main source of Sr, and Lesser Himalaya waters, where extremely radiogenic silicates (>0.8) are the main source of Sr. HHC waters are less important because of their low [Sr]. Rivers draining the Siwaliks foreland basin sediments have [Sr] ≈ 4 μmol/l and 87Sr/86Sr ≈ 0.725. Weathering of silicates in the Siwaliks and the flood plain results in a probably significant radiogenic (0.72–0.74) input to the Ganges and Brahmaputra (G-B), but quantification of this flux is limited by uncertainties in the hydrologic budget. The G-B in Bangladesh show strong seasonal variability with low [Sr] and high 87Sr/86Sr during the monsoon. Sr in the Brahmaputra ranges from 0.9 μmol/l and 0.722 in March to 0.3 μmol/l and 0.741 in August. We estimate the seasonally weighted flux from the G-B to be 6.5 × 108 mol/yr with 87Sr/86Sr = 0.7295.  相似文献   

2.
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3.
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.  相似文献   

4.
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   

5.
The age of the Katera Group, which occupies a large area in the western North Muya Range and occurs 100–150 km east of the Uakit Group, is a debatable issue. Based on geological correlations with reference sections of the Baikal Group and Patom Complex, the Katera and Uakit groups were previously considered nearly coeval units and assigned to Late Precambrian (Khomentovskii and Postnikov, 2002; Salop, 1964). This was supported partly by the Sm–Nd model datings (Rytsk et al., 2007, 2009, 2011). Finds of the Paleozoic flora substantiated the revision of age of the Uakit Group and its assignment to the Late Devonian–Early Carboniferous (Gordienko et al., 2010; Minina, 2003, 2012, 2014). We have established that Sr and C isotopic compositions in carbonates of these groups differ drastically, as suggested by their different ages. Sediments of the Nyandoni Formation (Katera Group), which contains carbonates characterized by minimum values of 87Sr/86Sr = 0.7056 and maximum values of δ13C = 4.9‰, were accumulated in the first half of Late Riphean (800–850 Ma ago), whereas the overlying Barguzin Formation (87Sr/86Srmin = 0.70715, δ13Cmax= 10.5‰) was deposited at the end of Late Riphean (700–750 Ma). Judging from the isotope data, the Nerunda Formation (Uakit Group), which contains carbonates with characteristics matching the most rigorous criteria of fitness for the chemostratigraphic correlation (Sr content up to 4390 μg/g, Mn/Sr < 0.1, δ18O = 23.0 ± 1.8‰), was deposited at the end of Vendian ~550–540 Ma ago). The sequence includes thick typical carbonate horizons with very contrast carbon isotopic compositions: the lower unit has anomalous high δ13C values (5.8 ± 1.0‰); the upper unit, by anomalous low δ13C values (–5.2 ± 0.5‰]). Their Sr isotopic composition is relatively homogeneous (87Sr/86Sr = 0.7084 ± 0.0001) that is typical of the Late Vendian ocean. The S isotopic composition of pyrites from the Nyandoni Formation (Katera Group) (δ34S = 14.1 ± 6.8‰) and pyrites from the Mukhtunny Formation (Uakit Group) (δ34S = 0.7 ± 1.4‰) does not contradict the C and Sr isotopic stratigraphic data.  相似文献   

6.
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   

7.
The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite concretions contain some dolomite admixture (Mg/Ca = 0.011?0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3–2.1). The Sr content is as much as 385–505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA fraction). In individual calcite samples, discrepancy between the measured 87Sr/86Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008–0.0033. The initial 87Sr/86Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with “mild” reductive conditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite-bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic 87Sr from the associated siliciclastic minerals, resulting in growth of the initial 87Sr/86Sr in concretions up to 0.7108–0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. Therefore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial 87Sr/86Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.  相似文献   

8.
The Xujiashan antimony deposit is hosted by marine carbonates of the Upper Sinian Doushantuo and Dengying Formations in Hubei Province, South China. Our Sr isotopic data from pre‐ and syn‐mineralization calcites that host the mineralization show that the pre‐mineralization calcite displays a narrow range of 87Sr/86Sr ratios (0.7096 to 0.7097), similar to the ratios of the Sinian seawater, and high Sr concentrations (2645 to 8174 ppm). In contrast, the syn‐mineralization calcite exhibits low Sr concentrations (785 to 2563 ppm) and high 87Sr/86Sr ratios (0.7109 to 0.7154), which is interpreted as the result of addition of radiogenic strontium during the antimony mineralization. The study of Sr isotopes suggests that their Sr component to the pre‐mineralization calcite derived directly from the host rocks (i.e. the Sinian marine carbonates), while radiogenic 87Sr for the syn‐mineralization calcite derived from the underlying Mesoproterozoic Lengjiaxi Group basement through hydrothermal fluid circulation along the major fault that hosts the mineralization. The Pb isotopic ratios of stibnite are subdivided into two groups (Group A and Group B), Group A is characterized by higher radiogenic lead, with 206Pb/204Pb = 18.874 to 19.288, 207Pb/204Pb = 15.708 to 15.805, and 208Pb/204Pb = 38.642 to 39.001. Group B shows lower lead isotope ratios (206Pb/204Pb = 17.882 to 18.171, 207Pb/204Pb = 15.555 to 15.686, and 208Pb/204Pb = 37.950 to 38.340). The single‐stage model ages of Group A are mainly negative or slightly positive values (‐258 to 3 Ma), while those of Group B range from 636 to 392 Ma, with an average of 495 ± 65 Ma. In addition, there are positive linear correlations among Pb isotopic ratios. These results suggest that the lead of Group A stibnite was mainly derived from the Sinian marine carbonates, and that of Group B stibnite from the underlying Lengjiaxi Group basement. This conclusion is consistent with the results of the Sr isotopes. These results indicate that the Xujiashan deposit is not syngenetic sedimentary and in situ reworked origin as previously considered. The metal (mainly Sb) of this deposit was not only derived from the Sinian host rocks, but also partly derived from the underlying Mesoproterozoic Lengjiaxi Group basement.  相似文献   

9.
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   

10.
《International Geology Review》2012,54(11):1350-1362
ABSTRACT

Recent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ26Mg values and higher 87Sr/86Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined 87Sr/86Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ26Mg values of ?0.60 to ?0.30‰ and relatively low initial 87Sr/86Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.  相似文献   

11.
238 marine carbonate samples were collected from seven sedimentary sections ofthe entire late Palaeozoic (Permian, Carboniferous and Devonian) in the Upper Yangtze Plat-form, southwest China. Based on the absence of cathodoluminescence and very low Mn (gener-ally<50 ppm) contents of the samples, it is thought that they contain information on the orig-inal sea water geochemistry. The results of isotopic analyses of these samples are presented interms of δ~(13)C and ~(87)Sr/~(86)Sr ratios versus geological time. The strontium data, consistent withother similar data based on samples from North America, Europe, Africa and other areas inAsia, support the notion of a global consistency in strontium isotope composition of marinecarbonates. The strontium data exhibit three intervals of relatively low ~(87)Sr/~(86)Sr ratios in thelate Middle Devonian to early Late Devonian, Early Carboniferous and Early Permian, corre-sponding to global eustatic high sea level stands. The lowest ~(87)Sr/~(86)Sr ratio recorded in theLate Permian was probably caused by substantial basalt eruptions in the Upper Yangtze Plat-form at the time. Three corresponding periods of relatively high δ~(13)C values at roughly the samethe intervals were caused by a relatively high rate of accumulation of organic carbon duringsea level rises at these times. The deposition of coal was probably responsible for the increaseof sea water δ~(13)C at other times. The δ~(13)C values drop dramatically near theDevonian/Carboniferous, Carboniferous/Permian and Permian/Triassic boundaries, con-sistent with other similar data, which further support the notion that geological time boundariesare associated with mass extinction and subsequent rejuvenation.  相似文献   

12.
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.  相似文献   

13.
In the last ten years, with important discoveries from oil and gas exploration in the Dabashan foreland depression belt in the borderland between Shanxi and Sichuan provinces, the relationship between the formation and evolution of, and hydrocarbon accumulation in, this foreland thrust belt from the viewpoint of basin and oil and gas exploration has been studied. At the same time, there has been little research on the origin of fluids within the belt. Based on geochemical system analysis including Z values denoting salinity and research on δ13C, δ18O and 87Sr/86Sr isotopes in the host rocks and veins, the origin of paleofluids in the foreland thrust belt is considered. There are four principal kinds of paleofluid, including deep mantle-derived, sedimentary, mixed and meteoric. For the deep mantle-derived fluid, the δ13C is generally less than ?5.0‰PDB, δ18O less than -10.0‰PDB, Z value less than 110 and 87Sr/86Sr less than 0.70600; the sedimentary fluid is mainly marine carbonate-derived, with the δ13C generally more than ?2.0‰PDB, δ18O less than ?10.0‰PDB, Z value more than 120 and 87Sr/86Sr ranging from 0.70800 to 0.71000; the mixed fluid consists mainly of marine carbonate fluid (including possibly a little mantle-derived fluid or meteoric water), with the δ13C generally ranging from ?2.0‰ to ?8.0‰PDB, δ18O from ?10.0‰ to ?18.0‰ PDB, Z value from 105 to 120 and 87Sr/86Sr from 0.70800 to 0.71000; the atmospheric fluid consists mainly of meteoric water, with the δ13C generally ranging from 0.0‰ to ?10.0‰PDB, δ18O less than ?8.0‰PDB, Z value less than 110 and 87Sr/86Sr more than 0.71000. The Chengkou fault belt encompasses the most complex origins, including all four types of paleofluid; the Zhenba and Pingba fault belts and stable areas contain a simple paleofluid mainly of sedimentary type; the Jimingsi fault belt contains mainly sedimentary and mixed fluids, both consisting of sedimentary fluid and meteoric water. Jurassic rocks of the foreland depression belt contain mainly meteoric fluid.  相似文献   

14.
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   

15.
《Precambrian Research》2002,113(1-2):43-63
Carbon, oxygen and strontium isotope compositions of carbonate rocks of the Proterozoic Vindhyan Supergroup, central India suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter Proterozoic. The carbonate rocks of the Lower Vindhyan Supergroup from eastern Son Valley and central Vindhyan sections show δ13C values of ∼0‰ (V-PDB) and those from Rajasthan section are enriched up to +2.8‰. In contrast, the carbonate rocks of the Upper Vindhyan succession record both positive and negative shifts in δ13C compositions. In the central Vindhyan section, the carbonates exhibit positive δ13C values up to +5.7‰ and those from Rajasthan show negative values down to –5.2‰. The δ18O values of most of the carbonate rocks from the Vindhyan Supergroup show a narrow range between –10 and –5‰ (V-PDB) and are similar to the ‘best preserved’ 18O compositions of the Proterozoic carbonate rocks. In the central Vindhyan and eastern Son Valley sections, carbonates from the Lower Vindhyan exhibit best-preserved 87Sr/86Sr compositions of 0.7059±6, which are lower compared to those from Rajasthan (0.7068±4). The carbonates with positive δ13C values from Upper Vindhyan are characterized by lower 87Sr/86Sr values (0.7068±2) than those with negative δ13C values (0.7082±6). A comparison of C and Sr isotope data of carbonate rocks of the Vindhyan Supergroup with isotope evolution curves of the latter Proterozoic along with available geochronological data suggest that the Lower Vindhyan sediments were deposited during the Mesoproterozoic Eon and those from the Upper Vindhyan represent a Neoproterozoic interval of deposition.  相似文献   

16.
《Applied Geochemistry》2003,18(1):117-125
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).  相似文献   

17.
A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes.The Sr isotope compositions of vadose cave dripwaters (mean 87Sr/86Sr = 0.7087) and phreatic groundwaters (mean 87Sr/86Sr = 0.7079) generally fall between values for host carbonates (mean 87Sr/86Sr = 0.7076) and exchangeable Sr in overlying soils (mean 87Sr/86Sr = 0.7088). Dripwaters have lower Mg/Ca and Sr/Ca ratios, and higher 87Sr/86Sr values than phreatic groundwaters. Dripwater 87Sr/86Sr values also inversely correlate with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times, and water-rock interaction with overlying soils and host aquifer carbonate rocks. Consistent differences in dripwater geochemistry (i.e., 87Sr/86Sr, Mg/Ca, and Sr/Ca) between individual caves are similar to compositional differences in soils above the caves. While these differences appear to exert significant control on local fluid evolution, geochemical and isotopic variations suggest that the controlling processes are regionally extensive. Temporal variations in 87Sr/86Sr values and Mg/Ca ratios of dripwaters from some sites over the 4-yr interval correspond with changes in both aquifer and climatic parameters. These results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records, as well as the understanding of controls on water chemistry for both present-day and ancient carbonate aquifers.  相似文献   

18.

Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable 87Sr/86Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted 87Sr/86Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with 87Sr/86Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.

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19.
Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ18Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ13C values together with 87Sr/86Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ18Ο and δ13C values and 87Sr/86Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ18Ο, δ13C and 87Sr/86Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.  相似文献   

20.
Strontium isotope compositions of ancient sulphate deposits not only provide chemostratigraphic information but also offer insight into the system in which the evaporites precipitated. Primary gypsum from two Middle Miocene (Badenian) sections in southern Poland shows steadily higher 87Sr/86Sr ratios than those expected from a marine‐derived formation. The ratios are interpreted as the result of increasing inflow into the basin at the time of gypsum precipitation. Palaeogeographic reconstructions suggest that riverine runoff sources were situated in the West and East European platforms (to the north and east, respectively) and the Carpathians (to the south), which are mostly composed of Mesozoic sedimentary rocks; their dissolution cannot be responsible for the higher 87Sr/86Sr ratios recorded. We conclude that Archaean and Palaeoproterozoic igneous and supracrustal rocks of the Ukrainian Shield were the source of the higher 87Sr/86Sr ratios recorded in the Badenian primary gypsum. A distinctive decreasing trend of 87Sr/86Sr ratios from western Ukraine to southern Poland is explained by a consistent direction of brine inflow during gypsum crystallization (typical cyclonic circulation controlled by the Coriolis effect).  相似文献   

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