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1.
We present strontium (Sr) isotope ratios that, unlike traditional 87Sr/86Sr data, are not normalized to a fixed 88Sr/86Sr ratio of 8.375209 (defined as δ88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87Sr-84Sr double spike. This technique yields two independent ratios for 87Sr/86Sr and 88Sr/86Sr that are reported as (87Sr/86Sr∗) and (δ88/86Sr), respectively. The difference between the traditional radiogenic (87Sr/86Sr normalized to 88Sr/86Sr = 8.375209) and the new 87Sr/86Sr∗ values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((87Sr/86Sr∗, δ88/86Sr)Seawater) and modern marine biogenic carbonates ((87Sr/86Sr∗, δ88/86Sr)Carbonates) with the corresponding values of river waters ((87Sr/86Sr∗, δ88/86Sr)River) and hydrothermal solutions ((87Sr/86Sr∗, δ88/86Sr)HydEnd) in a triple isotope plot. The measured (87Sr/86Sr∗, δ88/86Sr)River values of selected rivers that together account for ∼18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)‰). The average (87Sr/86Sr∗, δ88/86Sr)HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the (87Sr/86Sr∗, δ88/86Sr)Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)‰). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)‰). The difference between the estimated (87Sr/86Sr∗, δ88/86Sr)input and (87Sr/86Sr∗, δ88/86Sr)output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10-30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial exposure of abundant fine-grained material.  相似文献   

2.
Chemical and isotopic data for 23 geothermal water samples collected in New Zealand within the Taupo Volcanic Zone (TVZ) are reported. Major and trace elements including Li, B and Sr and their isotopic compositions (δ7Li, δ11B, 87Sr/86Sr) were determined in high temperature geothermal waters collected from deep boreholes in different geothermal fields (Ohaaki, Wairakei, Mokai, Kawerau and Rotokawa geothermal systems). Lithium concentrations are high (from 4.5 to 19.9 mg/L) and Li isotopic compositions (δ7Li) are homogeneous, ranging between −0.5‰ and +1.4‰. In particular, it is noteworthy that, except for the samples from the Kawerau geothermal field having slightly higher δ7Li values (+1.4%), the other geothermal waters have a near constant δ7Li signature around a mean value of 0‰ ± 0.6 (2σ, n = 21). Boron concentrations are also high and relatively homogeneous for the geothermal samples, falling between 17.5 and 82.1 mg/L. Boron isotopic compositions (δ11B) are all negative, and display a range between −6.7‰ and −1.9‰. These B isotope compositions are in agreement with those of the Ngawha geothermal field in New Zealand. Lithium and B isotope signatures are in a good agreement with a fluid signature mainly derived from water/rock interaction involving magmatic rocks with no evidence of seawater input. On the other hand, Sr concentrations are lower and more heterogeneous and fall between 2 and 165 μg/L. The 87Sr/86Sr ratios range from 0.70549 to 0.70961. These Sr isotope compositions overlap those of the Rotorua geothermal field in New Zealand, confirming that some geothermal waters (with more radiogenic Sr) have interacted with bedrocks from the metasedimentary basement. Each of these isotope systems on their own reveals important information about particular aspects of either water source or water/rock interaction processes, but, considered together, provide a more integrated understanding of the geothermal systems from the TVZ in New Zealand.  相似文献   

3.
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4.
The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in 18O and 13C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ13C and δ18O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ13C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ18O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ18O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C4 type plants, supporting a scenario of expansion of C4 plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The 87Sr/86Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic 87Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.  相似文献   

5.
The watershed in the southern Jiangxi Province (Jiangxi Province is called simply Gan) (SGW) and the watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) are two subtropical watersheds of the Yangtze River in China. Both watersheds have similar latitudes and climate, but distinct differences in basin lithology. These similarities and differences provide a good natural laboratory in which to investigate weathering processes and Sr end-members in river waters. This work aims to identify and contrast the sources, fluxes and controls on Sr isotopic composition in the river waters of these two areas. Results showed that the 87Sr/86Sr in the SGW waters ranged from 0.716501 to 0.724931, with dissolved Sr averaging 27 μg l− 1. Rhyolites and granites are two major sources for the dissolved Sr. The SGW waters receive 42% of their Sr from silicates weathering, 32% from carbonates and 3.2% from evaporites. 87Sr/86Sr in the CQW waters has a lesser variation from 0.707694 to 0.710039, but higher Sr contents (average of 208 μg l− 1). Dolomite, limestone and dolomitic limestone are major sources of Sr in the waters. The CQW waters receive 69% of their Sr from carbonates, 1.7% from silicates and 0.9% from evaporites. The chemical erosion rate and Sr flux in the CQW are 122 t km− 2 a− 1 and 0.079 t km− 2 a− 1, respectively, which are higher than those of the SGW (56 t km− 2 a− 1 and 0.021 t km− 2 a− 1, respectively). These data suggest that the intensive carbonates weathering occurred in the karstic area in the upper-reach of the Yangtze River exert great influence on the high Sr concentration and low Sr isotopic ratios in the River.  相似文献   

6.
《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   

7.
The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ34S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ34S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ34S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87Sr/86Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ18O consistent with a marine environment. Its δ13C is however lower than those of marine carbonates, suggesting a contribution of 13C-depleted carbon from degradation of organic matter. δ18O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ18O values (∼+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates.  相似文献   

8.
《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   

9.
Mg isotope ratios (26Mg/24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/24Mg ratio (expressed as δ26Mg) at −0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ26Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (δ26Mggrass-δ26Mgrain) of 0.21‰. The deep pore-waters (1-15 m deep) have δ26Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of 24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.  相似文献   

10.
Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ26Mgplant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ26Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ26Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ26Mgleaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ26Mgleaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo-Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ26Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo-magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ26Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ26Mg that is comparable to analytical uncertainties.  相似文献   

11.
《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   

12.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.  相似文献   

13.
Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater 87Sr/86Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle-Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ18O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ18O in some samples, particularly those from the Lower Ordovician, maximum δ18O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ18O records the effect of decreasing tropical seawater temperatures across the Middle-Late Ordovician transition superimposed on seawater δ18O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ13C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ13C excursion up to +7‰. Nevertheless, an underlying trend in mean δ13C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater 87Sr/86Sr and δ18O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle-Late Ordovician event.  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.
《Applied Geochemistry》2006,21(10):1626-1634
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.  相似文献   

16.
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.  相似文献   

17.
Over the last decade it has become apparent that Li isotopes may be a good proxy to trace silicate weathering. However, the exact mechanisms which drive the behaviour of Li isotopes in surface environments are not totally understood and there is a need to better calibrate and characterize this proxy. In this study, we analysed the Li concentrations and isotopic compositions in the various surface reservoirs (soils, rocks, waters and plants) of a small forested granitic catchment located in the Vosges Mountains (Strengbach catchment, France, OHGE http://ohge.u-strasbg.fr). Li fluxes were calculated in both soil profiles and at the basin scale and it was found that even in this forested basin, atmospheric inputs and litter fall represented a minor flux compared to input derived from the weathering of rocks and soil minerals (which together represent a minimum of 70% of dissolved Li). Li isotope ratios in soil pore waters show large depth dependent variations. Average dissolved δ7Li decreases from −1.1‰ to −14.4‰ between 0 and −30 cm, but is +30.7‰ at −60 cm. This range of Li isotopic compositions is very large and it encompasses almost the entire range of terrestrial Li isotope compositions that have been previously reported. We interpret these variations to result from both the dissolution and precipitation of secondary phases. Large isotopic variations were also measured in the springs and stream waters, with δ7Li varying from +5.3‰ to +19.6‰. δ7Li increases from the top to the bottom of the basin and also covaries with discharge at the outlet. These variations are interpreted to reflect isotopic fractionations occurring during secondary phase precipitation along the water pathway through the rocks. We suggest that the dissolved δ7Li increases with increasing residence time of waters through the rocks, and so with increasing time of interaction between waters and solids. A dissolution precipitation model was used to fit the dissolved Li isotopic compositions. It was found that the isotopic compositions of springs and stream waters are explicable by an isotopic fractionation of −5‰ to −14‰ (best fit −10.8‰), in agreement with Li incorporation into clay. In soil solutions, it was found that isotopic fractionation during secondary precipitation is larger (at least −23‰), suggesting a major role for different secondary phases, such as iron oxides that maybe incorporate Li with a higher isotope fractionation.  相似文献   

18.
A large collection of fluids (54 interstitial fluids and four expelled fluids) were sampled at the Manon site, at the outer edge of the Barbados accretionary complex. These warm fluids (up to 20°C) are expelled by sub-marine (5000 mbsl) mud volcanoes consisting of diapirs (unchanneled flow) and diatremes (channeled).Chlorine stable isotope ratios of these fluids were measured by IRMS with a reproducibility of ± 0.05‰ (1σ) versus SMOC (Standard Mean Ocean Chloride).A large range of δ37Cl between −5.3‰ and +0.1‰ is observed. Data from each volcanic structure describe a mixing between seawater and a low-δ37Cl fluid. The whole set of data is interpreted as the result of a mixing between two deep components and seawater. The two deep fluids are chemically distinct (e.g., in Ca, Mg, K, Li, Sr and Br contents and Br/Cl ratio). They display low and significantly different 87Sr/86Sr ratios (0.707790 and 0.707892, respectively) and δ37Cl values (−4.51 and −5.24‰, respectively).Physicochemical processes such as mineralogical transformation, diffusion, compaction or ion filtration are known to fractionate chlorine stable isotopes and can produce fluids with negative δ37Cl values. Ion filtration due to sediment compaction appears to be the more likely process to explain the negative δ37Cl values observed at the Manon site. A model for the generation of these signatures is proposed where a residual negative δ37Cl fluid reservoir is created at the bottom of the prism or the sediment pile. Further compaction/fracturing and/or dewatering of the slab may flush out these fluids and focus them towards the décollement zone. Mixing between the fluids and ultimately with seawater and water released during gas hydrate destabilizations may explain the data set within the individual cores and between the different structures.  相似文献   

19.
We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ25Mg = −0.14 ± 0.06; δ26Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ7Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ7Li = 3.5 ± 0.5‰, δ25Mg = −0.10 ± 0.03‰ and δ26Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ7Li and δ26Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ7Li and δ26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ7Li seen in some oceanic basalts.  相似文献   

20.
Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, 87Sr/86Sr ratios range from 0.7080 to 0.7090, and δ34S-SO4 values range from +10 to 30‰. A few waters have elevated δ13CDIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ18O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, −15 to −13‰ δ18O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ18O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ13CDIC = −14.7 to−11.1‰ and 87Sr/86Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.  相似文献   

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