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1.
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   

2.
THE EFFECT OF LESSER HIMALAYAN CALC-SILICATES ON THE ~(87)Sr/~(86)Sr OF THE BHOTE KOSI RIVER OF NEPAL—IMPLICATIONS FOR HIMALAYAN WEATHERING, THE MARINE ~(87)Sr/~(86)Sr RECORD AND GLOBAL CLIMATE1　VeizerJ .AnnuRevEPS ,1989,17:141～ 16 8. 2　KrishnaswamiS ,etal.EPSL ,1992 ,10 9:2 43～ 2 5 3. 3　EdmondJM .Science ,1992 ,2 5 8:15 94～ 15 97. 4　DerryLA .France LanordCEPSL ,1996 ,142 :5 9～ 74. 5　QuadeJ ,eta…  相似文献   

3.
Fluxes of Sr into the headwaters of the Ganges   总被引：1，自引：0，他引：1  
Himalayan weathering is recognized as an important agent in modifying sea water chemistry, but there are significant uncertainties in our understanding of Himalayan riverine fluxes. This paper examines causes of the variability, including that of the seasons, by analysis of downstream variations in Sr, 87Sr, and major ions in the mainstream, in relation to the composition of tributary streams from subcatchments with differing geologic substrates.Water samples were collected over four periods spanning the premonsoon, monsoon, and postmonsoon seasons. Uncertainties in the relative fluxes have been estimated, using Monte Carlo techniques, from the short-term variability of mainstream chemistry and the scatter of tributary compositions. The results show marked seasonal variations in the relative inputs related to high monsoon rainfall in the High and Lesser Himalaya, contrasting with the major contribution from glacial melt waters from the Tibetan Sedimentary Series (TSS) at times of low rainfall. Much of the spread in previously published estimates of the sources of Sr in Himalayan rivers may result from these seasonal variations in Sr fluxes.The annual fluxes of Sr into the headwaters of the Ganges are derived from the three main tectonic units in the proportions 35 ± 1% from the TSS, 27 ± 3% from the High Himalayan Crystalline Series (HHCS), and 38 ± 8% from the Lesser Himalaya. The particularly elevated 87Sr/86Sr ratios characteristic of the HHCS and the Lesser Himalaya enhance their influence on seawater Sr-isotope composition. The TSS contributes 13 ± 1%, the HHCS 30 ± 3%, and the Lesser Himalaya 57 ± 11% of the 87Sr flux in excess of the seawater 87Sr/86Sr ratio of 0.709.  相似文献   

4.
Sr and 87Sr/86Sr have been measured in the Yamuna river headwaters and many of its tributaries (YRS) in the Himalaya. These results, with those available for major ions in YRS rivers and in various lithologies of their basin, have been used to determine their contributions to riverine Sr and its isotopic budget. Sr in the YRS ranges from 120 to 13,400 nM, and 87Sr/86Sr from 0.7142 to 0.7932. Streams in the upper reaches, draining predominantly silicates, have low Sr and high 87Sr/86Sr whereas those draining the lower reaches exhibit the opposite resulting from differences in drainage lithology. 87Sr/86Sr shows significant co-variation with SiO2/TDS and (Na* + K)/TZ+ (indices of silicate weathering) in YRS waters, suggesting the dominant role of silicate weathering in contributing to high radiogenic Sr. This is also consistent with the observation that streams draining largely silicate terrains have the highest 87Sr/86Sr, analogous to that reported for the Ganga headwaters. Evaluation of the significance of other sources such as calc-silicates and trace calcites in regulating Sr budget of these rivers and their high 87Sr/86Sr needs detailed work on their Sr and 87Sr/86Sr. Preliminary calculations, however, indicate that they can be a significant source to some of the rivers.It is estimated that on an average, ∼25% of Sr in the YRS is derived from silicate weathering. In the lower reaches, the streams receive ∼15% of their Sr from carbonate weathering whereas in the upper reaches, calc-silicates can contribute significantly (∼50%) to the Sr budget of rivers. These calculations reveal the need for additional sources for rivers in the lower reaches to balance their Sr budget. Evaporites and phosphorites are potential candidates as judged from their occurrence in the drainage basin. In general, Precambrian carbonates, evaporites, and phosphorites “dilute” the high 87Sr/86Sr supplied by silicates, thus making Sr isotope distribution in YRS an overall two end member mixing. Major constraints in quantifying contributions of Sr and 87Sr/86Sr from different sources to YRS rivers are the wide range in Sr and 87Sr/86Sr of major lithologies, limited data on Sr and 87Sr/86Sr in minor phases and on the behavior of Sr, Na, and Ca during weathering and transport.The Ganga and the Yamuna together transport ∼0.1% of the global Sr flux at the foothills of the Himalaya which is in the same proportion as their contribution to global water discharge. Dissolved Sr flux from the Yamuna and its mobilization rate in the YRS basin is higher than those in the Ganga basin in the Himalaya, a result consistent with higher physical and chemical erosion rates in the YRS.  相似文献   

5.
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   

6.
METAMORPHISM IN THE LESSER HIMALAYAN CRYSTALLINES AND MAIN CENTRAL THRUST ZONE IN THE ARUN VALLEY AND AMA DRIME RANGE (EASTERN HIMALAYA)1　BrunelM ,KienastJR . tudep啨ｔｒｏ structuraledeschevauchementsductileshimalayenssurlatrans versaledel’Everest Makalu (N啨ｐａｌｏｒｉｅｎｔａｌ) [J].CanadianJ .EarthSciences,1986 ,2 3:1117～ 1137. 2　LombardoB ,RolfoF .TwocontrastingeclogitetypesintheHimalayas :implicationsfortheHimalayanorogeny…  相似文献   

7.
The crystallines in the Kumaon Himalaya, India are studied along Goriganga, Darma and Kaliganga valleys and found to be composed of two high-grade metamorphic gneiss sheets i.e. the Higher Himalayan Crystalline (HHC) and Lesser Himalayan Crystalline (LHC) zones. These were tectonically extruded as a consequence of the southward directed propagation of crustal deformation in the Indian plate margin. The HHC and its cover rocks i.e. the Tethyan Sedimentary Zone (TSZ) are exposed through tectonic zones within the hinterland of Kumaon Himalaya. The HHC records history of at least one episode of pre-Himalayan deformation (D1), three episodes of Himalayan deformation (D2, D3, D4). The rocks of the HHC in Kumaon Himalaya are thoroughly transposed by D2 deformation into NW-SE trending Sm (S1+S2). The extent of transposition and a well-developed NE-plunging L2 lineation indicate intense strain during D2 throughout the studied portion of the HHC. Ductile flow continued, resulting in rotation of F1 and F2 folds due NE-direction and NW-SE plunging F3 folds within the HHC. The over thickened crystalline was finally, superimposed by late-to-post collisional brittle-ductile deformation (D4) and exposed the rocks to rapid erosion.  相似文献   

8.
Re-Os analyses were performed on suspended loads and coarser grained bank sediments of the Brahmaputra River system. Re and Os concentrations of these sediments vary from 7 to 1154 ppt and from 3 to 173 ppt, respectively. 187Os/188Os ratios range from 0.178 to 6.8, and thus vary from nearly mantle to very radiogenic crustal values. Nevertheless, most of the sediments have 187Os/188Os ratios less than 1.5, and nearly all of the samples of the Brahmaputra main channel have ratios less than 1.2. Thus, as previously suggested, the Brahmaputra is much less radiogenic than the Ganga. The Siang River, the northern extension of the Brahmaputra, is quite radiogenic in Os despite receiving sediments from the Tsangpo River, which flows along a suture zone with ultramafic outcrops. The Brahmaputra main channel has a fairly constant 187Os/188Os ratio even though its tributaries contribute sediments with very heterogeneous Os isotopic compositions. These data, along with the corresponding Nd isotopic compositions, suggest that about 60-90% of the sediment in the Brahmaputra system is derived from Himalayan formations (Higher Himalaya and Lesser Himalaya) whereas 10-40% comes from ophiolite-bearing sequences, perhaps eastern equivalents of those of the Transhimalayan Plutonic Belt. Os data also confirm previously published Sr and Nd results, indicating that about half of the sediments delivered to the Brahmaputra are supplied by the Siang River, while the Himalayan and the eastern tributaries account for 40 and 10%, respectively.The lower 187Os/188Os of the Brahmaputra River compared to that of the Ganga is due to two factors. One is the more limited presence of the Lesser Himalaya and hence the lower black shale content of the eastern Himalaya. The other is the non-radiogenic Os supplied by the eastern and southern tributaries, reflecting the presence of mantle-derived lithologies in this region. Despite the lower sediment supply from these tributaries, they contribute greatly to the Os budget of the Brahmaputra River. This study indicates that the Brahmaputra River has little effect on the present-day seawater Os budget. However, reconsideration of this budget suggests that the Ganga, which provides the most radiogenic Os of major rivers studied to date, may have significant impact on the marine Os isotopic composition. The Indo-Asian collision cannot be excluded as an important cause of the increase in the marine 187Os/188Os over the past 16 million years until the contributions of all of the rivers draining the Himalayan Tibetan Plateau are known.  相似文献   

9.
The influx of Sr responsible for increase in marine Sr has been attributed to rise of Himalaya and weathering of the Himalayan rocks. The rivers draining Himalaya to the ocean by the northern part of the Indian sub-continent comprising the Ganga Alluvial Plain (GAP) along with Central parts of the Himalaya and the northern part of the Indian Craton are held responsible for the transformation of Sr isotopic signature. The GAP is basically formed by the Himalayan-derived sediments and serves as transient zone between the source (Himalaya) and the sink (Bay of Bengal). The Gomati River, an important alluvial tributary of the Ganga River, draining nearly 30,500 km2 area of GAP is the only river which is originating from the GAP. The river recycles the Himalayan-derived sediments and transport its weathering products into the Ganga River and finally to Bay of Bengal. 11 water samples were collected from the Gomati River and its intrabasinal lakes for measurement of Sr isotopic composition. Sr concentration of Gomati River water is about 335 μg/l, which is about five times higher than the world’s average of river water (70 μg/l) and nearly three times higher than the Ganga River water in the Himalaya (130 μg/l) The Sr isotopic ratios reported are also higher than global average runoff (0.7119) and to modern seawater (0.7092) values. Strong geochemical sediment–water interaction appearing on surface is responsible for the dissolved Sr isotopic ratios in the River water. Higher Sr isotopic rations found during post-monsoon than in pre-monsoon season indicate the importance of fluxes due to monsoonal erosion of the GAP into the Gomati River. Monsoon precipitation and its interaction with alluvium appear to be major vehicle for the addition of dissolved Sr load into the alluvial plain rivers. This study establishes that elevated 87Sr/86Sr ratios of the Gomati River are due to input of chemical weathering of alluvial material present in the Ganga Alluvial Plain.  相似文献   

10.
We examined the fluvial geochemistry of the Huang He (Yellow River) in its headwaters to determine natural chemical weathering rates on the northeastern Qinghai-Tibet Plateau, where anthropogenic impact is considered small. Qualitative treatment of the major element composition demonstrates the dominance of carbonate and evaporite dissolution. Most samples are supersaturated with respect to calcite, dolomite, and atmospheric CO2 with moderate (0.710-0.715) 87Sr/86Sr ratios, while six out of 21 total samples have especially high concentrations of Na, Ca, Mg, Cl, and SO4 from weathering of evaporites. We used inversion model calculations to apportion the total dissolved cations to rain-, evaporite-, carbonate-, and silicate-origin. The samples are either carbonate- or evaporite-dominated, but the relative contributions of the four sources vary widely among samples. Net CO2 consumption rates by silicate weathering (6-120 × 103 mol/km2/yr) are low and have a relative uncertainty of ∼40%. We extended the inversion model calculation to literature data for rivers draining orogenic zones worldwide. The Ganges-Brahmaputra draining the Himalayan front has higher CO2 consumption rates (110-570 × 103 mol/km2/yr) and more radiogenic 87Sr/86Sr (0.715-1.24) than the Upper Huang He, but the rivers at higher latitudes are similar to or lower than the Upper Huang He in CO2 uptake by silicate weathering. In these orogenic zones, silicate weathering rates are only weakly coupled with temperature and become independent of runoff above ∼800 mm/yr.  相似文献   

11.
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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.
A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with 87Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in 18O values and essentially no change in 13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower 13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in 18O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.  相似文献   

14.
《Applied Geochemistry》2002,17(3):285-300
Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members.  相似文献   

15.
The present published inventory of fluvial Sr and87Sr/86Sr data, combined with new information from the big rivers of Eastern Siberia (a combined total of ∼ 1,000 measurements), is used to investigate the probable origin of the large rise in the marine isotopic ratio, recorded in limestones, over the last ∼ 20 million years. With the exception of the data from the Ganga-Brahmaputra all measurements fall on what is proposed to be called the “Wickman trend”, essentially a mixing line between the limestone sink for Sr, with the integrated marine ratio, and the flux from the weathering of average continental crust. However, time-variations along this trend, i.e. changes in relative weathering intensity, cannot explain the observations from limestones. They can only be caused by very high and radiogenic fluxes of Sr as are occurring from the present Himalayan orogeny, lying far above the Wickman trend and caused by metamorphic remobilization of radiogenic Sr during underthrusting and subsequent unroofing associated with the collision of India with Eurasia. In general the variations in the ratio are therefore caused by specific tectonic events, not by general climatic variations in the intensity of aluminosilicate weathering.  相似文献   

16.
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   

17.
Large seasonal variations in the dissolved load of the headwater tributaries of the Marsyandi river (Nepal Himalaya) for major cations and 87Sr/86Sr ratios are interpreted to result from a greater dissolution of carbonate relative to silicate at high runoff. There is up to a 0.003 decrease in strontium isotope ratios and a factor of 3 reduction in the Si(OH)4/Ca ratio during the monsoon. These variations, in small rivers sampling uniform lithologies, result from a different response of carbonate and silicate mineral dissolution to climatic forcing. Similar trends are observed in compiled literature data, from both Indian and Nepalese Himalayan rivers. Carbonate weathering is more sensitive to monsoonal runoff because of its faster dissolution kinetics. Silicate weathering increases relative to carbonate during the dry season, and may be more predominant in groundwater with longer water-rock interaction times. Despite this kinetic effect, silicate weathering fluxes are dominated by the monsoon flux, when between 50% and 70% of total annual silicate weathering flux occurs.  相似文献   

18.
Marbles from Changpu (Dabie Shan, eastern China), subducted to 4.4 GPa, have 87Sr/86Sr values < 0.7040. These low 87Sr/86Sr values, which would imply a sedimentation age > 2 Ga if considered as primary signature, reflect fluid–rock interaction with a fluid from a low‐87Sr/86Sr source. The introduction of low‐87Sr/86Sr was paralleled by introduction of Mg and loss of Si, K and Na in such a way that carbonates from the purest marbles have the least evolved Sr isotopic composition. Introduction of Mg is also indicated by the distribution of calcite and dolomite. Calcite forms inclusions in garnet, whereas dolomite is restricted to the matrix. These chemical changes, inferred from the mineralogy, in combination with textural evidence require a mobile metamorphic fluid. P–T–X constraints for fluid generation and for permeability increase related to mineral reactions and phase transitions suggest that the marbles acquired their anomalous Sr‐isotopic composition during subduction below 60 km. The marbles with the least radiogenic Sr isotopic composition demonstrate that crustal rocks may lose their isotopic fingerprint during deep subduction.  相似文献   

19.
The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial87Sr/86Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.  相似文献   

20.
Isotopic characteristics of river sediments on the Tibetan Plateau   总被引：1，自引：0，他引：1  
We systematically collected 40 modern clastic sediment samples from rivers in different tectonic units of the Tibetan Plateau and measured their Sr–Nd isotopic compositions. The isotopic characteristics provide insight into the controversial paleo-tectonic affinity of terranes of the Tibetan Plateau and the provenance of Songpan–Ganzi flysch complex. The Qilian Terrane and Himalaya Terrane have more negative εNd(0) values (from ? 14.3 to ? 11.8 and from ? 20.64 to ? 13.26, respectively) and high 87Sr/86Sr values (from 0.719674 to 0.738818 and from 0.721020 to 0.824959, respectively), reflecting old and mature continental crust origin of these two terranes. The southern Lhasa Terrane is more radiogenic in εNd(0) values (from ? 8.82 to ? 3.8) and low in 87Sr/86Sr values (from 0.711504 to 0.719489), implying the combined impact of the Neo-Tethys mantle and Himalaya old continental crust. Sr–Nd isotopic compositions of the Qilian Terrane are similar to those in the Yangtze Craton, indicating that the Qilian Terrane was probably separated from the Yangtze Craton. Sr–Nd isotopic characteristics of the Songpan–Ganzi Terrane are similar to the Yangtze Craton and are remarkably different to those in the North China Craton, eastern Kunlun–Qaidam and the central Qiangtang metamorphic belt, implying that the widely distributed flysch complex of the Songpan–Ganzi Terrane was sourced from the Yangtze Craton.  相似文献   

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