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1.
In this study, we performed an integrated investigation of K and Mg isotopes in hydrothermally altered rocks from the giant Dexing porphyry Cu deposit in China. Both the altered porphyry intrusion and the surrounding wall rocks exhibit large variations in K and Mg isotope compositions, with δ41K values ranging between-1.02‰ and 0.38‰, and δ26Mg values ranging between-0.49‰ and 0.32‰. The δ41K and δ26Mg values of the majority of altered samples are higher than the isotopic baseline values for upper continental crust. We attribute the general increase in δ41 K and δ26Mg in altered rocks to hydrothermal alteration,which caused preferential incorporation of heavy K and Mg isotopes in alteration products, particularly phyllosilicates. However,a few altered samples show anomalously low δ41K and δ26Mg values. The δ41K and δ26Mg values do not correlate with K and Mg concentrations, or mineralogy of altered samples. The variable K-Mg isotope data likely reflect fluids of different physicalchemical properties, or different isotopic compositions. Based on the combined K-Mg isotope data, at least three groups of hydrothermal fluids are distinguished from the Dexing porphyry deposit. Therefore, K-Mg isotopes are potentially a novel tracer for fingerprinting fluids in complex hydrothermal systems.  相似文献   

2.
稳定同位素高度计极大地促进了全球较大山体隆升历史的研究,然而,古土壤氢氧同位素古高度计获得的古高度结果一般比通过古生物化石获得的要高得多.因此,需要在不同地区深入评估其现代高度关系及其影响因素,借此探讨稳定同位素古高度重建方法的校正.本研究测定了青藏高原东部东亚季风区龙门山29个表土叶蜡正构烷烃氢同位素值(δD_(wax))、22个土壤水氢同位素值(δD_(sw)),以及贡嘎山47个河水氢同位素值(δD_(rw))、55个泉水的氢同位素值(δD_(springw))和2个水文观测站的夏季降水氢同位素值(δD_(p)).龙门山断面表土δD_(wax)值分布在.160~.219‰(其δD_(wax)高程递减率为.18‰km.1(R2=0.83;p0.0001;n=29),比贡嘎山断面整体上偏正约40‰.气候和水汽来源的区域差异性可能导致了两个地区δD_(rw)值、表土δD_(wax)和ε_(wax)/p值的不同.另外,本研究发现两个断面表土δD_(wax)相对δD_(rw)、δD_(p)和δD_(sw)的分馏值(ε_(wax)/_(rw)、ε_(wax)/p和ε_(wax)/_(sw))整体上随海拔高度的升高而增加(两个断面分别高达34‰和50‰).通过这两个高程断面的表土δD_(wax)、δD_(p)、δD_(rw)(δD_(springw))和δD_(sw)的对比研究,讨论了水汽再循环、冰川融水、蒸散发、植被类型、纬度和山形等因素对ε_(wax)/p值随高程变化的综合影响.因此,在估算高海拔处的古高度时,如果仍然参考有些低海拔处估算的植物的ε_(wax)/p值,那么,以此推导的古降水δD_(p)偏负于实际值,因而高估古高度.  相似文献   

3.
The hydrogen isotopic composition(δD) of leaf wax long-chain n-alkanes(C27, C29, and C31) from lacustrine sediments has been widely applied to reconstruct terrestrial paleoclimatic and paleohydrological changes. However, few studies have addressed whether the aquatic-derived n-alkanes can affect the δD values of lake sedimentary long-chain n-alkanes, which are usually regarded as a recorder of the terrestrial hydrological signals. Here we systematically investigated δD values of long-chain n-alkanes from modern aquatic plants, both near-shore and off-shore surface sediments, surrounding terrestrial plant litters, as well as river water and lake water in Lake Qinghai and its satellite lakes on the northeastern Qinghai-Tibet Plateau. Our data showed that(i) δD values of long-chain n-alkanes from aquatic plants varied from-184‰ to-132‰ for n-C27, from-183‰ to-138‰ for n-C29, and from-189‰ to-130‰ for n-C31, respectively, with no significant differences among the three n-alkanes homologues;(ii) δD values of long-chain n-alkanes from aquatic plants were generally more positive than those from surrounding terrestrial plants, possibly because that they recorded the D-enrichment of lake water in this semi-arid region;(iii) δD values of long-chain n-alkanes from surface sediments showed significant differences among the three n-alkanes homologues, due to the larger aquatic input of n-C27 to the sedimentary lipid pool than that of n-C31, and(iv) n-C27 δD values of near-shore aquatic plants and near-shore sediments are more negative than those from off-shore as a result of lower δD values of near-shore lake water. Our findings indicate that in this region(i) the offset between sedimentary n-C27 and n-C31 δD values(ΔδDC27-C31) could potentially be used to evaluate if sedimentary long-chain n-alkanes are derived from a single source;(ii) while δD values of n-C27 may be influenced by lake water hydrological changes, sedimentary n-C31 is derived predominantly from terrestrial plants and thus its δD can serve as a relatively reliable indicator for terrestrial paleoclimatic and paleohydrological reconstructions.  相似文献   

4.
Hydrogen isotopes in lipid biomarkers can trace past changes in the hydrologic cycle. Recent studies have revealed the potential of hydrogen isotopes in microalgal lipids for quantitatively reconstructing water δ~2H(δD) values and salinity. In this study we collected suspended particles along a salinity gradient from the Changjiang River Estuary(CRE), and measured δD values in fatty acids in these particles. The results indicated that δD values of water were correlated highly with salinity from the CRE, in agreement with the results from other estuaries. δD values in palmitic acid and stearic acid had a positive correlation with δD values of water from the CRE. Nevertheless, in the CRE, hydrogen isotope fractionation in fatty acids relative to water increased as salinity increased, opposite the trend in hydrogen isotope fractionation with salinity found in microalgal culture and field studies. We attribute the increase in hydrogen isotope fractionation as salinity increased to light availability, which was likely lower in the particle rich mixing zone at the end of the estuary, and potentially as well to multiple sources of fatty acids in the CRE.  相似文献   

5.
Water stable isotopes(δ~2 H and δ~(18)O) can record surface water evaporation,which is an important hydrological process for understanding watershed structure and function evolution.However,the isotopic estimation of water evaporation losses in the mountain watersheds remains poorly explored,which hinders understanding spatial variations of hydrological processes and their relationships with the temperature and vegetation.Here we investigated δ~2 H,δ~(18)O,and d-excess values of stream water along an altitude gradient of 2130 to 3380 m in Guan'egou mountain watershed at the east edge of the Qinghai-Tibet Plateau in China.The mean δ~2 H(-69.6‰±2.6‰),δ~(18)O(-10.7‰±0.3‰),and dexcess values(16.0‰±1.4‰) of stream water indicate the inland moisture as the major source of precipitation in study area.Water stable isotopes increase linearly with decreasing altitudes,based on which we estimated the fractions of water evaporation losses along with the altitude and their variations in different vegetations.This study provides an isotopic evaluation method of water evaporation status in mountain watersheds,the results are useful for further understanding the relationship between hydrological processes and ecosystem function under the changing climate surrounding the Qinghai-Tibet Plateau.  相似文献   

6.
It is well known that the destruction of the North China Carton(NCC) is closely related to subduction of the PaleoPacific slab, but materials recording such subduction has not been identified at the peak time of decratonization. This paper presents data of whole-rock major and trace elements and Sr-Nd-Hf isotopes and zircon U-Pb ages and Hf-O isotopes for Mesozoic volcanic rocks from the Liaodong-Jinan region in the northeastern NCC, in order to trace the subduction-related materials in their source and origin. The Mesozoic volcanic rocks in the Liaodong-Jinan region are mainly composed of two series of rocks, including alkaline basaltic trachyandesite, trachyandesite and trachyte, and subalkaline trachyandesite and andesite. Zircon U-Pb dating yields eruption ages of 129–124 Ma for these rocks. The Early Cretaceous volcanic rocks are all enriched in LILEs(such as Rb, Sr, Ba and Th) and LREEs, depleted in HFSEs(such as Nb, Ta and Ti), indicating that they were originated from mantle sources that had been modified by subducted crustal materials. However, they have relatively heterogeneous and variable isotopic compositions. The alkaline basaltic trachyandesite, trachyandesite and trachyte have enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions and mantle-like δ~(18)O values, suggesting that they were derived from low-degree partial melting of an isotopically enriched lithospheric mantle source. In contrast, the subalkaline trachyandesite and andesite have relatively depleted isotopic compositions with zircon ε_(Hf)(t) values up to +5.2 and heavy zircon O isotopic compositions with δ~(18)O values of +8.1‰ to +9.0‰, indicating that they were originated from a lithospheric mantle source that had been metasomatized by melts/fluids derived from the recycled low-T altered oceanic basalt. All of these geochemical features suggest that the Early Cretaceous volcanic rocks in the Liaodong-Jinan region would result from mixing of mafic magmas with different compositions. Such magmas were originated from the enriched lithospheric mantle and the young metasomatized mantle, respectively, with variable extents of enrichment and depletion in trace elements, radiogenic isotopes and O isotopes. Importantly, the identification of the low-T altered oceanic crust component in the origin of Early Cretaceous volcanic rocks by the zircon Hf-O isotopes provides affirmative isotopic evidence and direct material records for Mesozoic subduction of the Paleo-Pacific slab that induced decratonization of the North China Craton.  相似文献   

7.
Multiple sulfur and oxygen isotope compositions in Beijing aerosol   总被引：1，自引：0，他引：1  
Multiple sulfur isotopes(32S, 33 S, 34 S, 36S) and oxygen isotopes(16O, 18O) in Beijing aerosols were measured with MAT-253 isotope mass spectrometer. The δ34S values of Beijing aerosol samples range from 1.68‰ to 12.57‰ with an average value of 5.86‰, indicating that the major sulfur source is from direct emission during coal combustion. The δ18O values vary from 5.29‰ to 9.02‰ with an average value of 5.17‰, revealing that the sulfate in Beijing aerosols is mainly composed of the secondary sulfate. The main heterogeneous oxidation of SO2 in atmosphere is related to H2O2 in July and August, whereas H2O2 oxidation and Fe3+ catalytic oxidation with SO2 exist simultaneously in September and October. Remarkable sulfur isotope mass-independent fractionation effect is found in Beijing aerosols, which is commonly attributed to the photochemical oxidation of SO2 in the stratosphere. In addition, thermochemical reactions of sulfur-bearing compounds might be also a source of sulfur isotope anomalies based on the correlation between ?33S and CAPE.  相似文献   

8.
Compared with the measureable but limited K isotope variation in geological samples,biological samples have much larger variations in δ~(41)K values:from-1.3‰ to+1.1‰ relative to the international K standard NIST SRM 3141a.Notably,higher plants generally have δ~(41)K values that are lower than igneous rocks,whereas sea plants(algae)have δ~(41)K values that are higher than seawater;the range in δ~(41)K values of plants encompasses the δ~(41)K values of both igneous rocks and seawater.Plant cells utilize different K uptake mechanisms in response to highand low-K conditions.In a low-K environment,plant cells use energy-consuming ion pumps for active uptake of K;plant cells in high-K environments use non-energy-consuming ion channels.Based on these facts and on K isotope data from sea and land plants,it is hypothesized that the different K uptake mechanisms are accompanied by distinct K isotope fractionation behaviors or vital effects.The enrichment of light K isotopes in terrestrial plants could be attributed to preferential transport of isotopically light K in the energy-consuming active uptake process by K ion pumps in the membranes of plant root cells.On the other hand,the enrichment of heavy K isotopes in algae may be caused by a combination of the lack of K isotope fractionation during K uptake from seawater via ion channels and the preferential efflux of light K isotopes across the cell membrane back to the seawater.The large variation of K isotope compositions in biological samples therefore may reflect the diversity of isotopic vital effects for K in organisms,which implies the great potential of K isotopes in biogeochemical studies.  相似文献   

9.
Along the Proterozoic stratigraphic sections of Jixian, Tianjin, the δ13C values of carbonates are mostly negative in the Changchengian (Changcheng System), varying approximately within a range of (0±1 )‰in the Jixianian (Jixian System), and have commonly positive values of (2±2)‰ in the Qingbaikouan (Qingbaikou System). From Chuanlinggou Formation to Dahongyu Formation, i.e. from ca. 1700 Ma to 1600 Ma, the δ13C values of carbonates increase from about -3‰ to 0‰, but oscillate with large excursion in Dahongyu Formation. Two large excursions occur in the Jixianian, one in Yangzhuang Formation and the other in the third sub-member of Wumishan Formation overlaying. These integrated and continuous records of carbon isotope reflect secular variations of δ13C value in ancient seawater from ca. 1700 Ma to ca. 800 Ma. The increase and subsequent oscillation of seawater δ13C value occurring from the ages of ca. 1700 to ca. 1600 Ma and <1300 Ma may signify two correlative global tectonic events.  相似文献   

10.
Tan  Liangcheng  Liu  Wen  Wang  Tianli  Cheng  Peng  Zang  Jingjie  Wang  Xiqian  Ma  Le  Li  Dong  Lan  Jianghu  Edwards  R. Lawrence  Cheng  Hai  Xu  Hai  Ai  Li  Gao  Yongli  Cai  Yanjun 《中国科学:地球科学(英文版)》2020,63(10):1622-1632
Evaluating anthropogenic impacts on regional vegetation changes during historical time is not only important for a better understanding of the Anthropocene but also valuable in improving the vegetation-climate models. In this study, we analyzed stable isotopes(δ~(18)O, δ~(13)C) and trace elements(Mg/Ca, Sr/Ca) of a stalagmite from Huangchao Cave in central Shandong, northern China.~(230)Th and AMS~(14)C dating results indicate the stalagmite deposited during 174BC and AD1810, with a hiatus between AD638 and 1102. Broad similarities of the δ~(18)O and trace elements in the stalagmite suggest they are reliable precipitation indexes. The δ~(13)C of the stalagmite, a proxy of vegetation change, was generally consistent with local precipitation and temperature variations on a centennial-scale before the 15th century. It typically varied from –9.6‰ to –6.3‰, indicating climate controlled C3 type vegetation during this period. However, a persistent and marked increasing trend in the δ~(13)C record was observed since the 15th century, resulting in δ~(13)C values from –7.7‰ to –1.6‰ in the next four centuries. This unprecedented δ~(13)C change caused by vegetation deterioration cannot be explained by climate change but is fairly consistent with the dramatically increasing population and farmland in Shandong. We suggest that the increasing deforestation and reclamation in central Shandong began to affect vegetation in the mountain region of central Shandong since the 15th century and severely destroyed or even cleared the forest during the 16th–18th century.  相似文献   

11.
Organic carbon isotope(δ13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ13Corg values of the late Ediacaran Dengying Formation range from -29‰ to -24‰. In the deep-water Longbizui section, δ13Corg values from time-equivalent strata of the Dengying Formation are mostly between –35‰ and -32‰. These new data, in combination with δ13Corg data reported from other sections in South China, reveal a 6‰–8‰ shallow-to-deep water δ13Corg gradient. High δ13Corg values(-30‰) occur mostly in shallow-water carbonate rocks, whereas low δ13Corg values(-32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ13Corg variations imply limited buffering effect from a large dissolved organic carbon(DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ13Corg values(-30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ13Corg data and may record the isotope signature of organic matter from primary(photosynthetic) production. In contrast, low δ13Corg values(-32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.  相似文献   

12.
The compound-specific stable carbon isotope compositions(δ~(13)C) of leaf wax n-alkanes from two short sediment cores recovered off the Pearl River estuary(PRE) were analyzed to check for their capability of indicating decadal scale catchment environmental change. Sedimentary long-chain n-alkanes exhibited an odd-over-even predominance, with a maximum at n-C_(29) or n-C_(31), indicating their leaf wax origin was from vascular plants. The δ~(13)C values of C_(29) and C_(31) n-alkane in all the sediment samples were in the range of -28.8‰ to -31.2‰, consistent with the C_3 plant-dominated vegetation in the Pearl River catchments. The time series of δ~(13)C records from the two cores were comparable and displayed a decreasing trend from the early 20 th century to the end of the 1970s, followed by a reversal in that change leading to continued increase for ca. 15 years. After being corrected for the effect of atmospheric CO_2 rise and δ~(13)C_(atm) decline, the δ~(13)C_(29) records largely retained their raw changing pattern; the post-1980 increase being more conspicuous. The slightly decreasing trend in corrected δ~(13)C records before around 1980 may have been caused by an increase in precipitation, whereas the subsequent increase of δ~(13)C is likely associated with the observed dry climate and/or intensive anthropogenic deforestation. Our results thus demonstrate that leaf wax n-alkanes buried in the sediments off the PRE may well reflect change in the regional climate and/or human activity in the river catchments over the past century.  相似文献   

13.
Iron isotopic composition of the upper continental crust(UCC) is critical for understanding Fe mobilization and migration through the Earth. Because rocks exposed at Earth's surface have heterogeneous δ~(56)Fe, finegrained clastic sediments can be used to estimate the average composition of UCC. In this study, we report δ~(56)Fe of loess-paleosol sequences from Yimaguan, Chinese Loess Plateau(CLP), to constrain the average Fe isotopic composition of UCC. The loess-paleosol sequences in this area formed in glacial-interglacial cycles and are characterized by varying degrees of weathering. Our data show that the loess-paleosol layers have extremely homogeneous Fe isotopic compositions with δ~(56)Fe ranging from 0.06‰ to 0.12‰, regardless of variations in the major element composition and weathering intensity. Our study indicates that since Fe isotopes are not significantly fractionated during loess deposition, the loess can be regarded as representative of UCC. It follows that the average δ~(56)Fe of UCC is 0.09‰± 0.03‰(2SD), consistent with previous estimates based on igneous rock data.  相似文献   

14.
The Leishan-Rongjiang antimony ore field(LAOF) is in a unique geotectonic location in the uplift between the Youjiang and Xiangzhong basins.This paper focuses on two representative deposits in the LAOF:the Bameng and Peize antimony(Sb) deposits.We analyzed fluid inclusions(FIs) in stibnite and coexisting quartz,as well as the sulfur isotopic composition of stibnite,to better understand the nature of the ore-forming fluid and the metallogenic process.The FIs data from samples of the stibnite and coexisting quartz indicate that the ore-forming fluids were characterized by low-temperature(150-210 ℃),low-salinity(1.5 wt%-6.0 wt%NaCl equiv.),and low-density(0.872-0.961 g/cm~3).The δ~(34)S values of stibnite(-8.21‰ to 3.76‰,average =-6.30‰)fall in between the sulfur isotopic compositions of the mantle and of biogenic sulfur in sedimentary rocks.However,the δ~(34)S_(∑s) values(-4.41 ‰ to +0.04‰,average =-2.49‰) of the ore-forming fluids are generally closer to the sulfur isotopic composition of the mantle source,indicating that the sulfur in the LAOF was mainly sourced from the mantle,but with possible involvement of biogenic sulfur.In addition,FIs petrography and ore deposit geology show that fluid boiling resulted from an abrupt decrease in pressure,which may have triggered the precipitation of stibnite.We conclude that low-temperature,dilute hydrothermal fluids with mixed origins migrated along the regional fault and interacted with the wall rock,extracting the ore-forming materials.Then,the oreforming fluids were injected into the fault fracture zones.  相似文献   

15.
High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite(very finely to finely crystalline, planar-s to nonplanar-a dolomite;type-1), patterned dolomite(finely crystalline, planar-s dolomite; type-2), and mottled dolomite(finely to medium crystalline,nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ13C and 87Sr/86 Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ18O values(.4.97‰ to.4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated(i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ18O values(.7.74‰ to.5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ18O values(–9.30‰to –7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.  相似文献   

16.
In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea(SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L~(-1). The hydrocarbon δ~(13)C values, ranging from-69.4‰ to-72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results(~(-1)83‰ to~(-1)85‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO_2 reduction. By isotopic measurement, five light δ~(13)C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ~(13)C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ~(13)C values of benthic Uvigerina peregrina are extremely depleted(as low as~(-1)5.85‰ PDB), while those of planktonic Globigerinoides ruber reach-5.68‰ PDB. Scanning electron micrograph(SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ~(13)C composition of benthic foraminifera. This means that the anomalous δ~(13)C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane(AOM). Furthermore, the negative δ~(13)C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.  相似文献   

17.
Major elements and carbon isotopes of dissolved inorganic carbon(DIC)have been measured in the waters of Changbaishan mountain,a volcanic area in northeastern China,between June and September 2016 to decipher the origin of the CO_2 involved in chemical weathering reactions.Spatial variations of major elements ratios measured in water samples can be explained by a change of the chemical composition of the volcanic rocks between the volcanic cone(trachytes)and the basaltic shield as evidenced by the variations in the composition of these rocks.Hence,DIC results from the neutralization of CO_2 by silicate rocks.DIC concentrations vary from 0.3 to 2.5 mmol/L and carbon isotopic compositions of DIC measured in rivers vary from-14.2‰to 3.5‰.At a first order,the DIC transported by rivers is derived from the chemical weathering’s consumption of CO_2 with a magmatic origin,enriched in~(13)C(-5%)and biogenic soil CO_2 with lower isotopic compositions.The highest δ~(13)C values likely result from C isotopes fractionation during CO_2 degassing in rivers.A mass balance based on carbon isotopes suggest that the contribution of magmatic CO_2 varied from less than 20%to more than 70%.Uncertainties in this calculation associated with CO_2 degassing in rivers are difficult to quantify,and the consequence of CO_2 degassing would be an overestimation of the contribution of DIC derived from the neutralization of magmatic CO_2 by silicate rocks.  相似文献   

18.
The Furongian Series of the Cambrian in the Tarim Craton(NW China) is mainly composed of crystalline dolostones,and is an important target for oil and gas exploration. The chronostratigraphic framework of the Furongian Series in the Tarim Craton has not yet been established due to the scarcity of fossils and the absence of the Steptoean Positive Carbon Isotope Excursion(SPICE), which is the primary global carbon isotope excursion of the Furongian Series. In this study, the SPICE was discovered by examining the carbon isotopes of the representative drilling wells in the West Platform and East Basin. A positive carbon isotope anomaly with δ~(13)C values ranging from 1.5‰ to 1.9‰(PDB), were found in the middle of the Xiaqiulitag Formation of the three drilling wells in the West Platform, and a positive excursion with δ~(13)C values up to 4.0‰(PDB), which is correlated with the SPICE, was found in the lower limestone of the Tuershaketag Formation in the East Basin. Based on the carbon isotopic data and chemostratigraphic correlation, the chronostratigraphic framework of the Furongian Series across different facies in the Tarim Craton is preliminarily established. The basal boundary of the Furongian Series was preliminarily defined by the occurrence of the SPICE. Aweak positive anomaly of δ~(13)C was observed at the base of the Penglaiba Formation of the three boreholes in the West Platform and the top of the Tuershaketag Formation of Tadong-2 well. We propose that the positive anomaly of the carbon isotope above the weak negative anomaly at the base of the Penglaiba Formation and the top of the Tuershaketag Formation can be used to define the Cambrian-Ordovician boundary in the Tarim Craton.  相似文献   

19.
The late Mesozoic antimony(Sb)mineralization belt in South China hosts a large portion of the world’s Sb reserves.However,the source and mineralization processes of these Sb deposits remain controversial.Here,we measured mercury(Hg)concentrations and isotopic compositions of stibnite in the Banpo Sb-only and Woxi Sbpolymetallic ore deposits,as well as associated rocks in the Yangtze Block in order to constrain the metal sources and ore formation processes in the South China Sb mineralization belt.Stibnite samples from both deposits exhibit significant enrichment in Hg(4.23–50.6 ppm)and have higher δ202Hg values(?0.47‰to 2.03‰)than the studied Precambrian basement rocks(?1.42‰to 0.59‰),Paleozoic sedimentary rocks(?2.40‰to?0.32‰),and other natural Hg reserves(e.g.,marine and continental systems).This indicates that significant mass-dependent fractionation of Hg isotopes occurred during hydrothermal processes.Negative to slightly positive Δ199Hg values of?0.17‰to 0.02‰were obtained for stibnite from the studied deposits,similar to values for the Precambrian basement rocks,but different from those of the Paleozoic sedimentary rocks and data previously reported for mantle materials.This suggests that Precambrian basement rocks were the source of Hg and associated metals.Our data and the tectonic evolution of South China indicate that late Mesozoic asthenospheric upwelling,in response to the Paleo-Pacific oceanic slab foundering,generated heat that drove the circulation of fluids in the basement and crustal basinal rocks.These fluids leached Sb,Hg,and other metals from the Precambrian basement rocks and formed the world-class Sb mineralization belt in South China.  相似文献   

20.
The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread meta-morphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sam-pled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a εNd -TDM plot. The first is defined by the εNd(210 Ma) values of -3.45 to -7.34 and TDM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with εNd(130 Ma) values from -15.24 to -16.61 and TDM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has εNd(130 Ma) values of -10.89 to -16.32 and Nd model ages (TDM) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.  相似文献   

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