Isotopic constraints on growth conditions of multiphase calcite–pyrite–barite concretions in Carboniferous mudstones     

R. Raiswell  S. H. Bottrell  S. P. Dean  J. D. Marshall†  A. Carr‡  & D. Hatfield 《Sedimentology》2002,49(2):237-254

Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ¹³C=?28·7‰ PDB and δ¹⁸O=?1·6‰ PDB through to δ¹³C=?6·9‰ PDB and δ¹⁸O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ¹³C=?12·0 to ?10·1‰ PDB and δ¹⁸O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ³⁴S=+24·8‰ CDT and δ¹⁸O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ³⁴S=+6 to +11‰ CDT and δ¹⁸O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.  相似文献   

Celestite formation, bacterial sulphate reduction and carbonate cementation of Eocene reefs and basinal sediments (Igualada, NE Spain)   总被引：5，自引：0，他引：5  

Conxita Taberner  James D. Marshall†  James P. Hendry‡  Catherine Pierre§  & M. F. Thirlwall¶ 《Sedimentology》2002,49(1):171-190

Petrographic and geochemical studies of an Upper Eocene reef and associated basinal sediments from the mixed carbonate–siliciclastic fill of the south‐eastern Pyrenean foreland basin near Igualada (NE Spain) provide new insights into the evolution of subsurface hydrology during the restriction of a marine basin. The reef deposits are located on delta‐lobe sandstones and prodelta marls, which are overlain by hypersaline carbonates and Upper Eocene evaporites. Authigenic celestite (SrSO₄) is an important component in the observed diagenetic sequences. Celestite is a significant palaeohydrological indicator because its low solubility constrains transportation of Sr²⁺ and SO₄^2? in the same diagenetic fluid. Stable isotopic analyses of carbonates in the reef indicate that meteoric recharge was responsible for aragonite stabilization and calcite cementation. Sulphur and oxygen isotope geochemistry of the celestite demonstrates that it formed from residual sulphate after bacterial sulphate reduction, but also requires that there was a prior episode of sulphate recycling. Meteoric water reaching the reef and basinal areas was most probably charged with SO₄^2? from the dissolution of younger Upper Eocene marine evaporites. This sulphate, combined with organic matter present in the sediments, fuelled bacterial sulphate reduction in the meteoric palaeoaquifer. Strontium for celestite precipitation was partly derived in situ from dissolution of aragonite corals in the reef and basinal counterparts. However, ⁸⁷Sr/⁸⁶Sr data also suggest that Sr²⁺ was partly derived from dissolution of overlying evaporites. Mixing of these two fluids promoted celestite formation. The carbonate stable isotopic data suggest that the local meteoric water was enriched in ¹⁸O compared with that responsible for stabilization of other reefs along the basin margin. Furthermore, meteoric recharge at Igualada post‐dated evaporite deposition in the basin, whereas other parts of the same reef complex were stabilized before evaporite formation. This discrepancy resulted from the spatial distribution of continental siliciclastic units that acted as groundwater conduits.  相似文献   

Changes in the mode of Southern Ocean circulation over the last glacial cycle revealed by foraminiferal stable isotopic variability     

Ulysses S Ninnemann  Christopher D Charles 《Earth and Planetary Science Letters》2002,201(2):383-396

Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in δ¹³C, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the δ¹³C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the ‘Southern Ocean low δ¹³C problem’. One explanation for this glacial distribution of δ¹³C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age δ¹³C values. We show here, however, that glacial-interglacial δ¹³C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of δ¹³C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of δ¹³C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean δ¹³C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well.  相似文献   

云南兰坪北部铜多金属矿化区成矿流体流动与矿化分带——流体包裹体和稳定同位素依据   总被引：13，自引：7，他引：13       下载免费PDF全文

徐启东  李建威 《矿床地质》2003,22(4):365-376

滇西兰坪盆地北部发育了一类受逆冲推覆构造控制的浅成热液Cu—Ag—Pb-Zn矿化，形成了白秧坪、富隆厂、吴底厂、麻栗坪及金满、科登涧等大-中型矿床和矿点，并存在矿化分带。文章利用这些矿化脉体的流体包裹体和热液方解石的碳氧同位素组成资料，研究成矿流体与矿化分带的关系。结果表明，成矿流体主要属于NaCl-H2O成分体系，盐度ω(NaCleq)为2％～11％，形成温度为170～300℃，形成于1．8～3．8km深度内，这些相似性说明这类矿化的发生具有相似的流体性质和沉淀机制。热液方解石在δ^13C-δ^18O图解中呈近水平线展布的型式，指示流体源自地壳浅部的地下水系统，与海相灰岩等围岩作用形成了溶解碳以[HCO3^]-为主的成矿流体，流体与岩石的相互作用可能是成矿流体沉淀的主要机理。从西到东，流体包裹体的盐度-温度由高到低变化与矿化分带和逆冲推覆构造的根带→中带→锋带相配套，显示重力驱动流动可能是主要的流体流动机制。成矿流体在不同构造部位流动的通畅及流体．岩石系统的封闭-开放程度等流体流动性质与矿化发生的强度和规模有关，兰坪北部逆冲推覆构造中带的流体通畅地流动及沉淀时处于相对开放状态，有利于该区形成较大规模的浅成热液多金属矿化。  相似文献   

Isotopic Tracing of Trophication Processes over the Last Millennia on Lake Chenghai     

吴敬禄  王苏民  夏威岚 《中国地球化学学报》2003,22(3):271-279

Multi-parameter studies (stable isotopes in carbonate and organic matter, pigment,organic carbon and nitrogen contents) from a 660-yr continuous sediment core from Lake Cheng-hai, a closed, eutropic lake in southern China, provide information on lake historical eutrophi-cation. During the last 660 years, great changes have taken place in productivity and eutrophi-cation of Lake Chenghai in response to human activities. In 1690, the productivity of the lakebegan to increase as Lake Chenghai became closed from agriculture in the lake‘‘ s watershed. In 1942, Lake Chenghai evolved to eutrophic state, marked by an increase in organic carbon, ni-trogen, CaC03, pigment contents and obvious negative values of stable isotopes, which is more or less simultaneous with the large-scale population immigration during the period. In 1984, in-tensive human activities induced modern lacustrine productivity and eutrophic level. Human-in-duced trophic changes during the past few decades have affected the Lake Chenghai ecosystem tosuch an extent that it has never experienced before in the last 660 years.  相似文献   

采用环境同位素方法研究北江大堤石角段基岩渗漏通道   总被引：7，自引：1，他引：7       下载免费PDF全文

陈建生  董海洲  陈亮 《水科学进展》2003,14(1):57-61

论述了采用环境同位素与人工示踪的方法探测北江大堤石角段频发"管涌"的原因,通过对洪水期间不同区域层位的稳定同位素δD(‰)与δ18O(‰)的分析确定了地面涌水的来源,结合孔中的人工示踪方法,最终确定出北江大堤石角段的"管涌"来自基岩断裂强渗漏通道,为下一步的加固工程提供了重要的科学依据。  相似文献   

稳定碳同位素在第四纪古环境研究中的应用   总被引：26，自引：8，他引：26  

王国安 《第四纪研究》2003,23(5):471-484

文章对陆相沉积物、湖沼相沉积物以及脊椎动物牙齿中的δ¹³C研究作了系统的综述,特别对存在的问题提出了一些看法,认为黄土-古土壤序列中有机质δ¹³C与碳酸盐δ¹³C变化趋势相反的原因是CO₂与碳酸盐交换平衡时的分馏系数在古土壤堆积时和黄土发育时不一致造成的;陆相沉积物的有机质δ¹³C较碳酸盐δ¹³C能更加直接地反映沉积时地表植被状况,陆相沉积物的碳酸盐δ¹³C在古生态中的应用还有待于对它的形成机制和形成过程进行进一步的研究;我国黄土高原末次冰期的气候不利于C4的生长.文章强调古环境、古生态研究应该从现代过程入手,了解现代植物δ13C与气候参数的关系以及现代C4植物的分布规律.同时还认为分子化石的稳定同位素研究这一新研究手段将会给第四纪研究带来更新的活力.  相似文献   

Fluid mixing as the mechanism of formation of the Dajing Cu-Sn-Ag-Pb-Zn ore deposit,Inner Mongolia ——Fluid inclusion and stable isotope evidence   总被引：2，自引：0，他引：2  

刘伟  李新俊  谭骏 《中国科学D辑(英文版)》2003,46(7)

Since the 1990s, interest in the magmatic fluids and their relation to mineralization has been re-aroused[1—6]. Studies on stable isotopes of low-sulfidation deposits commonly show the predominance of meteoric water[7]. Paradoxically, the evidence for me…  相似文献   

再论印度—亚洲大陆碰撞的启动时间   总被引：39，自引：6，他引：39       下载免费PDF全文

王成善  李祥辉等 《地质学报》2003,77(1):16-24

利用沉积响应来识别印度－亚洲大陆碰撞启动时间是最直接和有效的方法之一。西藏仲巴错江顶群被甄别为碰撞型三角洲沉积，曲下组可能代表了碰撞启动时期的建造，藏南定日的海相白垩－古近系沉积演化，锶和碳同位素变化也支持两大陆碰撞启动时间大约在K／T界线时期，对比喜马拉雅西段碰撞启动时间并考虑大印度北缘失掉的宽度，提出两大陆量可能的碰撞启动时间是65Ma左右。  相似文献   

Oscillatory zoning in garnet from the Willsboro Wollastonite Skarn, Adirondack Mts, New York: a record of shallow hydrothermal processes preserved in a granulite facies terrane     

C. C. Clechenko  J. W. Valley 《Journal of Metamorphic Geology》2003,21(8):771-784

Oscillatory zoning in low δ¹⁸O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (X_Adr = 0.13–0.36). The δ¹⁸O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with X_Adr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ¹⁸O indicate that a high Fe³⁺/Al, high δ¹⁸O fluid mixed with a lower Fe³⁺/Al and δ¹⁸O fluid. The high δ¹⁸O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ¹⁸O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.  相似文献