Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'

The δ¹³C_carb and ⁸⁷Sr/⁸⁶Sr secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local δ¹³C_carb fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative δ¹³C_carb excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590–544 Myr interval, and two age-groups at 660–610 and 740–690 Myr can be resolved.     

鄂尔多斯盆地奥陶系白云岩成岩相地球化学特征*

白云岩是鄂尔多斯盆地奥陶系海相碳酸盐岩层系最主要的岩石类型和天然气藏的主要储集层。文中探讨了鄂尔多斯盆地奥陶系马家沟组白云岩成岩相的岩相与地球化学特征及其识别方法。根据岩心及露头观察、薄片鉴定、全岩X衍射、元素分析和碳、氧、锶同位素测试数据,识别出8种白云岩成岩相类型(准同生云化相、膏盐溶角砾相、活跃回流云化相、隐伏回流云化相、海水云化相、表生岩溶角砾相、埋藏溶蚀相和埋藏重结晶相),其分别形成于蒸发海水、中等盐度海水、正常海水、早期淡水、表生淡水及地层水成岩环境,隶属于近地表、风化壳及中—深埋藏3个成岩系统。建立了Fe-Mn、Cs-Sc、(K×Tl)/Na-(Sc×Ni)/Sr、δ¹³C-⁸⁷Sr/⁸⁶Sr 成岩相判别模版。弱活动性元素Sc、Ni和稳定重同位素¹³C、¹⁸O富集于蒸发海水类成岩相,较活动性元素Cs富集于中等盐度及正常海水类成岩相;K、Tl富集于还原环境成岩相,Fe、Mn富集于氧化环境成岩相;表生类成岩相以富含大量吸附态分散元素和 ⁸⁷Sr 为特征。研究结果表明,近地表及风化壳成岩系统决定了研究区马家沟组白云岩的成岩相及其地球化学特征,且岩相学研究及地球化学实验是开展白云岩成岩机理研究的有效手段。     

塔里木盆地中央隆起区上寒武统—下奥陶统白云岩地球化学特征及白云石化流体演化规律*

以详细的岩石学研究为基础,综合利用碳、氧、锶同位素等地球化学资料,深入分析了塔里木盆地中央隆起区上寒武统—下奥陶统白云石化流体演化规律以及白云岩成因机制。结果表明,上寒武统白云岩主要由泥晶—粉晶白云岩、微生物白云岩和(残余)颗粒白云岩等原始结构保留较好的白云岩构成,其C、Sr同位素与同期海水相近,O同位素值偏正,属于同生/准同生期与轻微蒸发海水有关的白云石化的产物;下奥陶统白云岩以细晶自形—半自形白云石为主,原始结构保留差,其C、Sr同位素与同期海水近似,但O同位素值略微偏负,主要为浅埋藏期白云石化的产物。部分早期白云岩在中—深埋藏过程中受埋藏重结晶和构造—热液白云石化的影响,形成细晶—粗晶他形白云岩和缝洞鞍形白云石充填物,该阶段白云石化流体主要来自于地层内封存的海源流体、深部热液以及蒸发岩层间热卤水,多期多源流体的共同作用导致该类白云岩具有较宽的Sr同位素组成和明显负偏的O同位素值。总体上,研究区白云岩具有早期形成(近地表到浅埋藏期大规模交代)、中期加强(中—深埋藏期部分重结晶)、晚期改造(热液局部调整)的整体演化趋势。     

锶同位素地层学研究进展

锶同位素地层学（SIS）根据地质历史中的任何时间全球范围内海水的 锶同位素组成都是均一的，海水87Sr/86Sr比值随时间变化这一基本原理，利用代表原始海水的海相碳酸盐（以及磷酸盐和硫酸盐）的87Sr/86Sr比值确定海相地层的年代，研究海平面变化，造山运动，古气候等全球事件，在地层学，沉积学，石油地质学和矿床学等领域中有着广泛的应用前景，同位素地层学已在世界范围内得到地质学家的普遍关注，并已获得迅速的发展，国内的有关研究则刚刚起步，锶同位素地层学研究的难点主要在于对样品成岩蚀变的挖掘和分析技术上，同时还需要有良好的地层学研究基础，在样品选择上应充分考虑其原始组分的抗成岩蚀变能力，锶同位素地层学的研究需要地层学家，沉积学家和地球化学家的共同努力。     

Testing Sr/Sr as a paleosalinity indicator on mixed marine, brackish-water and terrestrial vertebrate skeletal apatite in late Paleocene-early Eocene near-coastal sediments, Mississippi

Mixed marine, brackish-water and terrestrial vertebrate skeletal apatite is found in the late Paleocene-early Eocene Tuscahoma and Bashi Formations of Mississippi. The co-Occurrence in these near-coastal marine deposits of skeletal apatite with different provenances offers a unique opportunity for unraveling the effects of diagenesis on apatite ⁸⁷Sr/⁸⁶Sr composition, with bearings on the use of this isotope pair as paleosalinity indicator. The results show that the Sr isotopes of all originally poorly ordered phases of skeletal apatite have extensively re-equilibrated with pore water with significantly lower ⁸⁷Sr/⁸⁶Sr ratios than late Paleocene-early Eocene seawater. For example, marine shark and ray teeth from the Bashi Formation all have clearly nonmarine ⁸⁷Sr/⁸⁶Sr ratios, 0.7073-0.7075, compared with a coeval seawater ratio in the range 0.7077-0.7078. However, separated shark tooth enamel, with a high original crystallinity, appears to have retained all or most of its in vivo Sr isotopic signature until the present. Here we also show that ⁸⁷Sr/⁸⁶Sr results on recent vertebrate skeletal material from different environments are in good agreement with expected values for respective habitat.

Earlier analyses of calcite material from the Bashi Formation indicated the existence of a shift in seawater ⁸⁷Sr/⁸⁶Sr to a lower value at the Paleocene-Eocene boundary. The strong influence of pore water with low, nonmarine ⁸⁷Sr/⁸⁶Sr on the apatite in the Bashi Formation suggests that the proposed isotopic shift may be an artifact related to diagenetic processes.     

西藏羌塘盆地半岛湖地区索瓦组锶同位素组成与演化

西藏羌塘盆地中生代发育广泛的海相碳酸盐地层,前人对该区地层的对比研究多局限于古生物地层学,本文尝试通过锶同位素地层学方法对上侏罗-下白垩统索瓦组地层进行全球对比。根据锶同位素地层学的基本原理,本文在对样品进行显微组构分析、阴极发光分析、Ca、Si、Sr和Mn等元素含量分析及评估其对同期海水代表性和样品溶解过程合理性的基础上,测试了羌塘盆地半岛湖地区上侏罗-下白垩统锶同位素组成,并建立了相应的演化曲线,进行了全球对比研究。结果表明,样品的锶同位素组成与前人已公布的同期海水锶同位素组成数据较为接近,其演化曲线与国际上已公布的锶同位素演化曲线有着较好的可对比性,晚侏罗-早白垩世时期全球海平面的变化和侵蚀作用的强弱是控制锶同位素演化曲线的主要因素。此外,通过对索瓦组海相碳酸盐锶同位素的研究,我们认为索瓦组的顶部可能延伸到早白垩世欧特里夫期,近而说明了羌塘盆地在早白垩世欧特里夫期还存在大规模的海相地层。     

川东下三叠统飞仙关组-嘉陵江组界线附近的锶同位素组成

作为化学地层学的重要分支之一,锶同位素地层学已经逐步成为全球海平面变化、造山运动、古气候和古环境等全球事件研究和对比的有效工具,根据锶同位素地层学的基本原理,在对样品进行沉积岩组构、化学成分分析和对同期海水代表性评估的基础上,测试了四川东部华蓥山地区下三叠统飞仙关组-嘉陵江组界线附近6个碳酸盐岩样品的锶同位素组成。结果表明,该界线附近的锶同位素组成与前人公布的早三叠世同期海水的锶同位素组成数据基本一致。锶同位素地层学研究的引入,可望为我国南方海相三叠系飞仙关组和嘉陵江组的年代地层学对比研究提供重要依据。     

川东下三叠统飞仙关组一嘉陵江组界线附近的锶同位素组成

作为化学地层学的重要分支之一,锶同位素地层学已经逐步成为全球海平面变化、造山运动、古气候和古环境等全球事件研究和对比的有效工具,根据锶同位素地层学的基本原理,在对样品进行沉积岩组构、化学成分分析和对同期海水代表性评估的基础上,测试了四川东部华蓥山地区下三叠统飞仙关组-嘉陵江组界线附近6个碳酸盐岩样品的锶同位素组成。结果表明,该界线附近的锶同位素组成与前人公布的早三叠世同期海水的锶同位素组成数据基本一致。锶同位素地层学研究的引入,可望为我国南方海相三叠系飞仙关组和嘉陵江组的年代地层学对比研究提供重要依据。     

北羌塘盆地中侏罗统夏里组蒸发岩锶同位素地球化学特征及物质来源

北羌塘盆地位于青藏高原的中部,属东特提斯构造域,是一个具有成盐远景的蒸发岩盆地.前人对盆地的研究多局限于层序地层、构造运动、油气成藏等,对于盆地内蒸发岩的研究,特别是其成矿流体来源的研究报道较少.文章通过对北羌塘盆地龙尾湖QY-1钻孔中硬石膏岩层锶同位素进行测定,首次对盆地中侏罗统夏里组蒸发岩成矿流体的来源进行探讨.结果表明,笔者所分析的样品的锶同位素比值变化范围为0.707475～0.709048,均值0.708331,与同时期全球海水锶同位素比值(0.706860～0.707081)相比略高,表明成矿流体来源主要是海水.结合前人对羌塘盆地构造运动与盆地演化的认识,认为陆源锶的输入造成了本区的锶同位素比值高于同期海水.此外,钻孔中的同位素组成与前人公布的全球中侏罗世(164～160.2 Ma)海水锶同位素曲线具有较好的对比性,中晚侏罗世羌塘盆地海侵海退作用的强弱是控制盆地夏里组锶同位素演化的主要因素.     

川东北普光地区新型杂卤石钾盐矿的物源:Sr、S同位素证据

新型杂卤石钾盐矿是近年来发现于四川盆地东北部早—中三叠世的一种可溶性海相固体钾盐,因其可利用性及低成本可开采特征,及其对海相找钾实现突破性进展提供的指示意义,引起了工业界及学术界的广泛关注。但当前以新型杂卤石钾盐矿为载体的海相成钾理论研究尚处于探索阶段,本文聚焦川东北普光地区中下三叠统新型杂卤石钾盐矿,将锶、硫同位素研究方法应用于杂卤石碎屑、石膏及"绿豆岩"等样品,以探讨新型杂卤石钾盐矿中不同组分的物源特征。分析结果显示,杂卤石碎屑和矿层底部硬石膏样品具有相似的δ³⁴S和⁸⁷Sr/⁸⁶Sr同位素特征,δ³⁴S分布在27.9‰~28.9‰之间,⁸⁷Sr/⁸⁶Sr介于0.70826~0.70829之间。该研究结果与同时期全球海相硫酸盐的δ³⁴S和全球海水的⁸⁷Sr/⁸⁶Sr的可对比性表明上述组分的主要物源应为同时期海水。该研究结果为进一步新型杂卤石钾盐矿成矿模型的建立及海相成钾理论的补充和完善提供了一定的参考。     

The Paleoceanography during the Time with High δ13C of Phanerozoic marine carbonates

Carbon isotopic composition of marine carbonates is a record for various important geological events in the process of earth development and evolution. The carbonates of Carboniferous, Permian and Triassic, as the transition from Paleozoic to Mesozoic-Cenozoic have very high ¹³C value. Taking this as the main point, and combined with the oxygen, strontium isotopic composition in carbonates, distribution of carbonate basin area through geologic time, the correlation of carbon isotopic composition of marine carbonates to sea level change, organic carbon burial flux, exchange of CO₂ content in atmosphere and ocean, and long cycle evolution of the earth ecosystems were approached. The results are shown as follows: ①The interval of ¹³C >3‰ during Phanerozoic was concentrated in Carboniferous, Permian and the beginning of Triassic, but the beginning of Triassic was characterized by higher frequency and larger fluctuations in ¹³C value during a short time, whereas the Carboniferous-Permian presented a continuously stable high ¹³C value, indicating a larger amount of organic carbon accumulation in this time interval. Relatively high ¹⁸O values during this time was also observed, showing a long time of glaciations and cold climate, which suggest a connection among rapid organic carbon burial, cold climate, as well as pCO₂ and pO₂ states of atmosphere. ②The over consumption of atmosphere CO₂ by green plants during the time with high ¹³C of seawater forced CO₂ being transferred from ocean to atmosphere for the balance, but the decrease in the seawater amount and water column pressure caused by the global cooling could weaken dissolution capacity of CO₂ in seawater and carbon storage of marine carbonates, and also reduce the carbonate sedimentary rate and decrease the carbonate basin area globally from Devonian to Carboniferous and Permian. During the middle-late Permian carbonate was widely replaced by siliceous sediments even though in shallow carbonate platform, which resulted in the decrease of marine invertebrates, suggesting the Permian chert event should be global. ③The Phanerozoic ⁸⁷Sr/⁸⁶Sr trend of seawater showed a sharp fall in Permian and drop to a minimum at the end of the Permian, indicting input of strontium from the submarine hydrothermal systems (mantle flux). Such process should accompany with a supplement of CO₂ from deep earth to atmosphere and ocean system, but the process associated with widespread volcanism and rises of earth’s surface temperature pricked up the mass extinction during the time of end Permian. ④Cold climate and increase of continental icecap volume, the amalgamation of northern Africa and Laurentia continentals were the main reasons responsible for the sea level drop, but the water consumption result from the significantly increased accumulation of organic carbon should also be one of the reasons for the sea level drop on the order of tens of meters. ⑤The mass extinction at the end Permian was an inevitable event in the process of earth system adjustment. It was difficult for marine invertebrates to survive because of the continuously rapid burial of organic carbon, and of the decrease of sea water amount and its dissolution ability to CO₂. At last, at the end of Paleozoic, the supplement of CO₂ to atmosphere and ocean by widely magma activities resulted in a high temperature of earth surface and intensified mass extinction.     

广西钦州石炭-二叠纪深海硅质岩的锶同位素组成及其地质意义

报道了广西钦州海槽早石炭世-早二叠世深水硅质岩的锶同位素测定结果和经年代校正后的87Sr/86Sr初始值,建立了锶同位素演化曲线;通过与同时代上扬子地区台地相碳酸盐岩87Sr/86Sr比值和演化曲线的对比,研究了二者的异同和造成差异的原因。深水硅质岩和同期台地相碳酸盐岩的锶同位素演化曲线在总体特征上具有一定的相似性,反映海平面变化对其影响的一致性。玄武岩喷发等局部事件和硅质岩中无法回避的陆源碎屑的影响是造成87Sr/86Sr比值和演化曲线差异的主要原因。硅质岩锶同位素演化曲线显示,海平面上升发生于早石炭世,在晚石炭世早期达到最大值,晚石炭世晚期,海平面有所下降,在其后的早二叠世,海平面再次上升。     

西藏岗巴晚白垩世—古近纪早期锶同位素地层

在地质历史时期中,海相碳酸盐的锶同位素组成具有全球基本一致的演化规律,并且海水锶同位素组成受锶的来源控制,其比值与海平面的变化有内在的联系。西藏岗巴地区晚白垩世—古近纪早期地层发育良好,笔者对宗山、宗浦两个剖面中未经蚀变的26个海相碳酸盐样品进行锶同位素测试,根据得出的数据绘制演化曲线与全球锶同位素曲线有较好的可对比性。曲线的负偏极值与地层界线较为一致,宗山剖面曲线的负向峰值（0.707612）与上/下白垩统界线相对应,宗浦剖面中,锶同位素曲线的负偏值（0.707761）与古新统/始新统界线相对应,晚白垩世—古近纪锶同位素演化曲线也与海平面变化相一致。另外,还讨论了岗巴地区锶同位素变化的原因。     

Isotope stratigraphy of Neoproterozoic cap carbonates in the Araras Group, Brazil

The Neoproterozoic carbonate sequence on the southeastern border of the Amazon Craton is divided into three lithostratigraphic units: a basal cap dolomite, an intermediate limestone, limestone-mudstone unit, and an upper dolarenite-dolorudite unit. Sections of the cap-carbonate were measured from the inner shelf to the outer shelf. Carbon isotope ratios (relative to PDB) vary between − 10.5 and − 1.7‰ in cap dolomite, and between − 5.4 and + 0.1‰ in laminated limestone and mud-limestone. Limestones and mud-limestones exhibit ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70740 to 0.70780. A comparative isotope stratigraphy between the inner-shelf and the middle-shelf basin shows differences in carbon isotope ratios: The cap dolomite and limestones have lower δ¹³C ratios on the border of the basin (inner shelf) than in the middle shelf of the basin. These lower values can be related to shallower environmental conditions and to a stronger influence of the continental border. The ⁸⁷Sr/⁸⁶Sr ratios are the same in both areas, and are consistent with seawater composition at around 600 Ma.     

Mobility and exile at 2nd century A.D. khirbet edh‐dharih: Strontium isotope analysis of human migration in western Jordan

The 2nd century A.D. Roman annexation of the Nabataean Kingdom may have resulted in significant settlement disruption. Here we investigate the possibility that political refugees occupied Khirbet edh‐Dharih after Roman takeover, using strontium isotope analysis of archaeological human dental enamel. We additionally discuss regional patterning of 87Sr/86Sr in western Jordan based on archaeological faunal dental enamel samples to understand strontium isotope spatial heterogeneity and to facilitate interpretation of the Dharih results. Strontium isotope analysis of small mammal dentition varied as expected based on bedrock geology, but small faunal sample sizes did not accurately reflect the broad range of biologically available strontium at Dharih. Only one person at Khirbet edh‐Dharh appears to be non‐local based on isotopic data. The status of the remaining individuals is unclear, as they could have originated from Dharih or the Nabataean capital city of Petra. Incorporating modern fauna and soil samples to establish the local value may assist in characterizing the local strontium in a geologically complex area such as Dharih. Futhermore, future analysis of elemental strontium to assess dietary composition and diagenetic contamination would facilitate interpretation of the strontium sources in the Dharih individuals. © 2008 Wiley Periodicals, Inc.     

Iron-Molybdenum Isotopes and the Chemical Evolution of Ancient-Oceans

Iron(Fe) is abundant in nature while molybdenum(Mo) is the most abundant transition metal in seawater. Due to their high sensitivity to the redox state of the environment, the isotopic compositions of Fe and Mo as well as variations have been widely used to probe the redox conditions and the evolution of ancient ocean chemistry in favor of improved analytical techniques. Here, we summarized isotopic fractionation mechanisms and natural distribution of both iron and molybdenum isotopes, and further we summarized and partially reinterpreted the redox evolution of ancient oceans through time based on available Fe-Mo data compiled in this study. The process that causes the largest iron isotope fractionation is redox reaction and the iron in oxidation state is generally enriched in ⁵⁶Fe. Biotic and abiotic pyrite formations also produce a large Fe isotope fractionations. Isotopic fractionation of molybdenum in seawater is mainly caused by the adsorption process of dissolved Mo onto ferromanganese oxides or hydroxides in sediments. Fe-Mn (hydro)oxides tend to adsorb isotopically light molybdenum resulting in the isotopic composition of Mo in seawater heavier. However, the Mo sinks in euxinic settings cause almost no molybdenum isotope fractionation. The Fe Mo isotope isotopic records through geological timegenerally suggest similar ocean redox evolution: Oceans older than 2.3 Ga was mainly dominated by ferruginous condition, and there was a slight increase in oxygen content between 2.6 and 2.5 Ga. Earth’s surface was initially oxidized during 2.3 to 1.8 Ga, during which euxinic deposition of sulfide was elevated. Euxinic waters may have expanded greatly between 1.8 and 0.8 Ga, and after that, Earth’s surface had being gradually oxidized and the euxinic waters shrank substantially.Finally, suggestions are proposed for further work on the Fe-Mo isotope research in the context of ancient ocean chemistry.     

Isotopic constraints on fluid/rock ratios in carbonate rocks: Barite-sulfide mineralization in the Schwaz Dolomite, Tyrol (Eastern Alps, Austria)

The significance of isotopic data on constraining the physical conditions of fluid-rock interaction and mineralization processes in carbonate rocks is discussed, based on the example of barite-tetrahedrite mineralization in Lower Devonian platform carbonates of the Western Greywacke Zone (Tyrol, Austria). Available strontium, oxygen, carbon and sulfur isotopic data are complemented with oxygen isotopic data for barite. Barites are homogeneous in δ¹⁸O_V-SMOW and δ³⁴S_CDT with values of + 15.4 and + 23.5‰, respectively. Their ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7128 and 0.7113 for the first generation and between 0.7117 and 0.7123 for younger remobilization. The dolomitic host rock shows a significant variation in Sr, O and C isotopic composition between non-mineralized and mineralized zones: ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7076 and 0.7133, δ¹⁸O_V-SMOW-values between +28.11 and +20.65‰, and δ¹³C_PDB-values between −1.15 and + 3.06‰. Fluid/rock volume ratios on the order of 1.3–3.2 are calculated for open-system behaviour by modelling Sr, O and C isotopic shifting capacities. The isotope data combined with other geological evidence support the following genetic model: Subsequent to synsedimentary sulfide mineralization during an Early Devonian rifting stage, collision tectonics in Carboniferous time led to the expulsion of Ba- and Sr-rich orogenic brines, which evolved from metamorphic fluids consisting essentially of H₂O and some CH₄, into an external sedimentary fold-and-thrust belt. The brines remobilized the synsedimentary sulfides, mixed with meteoric waters in the platform carbonates, reacted with evaporitic horizons and finally caused the recrystallization of dolomite and the precipitation of Sr-rich barite in structurally weak zones at 70–130°C. During the later Alpine orogeny supergene oxidation products were formed, and sulfates, sulfides and carbonates were further remobilized into late faults and fractures.     

Highly fractionated I-type granites in NE China (II): isotopic geochemistry and implications for crustal growth in the Phanerozoic

NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic.

Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7045±0.0015), positive _Nd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment.

The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic.     

重庆秀山寒武系锶同位素演化曲线及全球对比

本文评述了国际上已有的寒武纪海相碳酸盐的锶同位素演化曲线，公布了重庆秀山寒武系海相碳酸盐的锶同位素分析结果，尽管对样品进行了成岩蚀变检测和必要的筛选，但样品仍可能不同程度地经历了成岩蚀变，锶同位素组成与地层厚度投点图显得较为离散，笔者主要利用87Sr/86Sr比值较低的样品建立的寒武纪海水的锶同位素演化曲线在长期旋回上与国际上已公布的同时代锶同位素演化曲线仍具有较好相似性和可比性，说明全球事件是海水锶同位素组成的主要控制因素，海水的87Sr/86Sr比值在盆内对比，盆间对比，以及全球对比中都具有潜在价值。根据锶同位素地层学的基本原理，笔者尝试确定了重庆秀山寒武纪剖面一些关键点的年龄，累积厚度1340m处的年龄为503－509Ma，累积厚度1950m处的年龄为496－498Ma，这对秀山寒武纪剖面下，中寒武统和中、上寒武统界线的确定具有参考价值。     

Diagenetic mass transport in the southern San Joaquin basin, California, U.S.A.: Implications from the strontium isotopic composition of modern pore fluids

Three types of chemically and isotopically distinct pore fluids from the southern San Joaquin basin previously recognized by J.B. Fisher and J.R. Boles also have distinctive ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations. Meteoric fluids have stable isotopic compositions which lie on or near the meteoric water line and low chlorinities. Sr concentrations are between 0.01 and 2.6 mg l⁻¹, and ⁸⁷Sr/⁸⁶Sr ratios range from 0.7061 to 0.7078. Diagenetically modified connate marine fluids have δD-and δ¹⁸O-values more positive than −35‰ and 0‰, respectively, and have chlorinities generally comparable to seawater. Sr concentration are much higher than the meteoric group (16–198 mg l⁻¹), although the ⁸⁷Sr/⁸⁶Sr ratios (0.7070–0.7081) are not distinctive. Mixed meteoric-modified connate fluids have δD, δ¹⁸O and chlorinity intermediate between the meteoric and modified connate groups. Sr concentrations are also intermediate, between 16 and 22 mg l⁻¹, but ⁸⁷Sr/⁸⁶Sr ratios (0.7080–0.7087) are generally more radiogenic than either the meteoric or modified connate groups.

All of the fluids have ⁸⁷Sr/⁸⁶Sr ratios comparable to or lower than Tertiary seawater. Alteration of detrital plagioclase is the probable origin of the low isotopic ratios. Mass-balance calculations based on the Sr data suggest that essentially no transport of Sr occurred during diagenesis of sandstones containing modified connate pore fluids, while large amounts of Sr have been transported out of meteoric reservoirs by fluid flow. The chemically anomalous mixed meteoric-modified connate fluids contain the most radiogenic strontium in the basin. These fluids are spatially associated with major faults, and may represent clay mineral dehydration waters which have been transported upward from greater depth.

These results suggest that the three types of fluids identified by Fisher and Boles represent three distinct mass transport regimes: a largely stagnant deep-basin system containing modified connate pore fluids; an actively recharging meteoric system along the basin flanks; and a third system restricted to the southern basin which may be characterized by largescale cross-formational fluid flow, rather than dilution by meteoric waters.