共查询到20条相似文献，搜索用时 31 毫秒
1.
微亮晶（臼齿）碳酸盐成因及其在元古宙地球演化中的意义 总被引：14，自引：2，他引：14
本文应用多种技术方法,对微亮晶碳酸盐岩的成岩作用及其成因从宏观至微观进行了分析。根据岩石矿物学、地球化学和有机地球化学等研究证明,它是早期成岩过程中,在超高压的大气 CO_2,水平急剧下降的转折期,快速石化作用形成的具等粒结构μm级的微亮晶方解石集合体。MT 碳酸盐岩微亮晶成岩作用有两种类型:一是主要发生在海底沉积软泥中,在其底基质软泥中经差异压实作用形成褶皱肠状及复杂形的 MT 构造;二是直接在海水中或重力流搬运过程中形成的微亮晶球粒(核)。简言之,MT 碳酸盐岩石是在地球早期浅海环境下,由于微生物自养作用形成的地球化学成因产物。微亮晶碳酸盐岩是约束古大气圈和古海洋环境变化的最灵敏标志。通过详测 MT 丰度值和编制地球古大气圈 CO_2水平演化模式等,在前寒武纪发现有太古宙末期、早元古宙末期、中元古宙中期和新元古宙早-中期四个 MT 碳酸盐岩发育高峰值期,证明全球古大气圈 CO_2水平发生过四次不连续性、跳跃式下降周期,直到晚新元古宙未,全球雪球事件的发生,导致了 MT 碳酸盐岩的消失。其后,才进入显生宙的 CO_2低水平状态。MT 碳酸盐岩具有重要的油气资源远景,应予以重视。 相似文献
2.
柴达木盆地南翼山地区新近系储层类型主要为湖相碳酸盐岩.用碳、氧同位素和流体包裹体方法研究湖相碳酸盐岩的形成环境结果表明,碳酸盐岩总体形成于高盐度环境,局部地区浅层样品在成岩过程中遭受过大气淡水的改造.根据孔隙充填胶结物中流体包裹体的均一温度,推断胶结物形成于储层深埋阶段,随后地层发生抬升,导致现今较低深度的包裹体样品具有较高的均一温度. 相似文献
3.
Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah,USA) 
下载免费PDF全文
下载免费PDF全文 Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ13C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ13C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ13C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ34S) and silica (δ18O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments. 相似文献
4.
Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca-Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish-marine sediments whose pore waters abound in Ca2+ and Mg2+ originating in seawater and calcareous shells. Carbonates formed during Stage II (500–2000 m) include high Ca-Mg siderite, ankerite, Fe dolomite and Fe–Mg calcite in freshwater sediments. The assemblage of Stage II carbonates in brackish-marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage III (> 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks. 相似文献
5.
In Permian times the Baoshan Block of western Yunnan, southwest China formed the eastern part of the Cimmerian Continent. Most biogeographical and sedimentological data indicate that the Early Permian Dingjiazhai Formation formed on the block under conditions strongly influenced by the Permo-Carboniferous glaciation. After Early Permian rifting, with post-glaciation climatic amelioration, and as the Baoshan Block drifted northwards to approach South China and Indochina, faunal elements characteristic of Gondwana affinity decreased, while those of Cathaysian affinity increased. Finally, Late Permian faunas are characterized by exclusively Cathaysian elements. This shift of marine provinciality becomes an important indicator in understanding the Permian paleoclimatic evolution of the region. This research investigated the composition of carbonate grain associations and the early diagenetic features of limestones from the upper part of the Dingjiazhai Formation, and from the overlying Yongde and Shazipo formations. A sharp distinction in petrological and diagenetic features is recognized between the Dingjiazhai Formation and the two overlying formations. The Dingjiazhai carbonates are characterized by the bryonoderm (bryozoan-echinoderm)-extended facies of the heterozoan association, with no non-skeletal grains. Because early diagenetic cement was rarely formed, the Dingjiazhai carbonates experienced strong diagenetic compaction. In contrast, the Yongde and Shazipo carbonates show a chloroforam facies of photozoan association, with the common occurrence of non-skeletal grains. These carbonates were well cemented during early diagenetic processes. From comparison with Permian cool-water carbonates from northern Pangea and Tasmania, Australia, the Dingjiazhai carbonates are interpreted as deposits of warm-temperate conditions, while the overlying carbonates are considered to be deposits of subtropical or tropical conditions. This climatic interpretation, based on the petrographic features of the Permian carbonates, agrees well with existing biogeographical data from the region. 相似文献
6.
The carbonates in martian meteorite ALH84001 preserve a record of aqueous processes on Mars at 3.9 Ga, and have been suggested to contain signatures of ancient martian life. The conditions of the carbonate formation environment are critical for understanding possible evidence for life on Mars, the history of water on Mars, and the evolution of the martian atmosphere. Despite numerous studies of petrographic relationships, microscale oxygen isotope compositions, microscale chemical compositions, and other minerals associated with the carbonates, formation models remain relatively unconstrained. Microscale carbon isotope analyses of ALH84001 carbonates reveal variable δ13C values ranging from +27 to +64‰. The isotopic compositions are correlated with chemical composition and extent of crystallization such that the Mg-poor, early-formed carbonates are relatively 13C depleted and the Mg-rich, later forming carbonates, are 13C enriched. These data are inconsistent with many of the previously proposed environments for carbonate formation, and a new set of hypotheses are proposed. Specifically, two new models that account for the data involve low temperature (<100°C) aqueous processes: (1) the carbonates formed during mixing of two fluids derived from separate chemical and isotopic reservoirs; or (2) the carbonates formed from high pH fluids that are exposed to a CO2-rich atmosphere and precipitate carbonate, similar to high pH springs on Earth. 相似文献
7.
The Parnok deposit is made up of stratiform lodes of iron (magnetite) and manganese (oxide-carbonate, carbonate, and carbonate-silicate) ores localized among terrigenous-carbonate sediments (black shales) on the western slope of the Polar Urals. The lithological study showed that ore-bearing sediments were accumulated in a calm hydrodynamic setting within a relatively closed seafloor area (trap depressions). Periodic development of anaerobic conditions in the near-bottom seawater was favorable for the accumulation of dispersed organic matter in the terrigenous-carbonate sediments. Carbon required to form calcium carbonates in the ore-bearing sediments was derived from carbon dioxide dissolved in seawater. In the organic-rich sediments, carbonates were formed with the participation of carbon dioxide released by the destruction of organic matter. However, δ13C values (from 0.5 to ?4.4‰ PDB) suggest a relatively low fraction of the isotopically light biogenic carbon in the host calcite. The most probable sources of Fe and Mn were hydrothermal seepages at the seafloor. The Eh-pH conditions during stagnation were favorable for the precipitation of Fe and accumulation of Mn in a dissolved state. Transition from the stagnation regime to the concentration of oxygen in near-bottom waters was accompanied by oxidation of the dissolved Mn and its precipitation. Thus, fluctuations in Eh-pH parameters of water led to the differentiation of Fe and Mn. Initially, these elements were likely precipitated as oxides and hydroxides. During the subsequent lithification, Fe and Mn were reduced to form magnetite and rhodochrosite. The texture and structure of rhodochrosite aggregates indicate that manganese carbonates already began to form at the diagenetic stage and were recrystallized during the subsequent lithogenetic stages. Isotope data (δ13C from ?8.9 to ?17.1‰ PDB) definitely indicate that the oxidized organic matter of sediment served as the main source of carbon dioxide required to form manganese carbonates. Carbonates from host rocks and manganese ores have principally different carbon isotopic compositions. Unlike carbonates of host rocks, manganese carbonates were formed with an active participation of biogeochemical processes. Further processes of metagenesis (T ≈ 250–300°C, P ≈ 2 kbar) resulted in the transformation of textures, structures, and mineral composition of all rocks of the deposit. In particular, increase in temperature and pressure provided the formation of numerous silicates in manganese ores. 相似文献
8.
The differentiated Mesozoic alkali dolerite Prospect Intrusion contains a wide range of secondary minerals, including carbonates (primarily calcite), laumontite, prehnite and heulandite, whose stability relationships imply a formation temperature of <200°C. The δ18O data for carbonates define a higher temperature (160 – 195°C) suite, and a lower temperature (51 – 73°C) suite. The δ13C, δ18O and 87Sr/86Sr isotope systematics for these carbonates suggest derivation of the higher temperature group from magmatic fluids, whereas the other group had a major meteoric component that probably originated from porewater in the country rock. Source fluids for prehnite were meteoric rather than magmatic in origin based on their δD and δ18O ratios. Early in the intrusion's emplacement, CO2-rich hydrothermal fluids formed a carbonate rind sealing the upper part of the hydrothermal system and produced the higher temperature carbonates (calcite) and laumontite. Later, cooler fluids with a meteoric component infiltrated vesicles and fractures, depositing the lower temperature carbonates (calcite, aragonite), heulandite and prehnite. 相似文献
9.
Virginie Garnier Gaston Giuliani Daniel Ohnenstetter Anthony E. Fallick Jean Dubessy David Banks Hong Quang Vinh Thrse Lhomme Henri Maluski Arnaud Pêcher Kausar Allah Bakhsh Pham Van Long Phan Trong Trinh Dietmar Schwarz 《Ore Geology Reviews》2008,34(1-2):169
Marble-hosted ruby deposits represent the most important source of colored gemstones from Central and South East Asia. These deposits are located in the Himalayan mountain belt which developed during Tertiary collision of the Indian plate northward into the Eurasian plate. They are spatially related to granitoid intrusions and are contained in platform carbonates series that underwent high-grade metamorphism. All occurrences are located close to major tectonic features formed during Himalayan orogenesis, directly in suture zones in the Himalayas, or in shear zones that guided extrusion of the Indochina block after the collision in South East Asia. Ar–Ar dating of micas syngenetic with ruby and U–Pb dating of zircon included in ruby gives evidence that these deposits formed during Himalayan orogenesis, and the ages document the extensional tectonics that were active, from Afghanistan to Vietnam, between the Oligocene and the Pliocene.The petrography shows that ruby-bearing marbles formed in the amphibolite facies (T = 610 to 790 °C and P ~ 6 kbar). A fluid inclusion study defines the conditions of gem ruby formation during the retrograde metamorphic path (620 < T < 670 °C and 2.6 < P < 3.3 kbar) for the deposits of Jegdalek, Hunza and northern Vietnam.Whole rock analyses of non-ruby-bearing marbles indicate that they contain enough aluminum and chromiferous elements to produce all the ruby crystals that they contain. In addition, (C, O)-isotopic analyses of carbonates from the marbles lead to the conclusion that the marbles acted as a metamorphic closed fluid system that were not infiltrated by externally-derived fluids. The carbon isotopic composition of graphite in marbles reveals that it is of organic origin and that it exchanged C-isotopes with the carbonates during metamorphism. Moreover, the O-isotopic composition of ruby was buffered by metamorphic CO2 released during devolatilisation of marble and the H-isotopic composition of mica is consistent with a metamorphic origin for water in equilibrium with the micas. The (C, O, H)-isotopic compositions of minerals associated with marble-hosted ruby are all in agreement with the hypothesis, drawn from the unusual chemistry of CO2–H2S–COS–S8–AlO(OH)-bearing fluids contained in fluid inclusions, that gem ruby formed at P ~ 3 kbar and 620 < T < 670 °C, during thermal reduction of evaporite by organic matter, at high temperature-medium pressure metamorphism of platform carbonates during the Tertiary India–Asia collision. The carbonates were enriched in Al- and chromiferous-bearing detrital minerals, such as clay minerals that were deposited on the platform with the carbonates, and in organic matter. Ruby formed during the retrograde metamorphic path, mainly by destabilization of muscovite or spinel. The metamorphic fluid system was rich in CO2 released from devolatilisation of carbonates, and in fluorine, chlorine and boron released by molten salts (NaCl, KCl, CaSO4). Evaporites are key to explaining the formation of these deposits. Molten salts mobilized in situ Al and metal transition elements contained in marbles, leading to crystallization of ruby. 相似文献
10.
A study on the environmental conditions of the microsparite （Molar-tooth） carbonates 总被引：1，自引：0，他引：1
Along with the progress in research on the Precambrian, Molar-tooth carbonates (simplified as MT, or microsparite carbonates or MT structure) which were formed in the Middle-Late Proterozoic have become a hot subject recently. The Proterozoic Molar-tooth (MT) carbonate rocks refer to those Meso- to Neoproterozoic (1600-650 Ma) carbonates with MT structure, i.e., a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age, located in the environment of mid- to inner ramp and shallow platform. MTS, like a bridge connecting the inorganic world with the organic one, are closely related to the evolution of paleo-oceans, atmosphere and biosphere. Their development and/or recession are/is related to the origin of life and the abruption of sedimentary geochemistry events of marine carbonates. By using modern instruments and testing methods adequately, the contents of oxides in sandstones were measured and the REE distribution pattern curves were established; an accurate value of isotopic ratio of 87Sr/86Sr was obtained, that is, the age of MT formation is about 750-900 Ma; C and O isotopes of some fresh micrite limestone samples were analyzed; the energy spectrum analysis revealed that the MT consists mainly of microspar calcite, while as for its chemical composition, the matrix shows outstanding peaks of Ca, Mg, Al, Si, and K. The geochemical indicators proved that Neoproterozoic MT carbonates in the Jilin-Liaoning region were developed at the margin of a stable continent, in the torrid zone where the paleo-temperature was about 50℃, the seawater had normal salinity when MT was formed during the Wanlong period in southern Jilin and during the Yingchengzi and Xingmincun periods in eastern Liaoning. The sedimentary environment was located in the inner ramp. In summary, it is of great importance to understand the origin of MT, ascertain the paleo-climate and paleo-environment characteristics, constrain the age and the stratigraphic division and comparison of the Proterozoic so as to study the geochemical characteristics of MT carbonates and their formation environment. 相似文献
11.
K.L. Thomas-Keprta S.J. Clemett D.S. McKay S.J. Wentworth 《Geochimica et cosmochimica acta》2009,73(21):6631-5335
The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe3O4) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of magnetite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the compositional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates. 相似文献
12.
徐州—淮南地区新元古代臼齿碳酸盐岩成因探讨 总被引：8，自引：2，他引：8
臼齿碳酸盐岩是一种发育在中—新元古代，由微亮晶方解石组成的复杂褶皱构造，其成因一百多年来一直是个谜。通过对徐州-淮南地区新元古代臼齿碳酸盐岩特征、形成环境、分布时限研究，探讨了臼齿碳酸盐岩成因。该区臼齿碳酸盐岩发育在台地缓坡沉积体系中，可作为潮下浅水环境的标志，其形态在微层序中的分布反映沉积环境，具有重要的环境意义。建立了臼齿碳酸盐岩微相环境模式。快速石化作用形成均匀、等粒微亮晶方解石，是臼齿碳酸盐岩显著特征。现代类似于发育臼齿碳酸盐岩潮下环境没有这样的快速石化条件，它要求更高的超过饱和CaCO3沉淀。臼齿碳酸盐岩在本区分布时限介于850～720Ma间，在Sturtian冰期之前消失，与全球其他地区臼齿碳酸盐岩消失的时限一致，Sturtian冰期改变了海洋化学性质，是臼齿碳酸盐岩消失的根本原因。Sturtian冰期对海洋化学性质的影响，可能是揭开臼齿碳酸盐岩成因机理的一个重要新途径。 相似文献
13.
Results of the study of mineral, chemical, and isotopic compositions of carbonate rocks and ores from the Hukh Tag manganese ore manifestation suggest its hydrothermal origin. Secondary manganese carbonates and oxides were probably formed during several stages of tectonic reactivation with the participation of atmospheric (meteoric) waters. 相似文献
14.
疆东天山觉洛塔格地区梧桐窝子岩组构造环境探讨 总被引：7，自引：0，他引：7
梧桐窝子岩组为晚石炭世海底扩张喷发形成的一套中基性火山岩-细碧岩．硅质岩-碳酸盐岩建造．通过对剖面组合特征研究，认为该岩组是由辉长岩-辉绿岩-玄武岩．放射虫硅质岩组成的洋壳残片和岛弧物质的安山岩、凝灰岩、沉凝灰岩及弧前沉积的凝灰质砂岩、钙质砂岩、微晶大理岩、正常碎屑岩(糜棱岩、千糜岩)3部分组成的蛇绿混杂岩带．三所处构造背景、沉积环境不同，在岩石学、岩石化学及岩石地球化学特征上亦有较大差异． 相似文献
15.
新元古代晚期盖帽碳酸盐岩的成因与“雪球地球”的终结机制 总被引：5，自引：3，他引：5
新元古代晚期约635 Ma的地球发育了到达赤道附近的冰川作用，地质记录上表现为代表寒冷气候的冰期沉积杂砾岩，直接被代表温暖环境的碳酸盐岩层（常称盖帽碳酸盐岩）覆盖。由于盖帽碳酸盐岩奇特的岩石学和地球化学特征，引起了对其成因认识的巨大争论，提出了“雪球地球”和“甲烷渗漏”等假说。“雪球地球”假设可以解释一些令人困惑的地学现象，如低纬度和低海拔冰川沉积、盖帽碳酸盐岩、碳酸盐δ13C负漂移和条带状铁矿层等，但许多科学家对此提出了质疑。最近对盖帽碳酸盐岩的δ13C分析结果（最低达-41‰）、盖帽碳酸盐岩发育的类似现代冷泉碳酸盐岩沉积组构等似乎支持“甲烷渗漏”假说。 相似文献
16.
贵州松桃大塘坡地区南华纪早期冷泉碳酸盐岩地质地球化学特征 总被引：13，自引：2，他引：11
贵州松桃大塘坡地区的南华系是研究古代天然气渗漏及冷泉碳酸盐岩的理想地区之一.该区冷泉碳酸盐岩由两类岩石组成: 一是两界河组含砾砂岩中(Sturtian冰期早期) 的白云岩丘或透镜体; 二是之上的大塘坡组第一段黑色含锰岩系中的似层状或透镜状菱锰矿矿体及少量白云岩透镜体.该地区冷泉碳酸盐岩的碳同位素具明显的负偏特征, 充填在菱锰矿矿石气孔中的沥青碳同位素为-30.98‰, 并具有异常高的硫同位素正值.结合冷泉碳酸盐岩的空间分布、古天然气渗漏构造、菱锰矿的沉积有机质等特征分析, 认为其与现代冷泉碳酸盐岩的特征十分相近.两界河组的白云岩丘是目前所发现的最古老的冷泉碳酸盐岩, 与在它之上的菱锰矿矿体是同一个古天然气渗漏系统中不同时期的冷泉碳酸盐沉积.天然气泄漏形成的冷泉与大塘坡组烃源岩的形成关系密切. 相似文献
17.
This paper reports the genetic links among the depth distribution, mineralogy, and stable isotopic composition of diagenetic carbonates with sedimentation rates and types and preservation of organic matter in the terrigenous and biogenic sediments of Oligocene and Miocene age on the New Jersey slope. Calcites formed close to the sediment surface at sequence boundaries and maximum flooding surfaces, when the profile of early-diagenetic reactions was stabilized in the sediment column for extended periods. Dolomites precipitated in the sulfate reduction zone when diagenetic profiles stabilized during truncation, sequence boundary formation, and the deposition of lowstand sediments that overlie the sequence boundaries. Most dolomites occur in distal slope sediments that were deposited before the shelf had prograded into the study area. Siderites formed during a later stage of burial in the methanogenic zone; they are not directly genetically related to the sequence stratigraphy of the New Jersey slope. The diagene-tic dolomites and siderites occur in widely separated depth intervals below the present sea floor. The distribution of the diagenetic carbonates and their preferential occurrence in separated depth intervals resulted from different combinations of sedimentation rates and organic matter types and preservation. 相似文献
18.
《International Geology Review》2012,54(4):307-316
Stratabound barite and celestite deposits, related mainly to three evaporitic sequences, occur in the Mesozoic Neuquen Retroarc Basin, developed to the east of the Andean Cordillera of western Argentina. This basin is filled with Jurassic and Cretaceous marine to continental sediments that unconformably overlie basement rocks of Paleozoic to Triassic age. Celestite deposits formed by initial precipitation from seawater, with later crystallization during diagenesis and recrystallization related to Eocene intrusive activity. This is supported by evidence of evaporitic associations, textures, and Sr and S-isotope data. The barite deposits were deposited in a near-shore environment and could have formed as a result of interaction between barium absorbed in clay minerals (derived from weathering of basement rocks) and hypersaline seawater. This genetic model is supported by evidence such as the stratabound setting, textures, and Sr and S-isotope data. Carbon and oxygen-isotopic compositions of carbonates, which are interbedded with celestites, are in the range expected for marine carbonates. Lead-isotopic compositions of galenas from bedded and vein barite deposits of Upper Jurassic and Lower Cretaceous ages are consistent with Pb remobilization from a source located at high levels of the upper crust, possibly the basement rocks. 相似文献
19.
A. Stadnitskaia D. Nadezhkin V. Blinova J.S. Sinninghe Damsté 《Geochimica et cosmochimica acta》2008,72(7):1824-1836
Carbonate chimneys and other carbonate structures occur widespread in the Gulf of Cadiz and probably reflect the presence of cold seeps and associated release of methane in the geological past, possibly in the Early Pleistocene, but it is unclear under what conditions and by which processes these carbonates were formed. We studied a fossil methane-related carbonate crust collected from the Kidd mud volcano in the gulf. Concentrations of microbial lipids, their stable carbon isotope composition, sequences of fossil 16S rRNA genes of anaerobic methanotrophic archaea in combination with mineralogical and carbon and oxygen isotopic composition of carbonate were obtained for seven different horizons of the crust. This combination of organic and inorganic geochemical techniques with molecular ecological methods gave a consistent view on processes resulting in the formation of the crust and indicated that it took place in two phases and in a downward direction. Archaeal lipid biomarkers and fossil 16S rRNA gene sequence data revealed the dominance of archaeal ANME-2 group and elevated methane partial pressures during the formation of the top part of the crust. The lower part of the carbonate was likely formed in an environment with reduced methane fluxes as revealed by the dominance of fossil remains of ANME-1 archaea. The combination of these methods can be used as an effective tool to reconstruct in unprecedented detail the palaeo-biogeochemical processes resulting in the formation of carbonate fabrics. This interdisciplinary strategy may also be applied for other fossil methane-derived carbonates, generating new concepts and knowledge about past methane-related carbonate systems. 相似文献
20.
Depositional environments and facies of the Late Triassic Abu Ruweis Formation, Jordan 总被引：1，自引：0，他引：1
The Abu Ruweis Formation is composed of carbonates, evaporites, and mudstones, with some locally developed pelletic, oolitic and stromatolitic limestones. The lateral persistence of bedding, the purity of the evaporite rocks, the alternating arrangement of marine carbonates and evaporites indicates periodic deposition in subaqueous conditions (salina). Petrographic investigations, X-ray diffraction analysis as well as chemical analysis have shown that the outcropping evaporite beds are mainly composed of secondary gypsum, with rare anhydrite relics. Five microfacies of gypsum were recognized according to their fabrics: porphyroblastic and granoblastic gypsum showing polarization texture, gypsum pseudomorph after anhydrite laths, and satin spar gypsum. The textures they display indicate a hydration origin of precursor anhydrite, which is in turn rehydrated from primary gypsum. Some of these anhydrites were formed as a result of replacement processes of the carbonate sediments associated with the evaporites, as evidenced from the textural relationships of the carbonate and sulfate minerals. The O18 content ranges from 1.45 to 8.38% PDB and the C13 content ranges from −1.52 to 4.73% PDB. Trace elements analysis has shown that the Abu Ruweis dolomites are rich in strontium (up to 600 ppm), and sodium (up to 835 ppm). The isotope composition and trace elements content, as well as the petrographic characteristics point to a penecontemporaneous hypersaline dolomitization origin for the Abu Ruweis dolomites. The evaporites were deposited during a regressive lowstand systems tract, whereas the carbonates were deposited under shallow water marine conditions during a highstand systems tract. The Abu Ruweis succession represents a relatively stable arid climate within a rapidly subsiding basin. Restricted conditions were provided by the development of beach barriers. 相似文献