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1.
New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy 总被引：2，自引：0，他引：2
PHILIP A. SANDBERG 《Sedimentology》1975,22(4):497-537
Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non-skeletal grains) or to paramorphic replacement (proposed for non-skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non-skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non-skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non-skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic-Cretaceous time and by abundant younger, Mg-poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non-skeletal carbonates was probably during early to middle Cenozoic time. 相似文献
2.
The question of a primary versus diagenetic origin for the limestone-shale rhythms of the Blue Lias has been addressed through a study of pyrite abundance and isotopic composition. Pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent, less calcareous, sediment. The abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation. The isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate, part of which was subsequently reduced back to pyrite before preservation by burial. Acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution, hence supersaturation of sulphides could be maintained. By contrast, in adjacent less calcareous sediments, carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron-rich pore solutions. Continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and, with burial below the zone of bioturbation, the alkalinity so generated caused cementation of the limestones. Diagenetic cementation would be enhanced during an hiatus in sedimentation, an event which might be related to a Milankovitch forcing mechanism, but which would not be recorded in bioturbated, less calcareous sediment, thus leaving an imperfect record. Only cyclicity in pre-diagenetic sedimentation patterns may be safely related to a Milankovitch forcing mechanism as proposed by Weedon. 相似文献
3.
NILS-MARTIN HANKEN 《Sedimentology》1979,26(1):135-142
Moulds after aragonite fossils from two Upper Ordovician limestones in the Oslo Region are filled with well sorted clastic fine sand. The fossil moulds are thought to have been formed by selective dissolution of aragonite shell material by fresh water in the vadose zone. Internal sedimentation post-dates precipitation of a thin veneer of iron poor drusy calcite cement, but predates precipitation of ferroan blocky calcite cement. These age relationships and the texture of the fine sand suggests sedimentation in semiconsolidated sediment in the vadose zone of an island during early emergence. 相似文献
4.
安徽铜陵冬瓜山矿床中磁黄铁矿矿石结构特征及其成因意义 总被引：9，自引：3，他引：6
安徽铜陵冬瓜山矿床是长江中下游地区具有代表性的大型层状硫化物矿床,磁黄铁矿为矿床中的主要硫化物矿物.该矿床主要由层状硫化物矿体组成,伴有矽卡岩型和斑岩型矿体.在层状矿体上部,磁黄铁矿主要为块状构造,而层状矿体下部,磁黄铁矿多为层纹状、条带状构造,具有显著的沉积结构构造特征.野外地质观察及室内矿相学研究表明,层状矿体中磁黄铁矿矿石遭受了强烈的变质作用及热液交代作用.进变质过程中形成的结构主要为胶黄铁矿转变为黄铁矿以及进一步变质转变为磁黄铁矿、磁铁矿时形成的交代残留结构.退变质过程则以磁黄铁矿的退火、黄铁矿变斑晶的生长和单纯六方磁黄铁矿的形成为特征.岩浆热液对单纯六方磁黄铁矿的交代作用形成了单斜和六方磁黄铁矿的交生结构.这些结构特征表明层状矿体中的磁黄铁矿并不是岩浆热液成因,而主要为石炭纪同生沉积胶黄铁矿、黄铁矿在燕山期岩浆侵入所引起的热变质作用下脱硫所形成,并在热变质作用之后又受到岩浆热液的叠加交代.磁黄铁矿的结构特征显示冬瓜山矿床的形成经历了同生沉积、热变质、热液交代等多个阶段,支持其为同生沉积-叠加改造型矿床. 相似文献
5.
《International Geology Review》2012,54(1):65-72
Chert-plus-pyrite pebbles have been known for some time in a number of Witwatersrand conglomerates, but their distribution is highly variable. A wide variety of textures in chert-plus pyrite pebbles are documented here, and these textures place constraints on the origin of such pebbles. Replacement of chert pebbles by pyrite is indicated (pyrite grains and aggregates continue across many pebble boundaries), and both distributional and textural evidence favors a post-burial timing for this process. Significant mobility of sulfur after burial is indicated. Whether this replacement was diagenetic or metamorphic is not certain. 相似文献
6.
西藏北部西雅尔岗地区白垩-第三系红层的成岩作用及成岩环境 总被引：1，自引：0，他引：1
西雅尔岗地区位于羌塘高原腹地，地理范围为东经88°00′至89°00′，北纬32°40′至33°40′.大地构造位置从属于羌塘地块的玛依岗日一阿木冈基底隆断带.在晚白坚一第三纪时期，受青藏高原整沐抬升的影响，沉积了一套巨厚的红色陆屑建造，其基本特征为沉积厚度大、粗粒沉积所占比例高、沉积类型复杂、横向及纵向上相变快、岩石的颜色多为红色及紫色.沉积相类型主要为冲积扇、扇三角洲及湖泊.化石类型卖要为植物〔柏型枝)，孢粉和鸟足印化石.反映了在干操气候条件下，快速堆积的沉积特征. 根据对该区岩石学的研究，发现岩石的矿物组合和结构类型皆因成岩作用的影响而发生了很大的变化.矿物组合的变化是由于沉积后附加的陆源机械渗人粘土，在地表条件下不稳定矿物及岩屑的溶解作用以及在成岩过程中自生矿物的形成作用所造成的.导致结构类型改变的原因是(1)机械渗人的碎屑粘土及自生的粘土矿物形成孔隙间的粘土质“成岩杂基”.而这些孔隙在最初沉积时则没有杂基存在;(2)松散沉积物在成岩过程中，结构颖粒的溶解作用;(3)由于压实及压溶作用而形成粉砂级及其它细小的颗粒;(4)交代作用及重结晶作用.这些成岩过程明显地反映了整个区域岩石的演化历史. 显微镜、扫描电镜的观察以及红外光谱的侧定结果表明该区红层的形成是由于在成岩过程中赤铁矿的沉淀而造成的.在扫描电镜下可清楚地识别出赤铁矿的自生晶形.由此可以断定赤铁矿在其形成过程中显然是起了染色剂的作用.它的形成主要受孔隙水的氧化还原条件所控制，似乎于气候的关系不大，尽管它是产于干旱条件的沉积物中. 基于对该区岩石的成岩作用及成岩历史演化序列的研究，建立了该区沉积后的成岩环境模式，亦即(l)早期的表生成岩浅埋藏环境，该环境明显受到沉积环境、沉积作用控制以及气候因素的影响;(2)晚期的成岩深埋藏环境，该环境则主要受地温梯度、压力、温度和孔隙水的地球化学条件的控制. 相似文献
7.
L. Ya. Kizil’shtein 《Lithology and Mineral Resources》2006,41(6):570-575
The study of specific features of the pyritization of mollusk fossil shells has provided new evidence of the relationship between the generation of hydrosulfides during the bacterial reduction of sulfates and the composition of organic matter (OM) exploited by bacteria in processes of metabolism. The OM is represented by conchiolin of the ammonite shell frustule. Interaction between the bacterial H2S and Fe2+ fosters the pseudomorphous replacement of conchiolin by the colloidal iron monosulfide that is subsequently transformed into pyrite. Hydrogen sulfide and/or monosulfide migrate into diagenetic cracks and cavities formed in the clayey—carbonate matrix that fills up the interior cavity of a shell. We believe that the data reported in this communication should be taken into consideration in the study of formation constraints of vein and disseminated sulfide mineralization in sedimentary rocks during the early diagenesis and related problems of ore formation. 相似文献
8.
MICHEL LAMBOY 《Sedimentology》1993,40(1):53-62
Fabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with X-ray dispersive microanalysis. Replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests; replacement proceeds at constant volume with excellent preservation of the original microtextures. In some deposits, replacement of carbonate by apatite is the main phosphogenic process. However, in general, the process seems to be far less important than might be believed purely on the basis of thin section observations. In many phosphorites, internal or external apatite moulds of bioclasts are common, including very small particles such as coccoliths in phosphatized chalks. Apatite precipitation was typically followed by carbonate dissolution. Later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain. In these examples direct replacement of carbonate by phosphate cannot be demonstrated. 相似文献
9.
长江中下游成矿带庐枞盆地小包庄铁矿床地质特征研究 总被引：3，自引：0，他引：3
罗河铁矿床位于长江中下游成矿带内庐枞火山岩盆地的西北部,是成矿带内已发现规模最大的铁矿床。2013年在罗河铁矿床深部又勘探新发现了小包庄大型铁矿床,这是长江中下游成矿带内近年来重大找矿突破之一,具有重要的理论研究意义和勘探应用价值。本文在前人工作基础上,基于详细的钻孔观察和系统的岩相学、矿相学工作并结合电子探针测试分析,研究了小包庄铁矿床的矿化蚀变特征,厘定了矿床的成矿阶段,分析了成矿作用过程,并初步探讨了矿床成因。研究表明,罗河铁矿床和小包庄铁矿床为同一成矿系统在不同深度成矿作用的产物。小包庄铁矿床主矿体矿呈厚大的透镜状、似层状产于砖桥组地层中,位于罗河铁矿床主矿体之下约800~1000m,主要由浸染状矿体组成。矿床中金属矿物主要为磁铁矿和黄铁矿,非金属矿物主要为硬石膏、透辉石和碳酸盐,矿石的代表性矿物组合为磁铁矿-硬石膏-透辉石。矿石的结构构造主要有浸染状构造、脉状构造、块状构造、自形-半自形粒状结构、他形粒状结构和筛状结构等。矿床围岩蚀变强烈,主要蚀变类型有碱性长石化、透辉石化、绿泥石化、绿帘石化、碳酸盐化和硬石膏化。小包庄铁矿床形成经历了热液期的四个阶段,即碱性长石阶段、透辉石-硬石膏-磁铁矿阶段、绿泥石-绿帘石-碳酸盐阶段和硬石膏-黄铁矿-碳酸盐-石英阶段,其中,铁矿化主要发育于透辉石-硬石膏-磁铁矿阶段。通过矿床地质特征的分析以及与宁芜地区铁矿床的对比研究,本文认为小包庄铁矿床成矿物质和成矿流体来源于深部的闪长质侵入岩(?),而矿化发育在远离侵入岩或次火山岩之上的火山岩中,明显有别于宁芜地区玢岩铁矿床,类似于智利安第斯成矿带中部分产于安山质火山岩中的磁铁矿-磷灰石型矿床,是长江中下游成矿带中产于火山岩中的一类特殊类型的玢岩型铁矿。 相似文献
10.
《Journal of African Earth Sciences》2005,41(1-2):1-23
Field, petrographic and stable isotopic evidence indicate the former presence of widespread evaporites in the Neoarchaean Campbellrand Subgroup of South Africa. Calcitization of the vanished but once laterally-extensive evaporites was apparently driven by bacterial sulphate reduction of solid sulphate in association with organic diagenesis and pyrite precipitation within platform-wide microbialites and sapropels. This counters current interpretations that much of the calcite was precipitated directly on the seafloor or in primary voids in open marine conditions controlled by regional seawater chemistry. Rather, large-scale microbial mediation of ambient waters across a shallow to emergent platform raised carbonate alkalinity and removed kinetic inhibitors to carbonate formation.The low preservation potential of Precambrian solid sulphate is related in part to bacterial sulphate reduction within the microbially-dominated ecosystems of which cyanobacteria were a major component. Evidence for the former presence of solid sulphate in shallow Neoarchaean seas includes pseudomorphs after selenite, also recorded from the contemporaneous Carawine Dolomite of Australia, together with rock fabrics and textures typical of evaporite dissolution. Importantly, sulphur isotopes of pyrite samples from the Cambellrand carbonates show a wide range of values indicating biogenic fractionation of sulphate, a signature also seen in the Neoarchaean Belingwe Greenstone Belt of Zimbabwe, and the Mt McRae and Jeerinah shales of Western Australia.Mass microbial colonization across extensive Neoarchaean epeiric seas witnessed the microbiogeochemical transformation of the Earth’s hydrosphere, atmosphere and biosphere. The consequences for a reducing ocean would be the progressive oxidation of the major dissolved species in surface seawater, most notably of reduced sulphur and iron. Cyanobacterial photosynthetic oxidation of surface seawater drove formation of aqueous sulphate and permitted the precipitation of extensive evaporites in restricted basins, perhaps beginning the process of ridding the oceans of reduced sulphur. The first dramatic explosion of carbonate precipitation can be related to intense bacterial sulphate reduction in association with anoxic organic diagenesis and pyrite formation within the decaying interiors of microbialites and in sapropels. 相似文献
11.
The composition and structure of mineral substance in shells of three Jurassic ammonites (Cadoceras elatmae) from the Makar’ev South outcrop in Kostroma oblast, Russia, have been studied. The studied shells differ in degree of preservation.
The shell of one ammonite with partly retained internal structure has been replaced with quartz, carbonate-bearing apatite-(CaF),
and newly formed aragonite. Other shells without retained internal structure underwent phosphatization rather than replacement
with quartz. This difference serves as the basis for a discussion on fossilization conditions. Silicification of the shell
supports preservation of its wall and elements of the internal structure. It is suggested that quartz might be biogenic in
origin and was formed in cavities of phosphate substance, which had replaced the soft tissue of ammonites. 相似文献
12.
Clumped isotopes analyses in modern land snail shells are reported and used to interpret shell oxygen isotopes within the context of terrestrial paleo-climatology. Carbonate clumped isotopes thermometry is a new technique for estimating the temperature of formation of carbonate minerals. It is most powerful as an indicator of environmental parameters in combination with δ18O, allowing the partitioning of the δ18O signal into its temperature and water components. Results indicate that snail shell calcification temperatures are typically higher than either the mean annual or the snail activity season ambient temperatures. Small inter- and intra-snail variability suggests that shell aragonite forms at isotopic equilibrium so that the derived temperatures are an eco-physiological parameter reflecting snail body temperature at the time of calcification. We attribute these higher body temperatures to snail eco-physiological adaptations through shell color, morphology, and behavior. In combination with shell oxygen isotope composition, these temperatures allow us to calculate snail body water composition, which is in turn interpreted as a paleo-hydrological indicator, reflecting isotopic composition of local precipitation modified by local evaporation. 相似文献
13.
Shells of the aragonite bivalve Neomiodon (Great Estuarine Group, Jurassic, Scotland) replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure. The presence of these relics in such old altered shells, as well as the high Sr content of the replacement calcite, indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events, but rather a rapid, one-step process. Aragonite relics, once encased in neomorphic spar, will survive as unequivocal evidence of original aragonite mineralogy, barring total remobilization of the enclosing stable calcite, a generally unlikely event. The retention of this residual aragonite and high-Sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration (‘recrystallization’) invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones. Retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or, as in the case of our Neomiodon material, deeper burial environments. Pseudopleochroism of the replaced Neomiodon shells appears to be due to organic, largely graphitic, relics, not to the aragonite relics. 相似文献
14.
Alan C. Kendall 《Sedimentology》2001,48(1):29-55
Mississippian nodular anhydrites beneath an unconformity in the subsurface of southern Saskatchewan are locally replaced by calcite, pyrite and celestite. Triassic clastics above the unconformity are green, rather than red, and a usually developed subunconformity alteration zone (where carbonates are dolomitized, and porosity is filled with anhydrite) is absent. The unconformity lacks karstic features (unlike in the USA), and probably formed in a hyperarid climate. Mississippian anhydrites near the unconformity are not preferentially dissolved, nor were they extensively hydrated. Anhydrite calcitization occurred only after the unconformity was shallowly buried by redbeds, and it probably involved sulphate-reducing bacteria. Hydrogen sulphide, generated by bacteria, reduced redbed pigments. The replacement calcite contains pseudomorphs and relicts of anhydrite, and pseudomorphs of secondary gypsum. These indicate calcitization occurred only after original Mississippian gypsum was altered to anhydrite and this, in turn, was partially converted back to secondary gypsum beneath the unconformity. Replacement occurred concurrently with the formation elsewhere of the dolomitized zone beneath the unconformity. Sulphur isotopic ratios of replacement pyrite are depleted relative to Mississippian sulphate values, consistent with the activities of sulphate-reducing bacteria. Carbon isotopic ratios of replacive calcites, however, do not support this interpretation, and are identical to those of Mississippian limestones. Simple replacement of sulphate by pore-water bicarbonate (in equilibrium with host limestones) is unlikely because protons generated during the reaction should have created acidic conditions in which calcite would have dissolved. A full explanation of the calcitization remains elusive, but may involve replacement occurring in an active groundwater system and/or bacterial sulphate reduction occurring upstream of the site of calcitization. 相似文献
15.
J. D. HUDSON 《Sedimentology》1982,29(5):639-667
Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected-light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro-environments, while the sediment generally was not wholly anoxic. Most pyrite pre-dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from ?55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas-filled. In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro-nodules. Larger nodules show a variety of forms, some of which post-date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near ?20. Pyrite formed over a long time-span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void-fillings and concretions, and which help define the diagenetic history of a shale. 相似文献
16.
九种现代双壳类壳体物相组成的对比研究 总被引：2，自引：0，他引：2
利用X射线衍射分析技术,对黄东海常见的9种现代双壳类壳体进行了物相分析,结果表明这些双壳类壳体分别属于三种类型,即文石质壳、方解石质壳、文石+方解石混合质壳。其中,菲律宾蛤仔(Ruditapesphilippinarum)、中国蛤蜊(Mactrachinensis)、四角蛤蜊(Mactraveneriformis)、薄片镜蛤(Dosinialaminata)、毛蚶(Scapharcasubcrenata)属于文石质壳,长牡蛎(Crassostreagigas)、中国不等蛤(Anomiachinensis)、海湾扇贝(Argopectenirradieus)属于方解石质壳,紫贻贝(Mytilusedulis)属于文石+方解石质的混合质壳;双壳类壳体物相组成与其生活方式有着一定的联系,营底栖埋入式生活者趋于形成文石质壳,底栖固着生活方式者趋于形成方解石质壳或混合质壳;不同大小的菲律宾蛤仔(Ruditapesphilippinarum)的物相组成上几乎没有差别,表明其壳体物相组成不受生命效应的影响。 相似文献
17.
珍珠层中文石晶体择优取向的XRD极图分析 总被引：4，自引：0，他引：4
本文采用XRD极图方法对海水大珠母贝、企鹅珍珠贝及淡水三角帆蚌贝壳珍珠层中文石晶体的结晶学定向进行了研究。根据文石的 ( 0 0 2 )和 ( 0 12 )极图可以推断 ,三种贝壳珍珠层文石的a轴平行珍珠层面 ,c轴垂直珍珠层面。在珍珠层面上 ,二种海水壳珍珠层中文石的a轴亦存在强烈的择优取向现象 ,大珠母珍珠层中文石a轴沿与壳生长纹方向呈 2 0 0°左右的夹角分布 ,企鹅珍珠贝沿近平行壳生长纹的方向分布 ;淡水三角帆蚌珍珠层文石a轴亦平行珍珠层面分布 ,但其择优取向不如海水壳明显 ,在珍珠层面上沿各个角度上均有分布。 相似文献
18.
Aragonite and calcite preservation in sediments from Lake Iznik related to bottom lake oxygenation and water column depth 
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下载免费PDF全文 This study examines the forcing mechanisms driving long‐term carbonate accumulation and preservation in lacustrine sediments in Lake Iznik (north‐western Turkey) since the last glacial. Currently, carbonates precipitate during summer from the alkaline water column, and the sediments preserve aragonite and calcite. Based on X‐ray diffraction data, carbonate accumulation has changed significantly and striking reversals in the abundance of the two carbonate polymorphs have occurred on a decadal time scale, during the last 31 ka cal bp . Different lines of evidence, such as grain size, organic matter and redox sensitive elements, indicate that reversals in carbonate polymorph abundance arise due to physical changes in the lacustrine setting, for example, water column depth and lake mixing. The aragonite concentrations are remarkably sensitive to climate, and exhibit millennial‐scale oscillations. Extending observations from modern lakes, the Iznik record shows that the aerobic decomposition of organic matter and sulphate reduction are also substantial factors in carbonate preservation over long time periods. Lower lake levels favour aragonite precipitation from supersaturated waters. Prolonged periods of stratification and, consequently, enhanced sulphate reduction favour aragonite preservation. In contrast, prolonged or repeated exposure of the sediment–water interface to oxygen results in in situ aerobic organic matter decomposition, eventually leading to carbonate dissolution. Notably, the Iznik sediment profile raises the hypothesis that different states of lacustrine mixing lead to selective preservation of different carbonate polymorphs. Thus, a change in the entire lake water chemistry is not strictly necessary to favour the preservation of one polymorph over another. Therefore, this investigation is a novel contribution to the carbon cycle in lacustrine systems. 相似文献
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Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates 总被引：4，自引：0，他引：4
JAMES P. HENDRY 《Sedimentology》1993,40(1):87-106
Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ13C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ18O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with 18O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction. 相似文献
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Eclogites from the Jæren nappe in the Caledonian orogenic belt of SW Norway contain aragonite, magnesite and dolomite in quartz‐rich layers. The carbonates comprise composite grains that occur interstitially between phases of the eclogite facies assemblage: garnet + omphacite + zoisite + clinozoisite + quartz + apatite + rutile ± dolomite ± kyanite ± phengite. Pressure and temperature conditions for the main eclogite stage are estimated to be 2.3–2.8 GPa and 585–655 °C. Published ultrahigh pressure (UHP) experiments on CaO‐, MgO‐ and CO2‐bearing systems have shown that equilibrium assemblages of aragonite and magnesite form as a result of dolomite breakdown at pressures >5 GPa. As a result, recognition of magnesite and aragonite in eclogite facies rocks has been used as an indicator for UHP conditions. However, petrological testing showed that the samples studied here have not experienced such conditions. Aragonite and magnesite show disequilibrium textures that indicate replacement of magnesite by aragonite. This process is inferred to have occurred via a coupled dissolution–precipitation reaction. The formation of aragonite is constrained to eclogite facies conditions, which implies that the studied rocks have experienced metasomatic, reactive fluid flow during their residence at high pressure (HP) conditions. During decompression, the bimineralic carbonate aggregates were overgrown by rims of dolomite, which partially reacted with aragonite to form Mg‐calcite. The well‐preserved carbonate assemblages and textures observed in the studied samples provide a detailed record of the reaction series that affected the rocks during and after their residence at P–T conditions near the coesite stability field. Recognition of the HP mechanism of magnesite replacement by aragonite provides new insight into metasomatic processes that occur in subduction zones and illustrates how fluids facilitate HP carbonate reactions that do not occur in dry systems at otherwise identical physiochemical conditions. This study documents that caution is warranted in interpreting aragonite‐magnesite associations in eclogite facies rocks as evidence for UHP metamorphic conditions. 相似文献