“白云石（岩）问题”与湖相白云岩研究

 近30年来,微生物白云石模式推动“白云石（岩）问题”前进了一大步,大量的实验和实例证明微生物（硫酸盐还原菌、产甲烷古菌、中度嗜盐有氧细菌等）的代谢活动对于促进白云石低温沉淀起了关键作用。微生物白云石特殊的微结构、微形貌和稳定同位素特征是其主要识别标志。然而,该模式在微生物成岩规模、诱导机制、是否存在纳米细菌化石以及现代微生物白云石成因模式能否用来解释古代白云石的成因等方面仍存在较大争议。我国古代湖相白云岩因其时空分布广、成因机制多样等特点可为“白云石问题”的研究提供良好的条件。     

铁白云石矿物学特征及原生次生成因机制

长期以来,铁白云石的命名划分标准混乱,一直被当成高温及次生的产物.综述了国内外铁白云石的研究进展,详细探讨了铁白云石的矿物学特征,特别在其成因中加入了微生物因素,运用地质微生物学探讨原生铁白云石的成因机制.研究认为:（1）铁白云石命名划分依据应遵照国际矿物学会的原则,以铁是否替代白云石晶格中半数镁的位置为标准,分为白云石和铁白云石;（2）详述铁白云石矿物学及其晶体特征;（3）铁白云石成因分为原生沉淀及次生两种,微生物介导形成原生白云石的过程中,有助于铁进入到白云石晶格中,增加晶格中铁的含量.      

类白云石结构矿物的低温合成及其意义

<正>白云石(Ca Mg(CO3)2,dolomite)的低温形成毫无疑问的是矿物学上最有趣和最长久的难题之一。作为一种常见的碳酸盐矿物,白云石在地质历史时期大量发育,而在现代海洋沉积环境中很少出现,并且在实验室模拟海水条件下也几乎无法低温合成。事实上,"低温白云石问题"可能涉及一系列基本物理化学问题。如:白云石是否可以从溶液中直接沉淀和结晶,亦即无定形前驱体或者其他结晶前驱体是否必须存在。如果存在必要的前驱体,那么这些前驱体通过怎样的反应路径形成有序白云石;白云石是否仅在某些     

四川汉源-峨边地区上震旦统灯影组藻白云岩特征及成因研究

"藻白云岩"术语的提出已有五十年历史,震旦系藻白云岩以其巨大的厚度,精美的原生结构、构造,以及丰富的微生物化石和矿产资源而闻名,但至今其成因仍存在争议。以四川汉源-峨边地区上震旦统灯影组藻白云岩为研究对象,对其宏观的剖面、岩石学、原生构造特征,微观的白云石和微生物化石形态、原生结构特征,以及相应的地球化学特征进行了详细研究。结果表明:研究区灯影组藻白云岩为一套在潮坪和潟湖环境下形成的微生物（碳酸盐）岩的岩石类型组合,以藻黏结型的叠层状、纹层状、葡萄状和均一状（藻）白云岩为主要的次级类型;藻白云岩中主要发育原生的隐晶状微生物白云石和次生的纤维状拟晶白云石,其形成与蓝细菌、硫酸盐还原细菌、中度嗜盐好氧细菌和红藻等微生物的矿化作用密切相关:沉积-同生阶段,主要由微生物诱导矿化作用形成大量纳米似立方体粒状和（亚）微米级片状微生物白云石,同时共（伴）生纤维状文石和高镁方解石;同生-准同生阶段,主要由微生物影响矿化作用形成纳米球粒状和微米级不规则状、球状和卵状微生物白云石,同时纤维状文石和高镁方解石因微生物催化矿化作用和拟晶白云石化作用形成纤维状拟晶白云石;随后微生物白云石与拟晶白云石一起组成具纹层状、叠层状、均一状等构造的藻白云岩。对藻白云岩特征及成因的研究有助于理解微生物-矿物交互作用和过程的复杂性、多样性,也为前寒武纪微生物矿化作用、微生物白云石和拟晶白云石研究提供了新的实例。     

微生物成因白云石研究进展

"白云岩问题"一直是沉积地质学研究的热点和难点之一,白云岩在我国和世界范围内都是重要的油气储层。因此,深入认识白云岩成因对于碳酸岩油气勘探具有重要参考意义。白云岩成因有多种解释模式,如萨布哈蒸发模式、渗透回流模式、埋藏调节模式、混合水模式、潮汐泵模式等。近几十年来,随着低温白云石研究的不断深入,微生物白云石模式作为一种新的成因模式被提出并不断被完善。本文回顾了微生物成因白云石的研究进展,总结了低温白云石形成的3个动力学障碍(镁离子的高水合能、硫酸根的存在、碳酸根离子的低浓度和低活度),简要介绍了微生物成因白云石模式的建立、微生物成因白云石的生长过程及发育特征,系统分析了微生物在白云石形成过程中的调节作用,指出微生物(如硫酸盐还原菌、古甲烷菌)的存在可以改变溶液中的离子平衡,进而有利地克服白云石形成过程中的动力学障碍,并列举了低温微生物成因白云石的氧同位素指标在古温度恢复和过去气候变化研究中的应用,最后对微生物成因白云石相关研究方向(如多学科交叉、新技术应用等)加以展望。对微生物成因白云石模式的深入认识,将为正确解释"白云岩问题"提供新的途径,也将为石油学家关心的白云岩储层问题提供新的理论基础和研究思路。     

米粒状白云石及其出溶成因

米粒状白云石产于奥陶系近岸碳酸盐岩序列的云斑(生屑)泥晶灰岩、砾屑灰岩和叠层石灰岩中,在虫孔和砾屑粒间最为富集,外形为伸长菱面体,薄片中呈米粒状.晶体无环带或雾心,但内部含大量呈规则分布的细小方解石“包体”,结晶C轴垂直于晶体延长方向.与背景方解石泥晶相比,Fe、Mn、Na和Ba含量较高,Sr含量较低.C轴的固定取向排除了石膏假晶的可能,也不是普通白云石化或交代钡白云石的结果,推测是沉淀镁方解石重结晶后在压力增加的条件下经出溶作用形成.     

出溶白云石述评

本文介绍了出溶白云石的定义和形成过程,并用晶体化学的观点强调了镁方解石在压力增高的情况下会通过阳离子的有序调整采取出溶形式向稳定矿物相白云石和方解石转变.温度升高虽然可使出溶速度加快,但低温对离子的有序调整更为有利.在自然界,不仅在变质条件下白云石可以从镁方解石中出溶出来,在正常海洋环境中,镁方解石沉积也可被适当压力诱发产生出出溶白云石.     

青海湖底沉积物中球状白云石集合体的发现及其地质意义

青海湖为盐度低于海水的微咸水湖。为了探索白云石的成因,在青海湖二浪尖以北约15 km的湖底采集了长25 cm的沉积物柱状样品。对样品所做的X-衍射矿物物相定量分析表明其中含有2.2％的白云石。文献中也曾报道青海湖底沉积物中含有高达10％的白云石矿物。对这些含白云石矿物的湖底沉积物所进行的扫描电镜研究发现了其中呈球状、椭球状,大小1~2 μm的白云石集合体及其伴生的草莓状黄铁矿的存在。其特征与文献中报道的现代近海高盐度泻湖和萨勃哈中发现的微生物成因的白云石极为相似。这是内陆低盐度环境中此类白云石的首次发现,对于探讨不同盐度环境中白云石的形成机理,促进“白云石问题”的深入研究具有重要的理论意义。     

华北秦皇岛地区柳江盆地马家沟组碳酸盐岩多期白云化作用

华北秦皇岛地区的中奥陶统马家沟组以广泛发育海相白云质灰岩、灰质白云岩和白云岩为特征。在石门寨奥陶系亮甲山剖面,马家沟组自下而上识别出四类碳酸盐岩:含白云石泥晶灰岩（类型I）、细-粉晶白云岩（类型II）、“麦粒状”细-粉晶白云岩（类型III）和钙质泥晶白云岩（类型IV）。类型I主要由泥晶方解石构成,含三叶虫和介形类生物碎片,少量自形的粉晶白云石呈“漂浮状”分布于压溶缝合线内,基质中少见。岩相学和地球化学特征表明此类白云石形成于埋藏成岩期压溶作用之后,压溶缝为云化流体提供通道,压溶缝内泥质组分的成岩转化可能为白云化作用提供了部分镁离子来源;类型II白云岩主要由自形、半自形不等粒粉晶-细晶白云石构成,白云石普遍具有“雾心亮边”,在背散射和阴极发光照片中白云石可见清晰的多圈亮、暗相间环带。环带和带间主量元素的差异表明白云石经历了埋藏成岩期多期成岩流体的改造;类型III白云岩中白云石呈单向延伸的“米粒”或“麦粒”状,粉晶为主,晶体长轴方向具有垂直结晶轴c的特点,白云石具富铁、贫锰、锶的特点,长、短对角线上钙、镁离子的微小差异以及阴极发光特征表明此类白云石也经历了埋藏成岩期的改造,成岩流体使白云石发生微溶作用可能是导致白云石晶体单向延伸且光学性质固定取向的主要原因;类型IV为钙质泥晶球粒白云岩,含石膏假晶,白云石多为微晶和微亮晶,球粒也多由微晶白云石构成,溶孔发育,但全被亮晶贫铁方解石充填,此类岩石的白云化作用发生得很早,可能形成于潮上带澙湖或潮坪环境。综上所述,研究区马家沟组碳酸盐岩具有经历了不同类型及多期白云化作用的特点。自剖面底部向顶部,白云石的有序度由0.8降至0.47,而去云化作用则呈现逐渐增强的趋势。     

微生物白云岩模式研究进展

白云石（岩）问题一直是沉积学领域长期关注的研究主题之一。近年来,在研究含有白云石的现代自然环境和促进原生白云石的沉淀实验中,都加入了微生物因素,并取得了令世人关注的效果,这无疑为白云石（岩）的成因研究提供了新思路与新途径。在前人研究基础上,总结现有的观察资料和实验结果,将微生物促进白云石沉淀的机理模式归纳为厌氧模式和需氧模式2种,并分别介绍这两种模式中硫酸盐还原细菌、产甲烷菌和嗜盐好氧细菌促进白云石沉淀的机理;与微生物相关的矿物形态学特征中,球形和哑铃形白云石及白云石最初的成核阶段所形成的纳米球粒状结构具有一定代表意义,尤其是纳米球粒状结构可以作为生物矿物学上微生物白云石的标志性结构。通过这些特殊的形态特征来寻找微生物作用的证据,或可为古代相似成因白云石（岩）的成因研究提供一种标志。     

Dolomite in caves: Recent dolomite formation in oxic, non-sulfate environments. Castañar Cave, Spain

Dolomite is a common mineral in the rock record but rare in recent superficial environments. Where it does occur, it is related to anoxic, sulfate-rich environments and microbial activity. The occurrence of some dolomite deposits in caves, however, indicates that dolomite formation is also possible in oxic, non-sulfate settings. Dolomite is forming at 17 °C and in oxic-vadose conditions on the host rocks and aragonite speleothems of the Castañar Cave, Cáceres, Spain. It appears as spheroids and dumbbells 50–300 μm in diameter that internally consist of micron-sized rhombic to rounded crystals. Initially this dolomite is Ca-rich, non-stoichiometric and poorly ordered. Mg-rich solutions allow the precipitation of metastable Mg-rich carbonates, such as huntite. This soon transforms into this Ca-rich dolomite, which later “ages” to form a more stoichiometric dolomite. These dolomites show similarities to those grown under anoxic, sulfate-rich conditions and their presence in caves provides a different setting that may contribute to the understanding of the “dolomite problem”, including their initial formation and later recrystallization processes.     

Dolomite Mountains and the origin of the dolomite rock of which they mainly consist: historical developments and new perspectives

Beginning in the late 18th Century, the Dolomite Mountains in Northern Italy have been the location for major sedimentological developments, from the discovery of the mineral dolomite to the formulation of the coral-reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region. Further, the Dolomite Mountains have inspired voluminous research into the origin of dolomite, questioning whether dolomite is a primary precipitate or a secondary replacement product. Recently, with the recognition that microbes can mediate dolomite precipitation, a new geomicrobiological approach, combining the study of modern natural environments with bacterial culture experiments, is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record. This three-pronged methodology applied to the study of dolomite formation holds great promise for future research into the 'Dolomite Problem' and provides a new impetus to revisit the Dolomite Mountains in the 21st Century.     

塔里木盆地和田1井中寒武统膏岩层段发现原生白云石

岩石学与地球化学研究表明,塔里木盆地和田1井中寒武统膏岩层中白云石为原生白云石。岩石学特征上,白云石晶体微小,属隐晶—微晶(5μm),呈他形—次圆形,并且不含灰质前驱物。白云石与自生石膏有密切联系,根据其与石膏的关系及岩石学特征,可以将其分为两类:A类原生白云石,包裹在石膏内,晶体较大,一般大于3μm;B类原生白云石,石膏分散在其中,具有极好的长柱状晶形,白云石晶体较小,一般小于3μm。地球化学特征上,两类白云石差别也较明显。A类白云石具有高的n(Mg)/n(Ca)比,平均为0.96,接近化学计量白云石。Na_2O,Sr O,Fe O,Mn O平均含量分别为160μg/g,270μg/g,2250μg/g,120μg/g。δ~(13)C(PDB)与δ~(18)O(SMOW)平均值分别为0.3‰,24.6‰;与A类原生白云石相比,B类原生白云石n(Mg)/n(Ca)比较低,平均值为0.90。具有较高的Na_2O,Fe O,Mn O含量与稍低的Sr O含量,平均值分别为290μg/g,2540μg/g,200μg/g,200μg/g。δ~(13)C(PDB)与δ~(18)O(SMOW)变化不大,δ~(13)C稍低,平均值为-0.3‰,δ~(18)O值较高,平均值为25.2‰。研究表明,文中白云石为原生沉淀,而非交代前驱物灰岩形成,Mg的来源为强烈蒸发形成的高盐度咸水。微生物可能参与了白云石沉淀过程。     

Microbial dolomite in the sabkha environment of the middle Cambrian in the Tarim Basin,NW China

In the Tarim Basin, dolomite, which formed during the middle Cambrian associated with evaporites, has been attributed to the sabkha-style dolomite formed during the syndepositional period. The sedimentary microfacies suggests dolomite formation in the middle Cambrian is an ancient analogue of the sabkha of Abu Dhabi. Poorly crystallised dolomite spheroids or ovoids within or on the surface of dolomite crystals are a common phenomenon that can be widely observed in different stromatolites in the upper part of the intertidal zone and strongly resemble the morphology in modern sabkha dolomite-producing microbial mats and in microbial culture experiments. These lines of evidence suggest organic substrates for dolomite nucleation. Dolomite formation in the middle Cambrian in the Tarim Basin has been considered a classic analogue for carbonate and evaporate assemblages. The extent of microbial dolomite in ancient sabkha environments is proposed as an alternative model for dolomite formation, in which the mineral properties of organic substrates play a crucial role.     

微生物白云石模式评述

Vasconcelos等在实验室模拟巴西Lagoa Vermelha潟湖条件,成功地沉淀出白云石,由此提出了微生物白云石模式。该模式在南澳大利亚、阿拉伯海湾、西印度外陆棚边缘以及意大利南部等地区得到了不同研究者的肯定,我国鄂尔多斯盆地奥陶系马家沟组五段白云岩也可以解释为微生物成因。此模式对白云石形成机理的研究具有突破性的意义。当硫酸盐还原细菌利用SO42-时,也吸收了Mg2+,因为Mg2+与SO42-形成了很强的离子对。微生物由于其新陈代谢作用利用SO42-而克服了动力障碍,同时从离子对中释放出Mg2+。在白云石形成的化学反应中,微生物因素把硫酸根离子从反应的抑制剂变成了催化剂。白云石一旦成核,就会在埋藏过程中长大。尽管不是所有的白云石都可以用微生物白云石模式来解释,但野外研究和模拟实验都证明这一模式是有说服力和生命力的。     

Calcium and magnesium‐limited dolomite precipitation at Deep Springs Lake,California

Dolomite [Ca,Mg(CO₃)₂] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay‐fraction dolomite ooze. Based on δ¹⁸O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na‐SO₄‐dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg²⁺ and Ca²⁺ concentrations are ca 1 and 0·3 mm , respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca²⁺ (below 0·01 mm ). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca²⁺ and Mg²⁺ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ¹³C values near to equilibrium with atmospheric CO₂ (ca 3‰) contrasts the formation of isotopically light (organically derived), hard‐lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C‐rich sediments.     

Microbially mediated dolomite in Cambrian stromatolites from the Tarim Basin,north‐west China: implications for the role of organic substrate on dolomite precipitation

Dolomite [CaMg(CO₃)₂] is abundant in sedimentary rocks throughout the geological record, but it is rarely found in modern sediments. Also, it cannot be precipitated under low‐temperature conditions in the laboratory without microbial mediation and, as a result, its origin remains a long‐standing enigma. This study reports biologically mediated dolomite precipitation in ancient microbial mats and biofilms from the Cambrian Tarim Basin. The ambient temperature at the time of dolomite precipitation was estimated from δ¹⁸O values from early diagenetic dolomite, and the presence of structures associated with extracellular polymeric substances (EPS), is composed of fibres arranged in a reticular pattern, would favour epitaxial crystallization of dolomite on an organic substrate. In addition, poorly crystallized dolomite formed nanocrystal aggregates that strongly resemble the morphology and size distribution observed in microbial culture experiments. These lines of evidence confirm that microbial structures can be preserved in ancient dolomite and validate their use as biosignatures.     

Microbial dolomite crusts from the carbonate platform off western India

Abstract The occurrence of Late Pleistocene dolomite crusts that occur at 64 m depth on the carbonate platform off western India is documented. Dolomite is the most predominant mineral in the crusts. In thin section, the crust consists of dolomitized microlaminae interspersed with detrital particles. Under scanning electron microscopy, these laminae are made up of tubular filaments or cellular structures of probable cyanobacterial origin. Dolomite crystals encrust or overgrow the surfaces of the microbial filaments and/or cells; progressive mineralization obliterates their morphology. Well-preserved microbial mats, sulphide minerals (pyrrhotite and marcasite) and the stable isotope composition of dolomite in the crusts indicate hypersaline and anoxic conditions during dolomite formation. The crusts are similar to dolomite stromatolites, and biogeochemical processes related to decaying microbial mats under anoxic conditions probably played an important role in dolomite precipitation. The dolomite is therefore primary and/or very early diagenetic in origin. The dolomite crusts are interpreted to be a composite of microbial dolomite overprinted by early burial organic dolomite. The results of this study suggest that a microbial model for dolomite formation may be relevant for the origin of ancient massive dolomites in marine successions characterized by cryptalgal laminites. The age of the crusts further suggests that the platform was situated at shallow subtidal depths during the Last Glacial Maximum.