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1.
鱼骨状方解石是一种特殊的碳酸盐沉积,由锯齿状亮暗交互的亚毫米级条带组成,主要见于太古宙。以往认为,鱼骨状方解石属无机化学沉淀成因,形成于水体缺氧、碳酸盐过饱和、富Fe~(2+)、Mn~(2+)等碳酸盐沉淀抑制剂的环境条件;在地质记录中其丰度随时间的减少反映了海洋的长期氧化趋势。文中首次在华北地台中元古界高于庄组四段微生物礁内发现了大量鱼骨状方解石。宏观观察表明,这些鱼骨状方解石主要以微生物礁孔洞充填物形式产出,明显区别于太古宙以海底沉淀形式直接产出在海底的鱼骨状方解石。显微研究发现,鱼骨状方解石晶体纤维具有沿晶体生长方向旋转消光特征,证明其内部亚晶的光学C轴从纤维底部的随机排列逐步旋转至上部垂直纤维生长方向。这符合球状晶体生长模式,需要方解石沉淀抑制剂的参与。鱼骨状方解石产出具有丰度随时间减少以及产出形式由海底沉淀向孔洞胶结物转变的特征。这些特征与海洋氧化逐渐增强以及具氧化还原敏感属性的碳酸盐沉淀抑制剂逐渐从水体中移除相吻合。笔者认为鱼骨状方解石的沉淀抑制剂为Fe~(2+)和Mn~(2+),这与微生物岩无明显Ce异常和Fe~(2+)极强的抑制能力相一致。因此,鱼骨状方解石可用于指示缺氧环境条件。此外,显微和超微研究也表明鱼骨状方解石晶体内存在有大量与其生长方向一致的菌丝体残余和与之密切伴生的有机矿物,表明微生物为鱼骨状方解石成核和初始沉淀提供了重要垫板。 相似文献
2.
BIF在全球广泛分布,BIF型铁矿是铁的重要来源。根据产出的构造背景将其分为阿尔戈玛型（Algoma-type）和苏必利尔湖型（Lake Superior-type）。BIF主要产出于前寒武纪的古老克拉通和/或年轻地体,形成时代集中在3.0～2.0 Ga,峰期为2.5 Ga左右。前人对BIF型铁矿的成因研究着重于BIF的物质来源和Fe 2+ 氧化沉淀机制两个方面,但都尚未达成共识。物质来源的观点主要有大陆风化剥蚀、海底热液、海底热液和海水的混合物、热液淋滤洋壳、既有大陆物质来源又有热液来源,沉淀机制主要有生物沉淀和非生物沉淀两种认识,前者是指Fe 2+ 利用微生物（如蓝藻）光合作用产生的O 2氧化成Fe 3+,或Fe 2+ 直接被微生物代谢氧化,后者主要包括热液与海水混合、密度流作用、相分离、紫外线引起Fe 2+ 氧化沉淀等。 相似文献
3.
海相碳酸盐的沉淀方式被认为与水体氧化还原条件密切相关,即太古宙至古元古代缺氧的铁化海水中碳酸盐沉淀抑制剂Fe 2+和Mn 2+强力抑制灰泥在水柱中成核,但允许文石直接在海底生长,从而导致大量文石以海底沉淀方式产出,而新元古代适度的氧化海水则有利于灰泥以水柱沉淀方式形成。然而,碳酸盐沉淀方式的长期变化还可能受控于其他因素,其与海水氧化还原条件之间的关系还需要通过大量具体实例来验证。针对上述科学问题,笔者选择碳酸盐沉淀方式尚处于过渡时期的华北中元古界碳酸盐岩为研究对象,开展碳酸盐沉淀方式及与之对应的氧化还原条件研究。结果表明,华北高于庄组三段(约1.56 Ga)、雾迷山组四段下部(约1.48 Ga)和铁岭组二段(约1.44 Ga)发育大量灰泥水柱沉淀,其Ⅰ/(Ca+Mg)值较高(普遍大于0.5 μmol/mol)、Ce负异常(低至0.8),指示适度氧化的条件;而高于庄组四段下部(约1.55 Ga)和雾迷山组二段中部(约1.50 Ga)则发育大量纤维状文石海底沉淀,其Ⅰ/(Ca+Mg)值约为0,指示次氧化至缺氧的环境。因此,本研究首次用大量实例证实了前寒武纪海水氧化还原条件对碳酸盐沉淀方式的重要调控作用,并且后者可作为海水氧化还原条件分析的重要指标,适用于高效开展长序列、多剖面的低氧背景下前寒武纪碳酸盐岩地层的氧化还原条件分析。 相似文献
4.
海相碳酸盐的沉淀方式被认为与水体氧化还原条件密切相关,即太古宙至古元古代缺氧的铁化海水中碳酸盐沉淀抑制剂Fe2+和Mn2+强力抑制灰泥在水柱中成核,但允许文石直接在海底生长,从而导致大量文石以海底沉淀方式产出,而新元古代适度的氧化海水则有利于灰泥以水柱沉淀方式形成.然而,碳酸盐沉淀方式的长期变化还可能受控于其他因素,其... 相似文献
5.
碳酸盐岩的Fe/Mn元素比值,作为一项新的地球化学指标,可以用于恢复海洋的氧化还原状态.在氧化条件下,Fe 3+和Mn 4+均不可溶,因此氧化海水中的溶解Fe和Mn的含量均很低.Fe 3+和Mn 4+在还原条件下可以被细菌还原为可溶的Fe 2+和Mn 2+,而氧化还原电位的计算表明,Mn 4+的还原要早于Fe 3+的还原,因此细菌的Mn还原过程发生在沉积物的更浅层.可溶的Fe 2+和Mn 2+向上扩散到海水中,替代碳酸盐岩晶格里的Ca 2+,因此碳酸盐岩晶格中的Fe 2+和Mn 2+的含量受控于来自沉积物孔隙水的扩散,而后者又与水岩界面的氧化还原状态相关.因此可以预测,随着海水变得逐渐缺氧,碳酸盐岩中的Fe/Mn比值会逐渐增高.为了验证这一假说,我们分析了中元古代高于庄组白云岩的Fe/Mn比值.研究发现,几乎所有的样品的Fe/Mn比值介于20～30之间,显著高于泥盆纪末期深水碳酸盐岩和浅水台地碳酸盐岩的Fe/Mn比值.高于庄组碳酸盐岩高的Fe/Mn比值一方面可能指示了中元古代低的大气氧气浓度和海洋的广泛缺氧,也可能反映了白云岩形成于缺氧的沉积物空隙水里. 相似文献
6.
城口锰矿是成岩作用的产物,除叠层石外其它如鲕粒、豆粒、球粒等主要源自碳酸盐台地,由合适的海流搬运而来,后被原地生长的微生物韵律性包覆而构成稳定的“滩”。Mn 2+主要来源于下部富含有机质、形成于深水还原环境之泥岩的粒间水,其极有利于来自陆地和海底喷发的锰质的富集和保存,在成岩作用时受上覆重荷影响使富含Mn 2+的酸性粒间水被排驱,向压力较小的孔隙度大、渗秀性好的“滩”内转移,使“滩”相弱碱性碳酸盐层的Ca 2+、Mg 2+被Mn 2+交代而成矿。 相似文献
7.
采用场发射电镜（JSM-6700F）和透射电镜（JEM-2000FX）研究了富钴结壳中水羟锰矿的形态和结构。结果表明:水羟锰矿单体片径一般为30～50 nm,集合体成片状或鳞片状;获得了水羟锰矿 d=0.142 9 nm的单晶和三连晶电子衍射结构;水羟锰矿中Fe 3+、Co 3+、Ni 3+为高自旋态离子,Co 3+、Ni 3+、Cu 3+易与Mn 4+形成类质同像置换,部分Fe 3+与Mn 4+可能存在有限的类质同像置换,置换反应为3 Mn 4+=4Co 3+（Ni 3+、Cu 3+、Fe 3+）,体系总电价平衡。 相似文献
8.
微生物介导碳酸盐矿物沉淀为“白云岩”问题的解决带来了希望。本次研究利用好氧、中度嗜盐细菌,在Mg 2+/Ca 2+值分别为3,5和7的溶液中合成碳酸盐,记录了每隔24 h溶液中的pH值,并取样分析了溶液中主要阳离子的变化情况。了解溶液pH值和阳离子变化,有利于我们了解微生物作用下碳酸盐矿物形成的过程,对理解微生物作用下碳酸盐的形成机制具有重要意义。实验结果表明,合成的矿物主要为一水碳酸钙、方解石及原白云石。3种溶液的pH值在前4 d均明显降低,溶液整体呈酸性,4 d后溶液pH值降低的趋势减缓,10 d后溶液pH值缓慢上升,呈近中性或弱碱性。Ca 2+,Mg 2+均呈现先下降后略微上升的趋势,Ca 2+,Mg 2+后期略微上升可能是由于细菌活性降低后不再消耗Ca 2+,Mg 2+,溶液中的H +,Na +和K +等阳离子在阳离子交替吸附的作用下替换出了一部分吸附在培养基上... 相似文献
9.
金顶铅锌矿床位于中国西南中新生代兰坪盆地，是亚洲第二大铅锌矿床，其巨大的储量和复杂的地质成矿史吸引了众多学者的研究，但在矿床成因和矿化过程的精细描述等方面仍存在较大争议。本文通过详细的野外地质调查并结合硫化物的LA-(MC)ICP-MS微量元素和硫、铅同位素分析对该矿床进行了详细解剖。金顶铅锌矿床的含矿地层为景星组砂岩、云龙组含砾砂岩及灰岩角砾岩。成矿过程可分为2个主要阶段：第Ⅰ阶段早期为浸染状的黄铁矿-白铁矿-闪锌矿-方铅矿，晚期硫化物呈块状且出现天青石；第Ⅱ阶段为闪锌矿-方铅矿-黄铁矿-天青石-重晶石-方解石-石膏阶段。成矿过程中硫化物的原位微量元素和S-Pb同位素表现出规律演化特征。主要金属元素除了以简单的类质同象形式赋存于闪锌矿中，Zn 2+与微量元素存在4Zn 2+?2Fe 2++Ge 4+、2Zn 2+?Fe 2++Mn 2+、3Zn 2+?In 3++Sn 3+等复杂的替代关... 相似文献
10.
为了研究金在黄铁矿表面沉淀的机理,于室温、常压,在氯化物溶液中进行了黄铁矿粉末吸附金的实验。在不同pH的溶液中,黄铁矿均可吸附金,而且pH值明显地影响吸附速率。扫描电镜观察表明,反应后黄铁矿粒表面有金晶体形成。XPS研究得知,黄铁矿光片与含金氯化物溶液反应后表面有A 0存在;硫在反应初期为S 0、S 2O 32-,随后转变为SO 42-,而铁成为Fe 3+.黄铁矿中的Fe 2+和S 22-是溶液中金的还原剂。金在黄铁矿表面沉淀可能涉及吸附、还原和晶体生长等过程。 相似文献
11.
A decrease in temperature (Δ T up to 45.5 °C) and chloride concentration (ΔCl up to 4.65 mol/l) characterises the brine–seawater boundary in the Atlantis-II, Discovery, and Kebrit Deeps of the Red Sea, where redox conditions change from anoxic to oxic over a boundary layer several meters thick. High-resolution (100 cm) profiles of the methane concentration, stable carbon isotope ratio of methane, and redox-sensitive tracers (O 2, Mn 4+/Mn 2+, Fe 3+/Fe 2+, and SO 42−) were measured across the brine–seawater boundary layer to investigate methane fluxes and secondary methane oxidation processes. Substantial amounts of thermogenic hydrocarbons are found in the deep brines (mostly methane, with a maximum concentration up to 4.8×105 nmol/l), and steep methane concentration gradients mainly controlled by diffusive flow characterize the brine–seawater boundary (maximum of 2×105 nmol/l/m in Kebrit Deep). However, locally the actual methane concentration profiles deviate from theoretical diffusion-controlled concentration profiles and extremely positive δ13C–CH4 values can be found (up to +49‰ PDB in the Discovery Deep). Both, the actual CH4 concentration profiles and the carbon-13 enrichment in the residual CH4 of the Atlantis-II and Discovery Deeps indicate consumption (oxidation) of 12C-rich CH4 under suboxic conditions (probably utilizing readily available—up to 2000 μmol/l—Mn(IV)-oxihydroxides as electron acceptor). Thus, a combined diffusion–oxidation model was used to calculate methane fluxes of 0.3–393 kg/year across the brine–seawater boundary layer. Assuming steady-state conditions, this slow loss of methane from the brines into the Red Sea bottom water reflects a low thermogenic hydrocarbon input into the deep brines. 相似文献
12.
Acidithiobacillus ferrooxidans are commonly present in acid mine drainage (AMD). A. ferrooxidans derive metabolic energy from oxidation of Fe 2+ present in natural acid solutions and also may be able to utilize Fe 2+ released by dissolution of silicate minerals during acid neutralization reactions. Natural and synthetic fayalites were reacted in solutions with initial pH values of 2.0, 3.0 and 4.0 in the presence of A. ferrooxidans and in abiotic solutions in order to determine whether these chemolithotrophic bacteria can be sustained by acid-promoted fayalite dissolution and to measure the impact of their metabolism on acid neutralization rates. The production of almost the maximum Fe 3+ from the available Fe in solution in microbial experiments (compared to no production of Fe 3+ in abiotic controls) confirms A. ferrooxidans metabolism. Furthermore, cell division was detected and the total cell numbers increased over the duration of experiments. Thus, over the pH range 2–4, fayalite dissolution can sustain growth of A. ferrooxidans. However, ferric iron released by A. ferrooxidans metabolism dramatically inhibited dissolution rates by 50–98% compared to the abiotic controls. Two sets of abiotic experiments were conducted to determine why microbial iron oxidation suppressed fayalite dissolution. Firstly, fayalite was dissolved at pH 2 in fully oxygenated and anoxic solutions. No significant difference was observed between rates in these experiments, as expected, due to extremely slow inorganic ferrous iron oxidation rates at pH 2. Experiments were also carried out to determine the effects of the concentrations of Fe2+, Mg2+ and Fe3+ on fayalite dissolution. Neither Fe2+ nor Mg2+ had an effect on the dissolution reaction. However, Fe3+, in the solution, inhibited both silica and iron release in the control, very similar to the biologically mediated fayalite dissolution reaction. Because ferric iron produced in microbial experiments was partitioned into nanocrystalline goethite (with very low Si) that was loosely associated with fayalite surfaces or coated the A. ferrooxidans cells, the decreased rates of accumulation of Fe and Si in solution cannot be attributed to diffusion inhibition by goethite or to precipitation of Fe–Si-rich minerals. The magnitude of the effect of Fe3+ addition (or enzymatic iron oxidation) on fayalite dissolution rates, especially at low extents of fayalite reaction, is most consistent with suppression of dissolution by interaction between Fe3+ and surface sites. These results suggest that microorganisms can significantly reduce the rate at which silicate hydrolysis reactions can neutralize acidic solutions in the environment. 相似文献
13.
Archean carbonates commonly contain decimetre- to metre-thick beds consisting entirely of fibrous calcite and neomorphosed fibrous aragonite that precipitated in situ on the sea floor. The fact that such thick accumulations of precipitated carbonate are rare in younger marine carbonates suggests an important change in the modes of calcium carbonate precipitation through time. Kinetics of carbonate precipitation depend on the concentration of inhibitors to precipitation that reduce crystallization rates and crystal nuclei formation, leading to kinetic maintenance of supersaturated solutions. Inhibitors also affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong calcite-precipitation inhibitor, is thought to have been present at relatively high concentrations in Archean seawater because oxygen concentrations were low. The rise in oxygen concentration at 2.2-1.9 Ga led to the removal of Fe2+ from seawater and resulted in a shift from Archean facies, which commonly include precipitated beds, to Proterozoic facies, which contain more micritic sediment and only rare precipitated beds. 相似文献
14.
When Pseudomonas aeruginosa PAO1 biofilms (attached to Sepharose surfaces) were subjected to dissolved Fe 3+, most Fe was removed from solution within 25 h by surface complexation with negatively charged functional groups on the bacterial cell wall via a nucleation and mineralization process. Chemical formation of Fe-(hydr)oxides was partially responsible for dissolved Fe removal, which stemmed from a pH increase, facilitated by microbial activity. PAO1 used Fe 3+ as an electron acceptor producing Fe 2+ under localized anaerobic conditions over the first 50 h. The high ratio of Fe 2+ to total Fe in solution produced a high proportion of Fe(II) in Fe precipitates; however, as the formation of Fe-(hydr)oxides started after 50 h, the Fe 2+ content in solution began to diminish. Biofilms can so influence the local chemical conditions and metal speciation that the bulk solution phase is also affected, thereby mediating a wide-range (bio)geochemical cycling of iron. Long-term survival of natural biofilms, even under strict oligotrophic conditions, could have a broad lasting effect on the bulk geochemical environment. 相似文献
15.
The reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities (i.e. S=5, 15, 25, 35, 44) were determined at 25°C and 10 −2.5-atm. CO 2 partial pressure using a constant disequilibrium seeded technique. The rate data were fitted to an empirical rate law of the form: where n is the empirical reaction order; and k is the rate constant. Calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated, while those for aragonite show an increase in going from the higher (i.e. S=35, 44) to the lower (i.e. S=5, 15, 25) salinity range. This study also confirms previously published findings that above a given saturation state (i.e. Ωc>/2.6) aragonite precipitates more rapidly than calcite at 25°C. The incorporation of Sr2+ in aragonite and Mg2+ in calcite overgrowths are independent of the precipitation rate. The partition coefficient of Sr2+ in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44. However, the Mg2+ partition coefficient in calcite, increases with decreasing salinity of the parent seawater solutions, possibly as a result of variations in the sulfate content of the solutions and solids. The experimental results were discussed in the context of a number of geological environments. 相似文献
16.
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
17.
Based on the petrology of hydrothermally altered Archean basaltic greenstones, thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high-temperature (∼350 °C) hydrothermal fluid in an Archean subseafloor basalt-hosted hydrothermal system. The results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high-CO 2 condition, and predict a generation of SiO 2-rich, Fe-poor hydrothermal fluids in the Archean subseafloor hydrothermal system. The chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic-neutral seawater are characterized by inverse pH and chemical polarity to modern hydrothermal systems, leading to extensive precipitation of silica and iron oxyhydroxides on/under the seafloor. Such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike, the mechanism of silicification, and the pH-controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the Archean ocean. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life. 相似文献
18.
This paper discusses some major research to be carried out in the next five years in the newly established Laboratory of Ocean Lithosphere and Mantle Geodynamics. By using our existing sample collections of global mid-ocean ridge basalts, gabbros and abyssal peridotites from the Pacific, Atlantic and Indian oceans, the research includes: ①Using Ti-Zr-Hf stable isotope methods to test the hypothesis that the observed huge Nb-Ta and Zr-Hf fractionations result from mass-dependent fractionation under mantle magmatic conditions; ②Using a MORB sample suite of uniform ratios of incompatible elements and Sr-Nd-Pb isotopes with large major element compositional variation to test the common hypothesis of iron isotope fractionation, i.e, the affinity of heavy Fe with ferric Fe (Fe 3+), and both heavy Fe and ferric Fe (Fe 3+) being more incompatible than light Fe and ferrous Fe (Fe 2+) during magma evolution; while using an incompatible trace element and Sr-Nd-Pb isotope highly variable MORB suite to test the same hypothesis during low-degree mantle melting (i.e, the effect of mantle metasomatism); ③Proposing and testing the hypothesis that the high oxygen fugacity of the Earth’s mantle is a consequence of plate tectonics by subducting partially serpentinized oceanic mantle lithosphere with abundant ferric Fe (e.g. Fe 3+/SFe>2); ④The recent work by Andersen et al. (Nature, 2015) is a milestone contribution by using U isotope variation in oceanic basalts to hypothesize that the O 2-rich atmosphere since the late Archean (abont 2.4 Ga) mobilized the water soluble U (6+ vs. 4+) from continents, transported to the ocean and subducted with sediments to the upper mantle, which explains the low Th/U in MORB (<2.5) and the high Th/U (>3.5) ocean island basalts (OIB) do not see such U addition effect probably because OIB source materials are all ancient (> abont 2.4 Ga) if there were subducted component. The Cenozoic alkali basalts from eastern China are ideal materials for evaluating the significance of the subducted seafloor materials for the petrogenesis of OIB and enriched MORB by using the U isotope approach, which is expected to revise and improve the Andersen et al hypothesis. 相似文献
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