The impact of solution chemistry on Mytilus edulis calcite and aragonite

Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Ga solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios.Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Mytilus edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.     

Calcium-isotope fractionation in selected modern and ancient marine carbonates

The calcium-isotope composition (δ^44/42Ca) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts, non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca²⁺_aq-CaCO₃-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower δ^44/42Ca values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against ⁴⁴Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ^44/42Ca values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from δ¹⁸O values and Mg concentrations. The reconstruction of secular variations in the δ^44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ^44/42Ca values of Cretaceous skeletal calcite suggest that the δ^44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.     

鲕粒原生矿物识别及对海水化学成分变化的指示意义

鲕粒是碳酸盐沉积过程中一类非常特殊的颗粒类型, 为研究当时的沉积背景、水动力条件、气候环境, 甚至储层特征提供了重要线索。然而, 鲕粒的矿物组成及控制因素问题, 长期受到忽视。组成鲕粒的原生矿物类型在地质历史时期呈周期性变化, 在显生宙表现为三个以文石和高镁方解石占主导的时期以及两个以低镁方解石占主导的时期, 这也被称作“文石海”和“方解石海”时期。原生矿物的组成, 制约着鲕粒的纹层结构、保存程度以及成岩特征, 还蕴含着海水化学成分变化的线索。鲕粒原生矿物识别主要依据:①原生纹层结构;②保存程度;③微量元素浓度, 尤其是Sr-Mg的浓度。文石质鲕粒受文石不稳定性的影响, 原生结构保存程度较差;一般保存有典型的文石残余纹层结构(例如砖砌结构、溶解变形结构以及偏心结构等);在封闭成岩环境下原生矿物为文石质的鲕粒Sr浓度往往大于2 000 ppm;纹层结构主要为切线状(占主导)和放射状。方解石质鲕粒包括低镁方解石和高镁方解石两种类型:低镁方解石为稳定矿物, 原生结构一般保存良好。尽管高镁方解石也为亚稳定矿物, 但成岩转换后的保存程度好于文石。两者Sr含量一般均低于1 000 ppm, Mg含量一般在0~20 mol % MgCO₃(两者以4 mol % MgCO₃为界)。高镁方解石受成岩作用影响, 在纹层中往往保留有微粒白云石包裹体;海相地层中保存的方解石质鲕粒为放射状或同心-放射状结构。另外还存在一类由两种矿物共同构成的双矿物鲕粒, 可以通过分析两类纹层在结构和保存特征上的差异进行区分。鲕粒原生矿物成分随时间的波动变化受到海水化学条件, 尤其是Mg/Ca比值, 大气二氧化碳分压以及碳酸盐饱和度的控制。Mg/Ca比值的波动决定着鲕粒原生矿物类型的长期变化规律。一些突发性事件可能会扰动(区域)短时间尺度下鲕粒原生矿物的组成, 造成鲕粒原生矿物的转换。通过研究碳酸盐鲕粒原生矿物特征以及控制因素进而了解海水的化学特征, 是独立于古生物学和地球化学分析之外的一种较为可靠的沉积学方法。     

巴布亚新几内亚合恩半岛晚第四纪上升珊瑚礁造礁珊瑚的成岩历史

更新世以来,剧烈的构造运动已将巴布亚新几内亚合恩半岛东北海岸的晚第四纪珊瑚礁阶地抬升上千米.阶地中造礁珊瑚的成岩变化和成岩产物的组构特征反映了该礁的成岩历史,充分体现该区快速构造上升的影响.海水潜流带和淡水渗流带为上升礁的主要成岩环境.生物钻孔、生物碎屑填隙、珊瑚文石针粗化、珊瑚骨骼的溶解和新生变形转化,以及其不同矿物成分和组构的种种胶结物的胶结作用是造礁珊瑚经历的主要成岩作用.地球化学资料表明其成岩变化发生于开放的化学体系之中.     

The mineralogical composition of precursor sediments of calcareous rhythmites: a new approach

Previous studies in Silurian carbonates from Gotland (Sweden) have led to a model for the development of limestone-marl alternations. This model postulates that early diagenesis of precursor sediments without strong primary differences can result in a differentiation by selective dissolution of aragonite in marl beds and reprecipitation of calcite cement in limestone beds. This model is described as a set of mathematical equations that quantify the diagenetic processes (aragonite dissolution and calcite reprecipitation) that occur during the formation of limestone-marl interbeds from a hypothetical homogeneous precursor sediment. The calculations demonstrate that resulting hypothetical limestone-marl alternations show characteristic mathematical relationships between the ratios of the bed thicknesses of limestones and marls on one side, and the carbonate contents, on the other. By reversing this model, the original mineralogical composition of the precursor sediment of real-world rhythmic successions can be determined. In this study, alternations from the Silurian of Gotland, the Cambrian, Devonian, and Mississippian of North America, the Jurassic of France and Germany, and the Cretaceous of France are shown to exhibit mathematical relationships similar to those calculated for hypothetical precursor sediments without primary differences. Therefore, the mineralogical composition of their precursor sediments can be estimated. In contrast, the clear mismatch shown by the Lower Jurassic Belemnite Marls from Dorset indicates that these rhythms did not suffer an early diagenetic overprint. Our model helps to differentiate between rhythmites with strong depositional variations and those without; however, it cannot indicate whether a given alternation is the product of rhythmic diagenesis of a homogeneous precursor sediment or the result of diagenetic enhancement of subtle underlying sedimentary rhythms. For horizontally correlated patterns, such as laterally extensive beds and layers of nodules, an a priori unknown external signal has to be assumed.     

西沙群岛晚第四纪碳酸盐岩淡水成岩作用

基于西沙永兴岛上最新钻孔（SSZK1）取得的55.92 m岩芯的 U?Th定年、矿物、薄片、主微量元素及碳氧稳定同位素等资料,开展了西沙群岛晚第四纪碳酸盐岩沉积相和淡水成岩作用的研究。根据不同的矿物组成特征,可将SSZK1钻孔岩芯分为上、中、下三段: 下 段（33.89~55.92 m,主要为低镁方解石）、中 段（18.39~33.89 m,主要为文石和低镁方解石）、上段（0~18.39 m,主要为文石、高镁方解石和低镁方解石）。由于下段碳酸盐岩几乎全为稳定的低镁方解石组成,碳氧同位素值的严重负偏和小幅度变化,可推断其经历了程度较大的淡水成岩作用。中段和上段还存在不稳定的文石和高镁方解石,碳氧同位素值相对下段正偏和高幅高频变化,推测其淡水成岩作用的程度比下段要小。中段碳氧同位素值高幅高频变化同时也说明该段的矿物纵向变化较复杂。这种矿物组成的复杂变化可能是由于晚第四纪海平面频繁变化,该段被大气水渗流带和潜流带交替占据引起的。主微量元素的变化同时受到淡水成岩作用和沉积环境的影响。在中段、下段中可识别出sq1、sq2、sq3、sq4四个完整的相旋回。Na₂O,S,Sr 和碳氧同位素受到的淡水成岩作用而被消耗和负偏,且由于老一期的旋回经历了更长时间的淡水成岩作用,新、老旋回间的 Na₂O,S,Sr含量值和碳氧同位素值有明显差异。利用新、老时期形成的旋回间淡水成岩作用剩余Na₂O,S,Sr含量和碳氧同位素值的差别可以将新、老两个旋回区分开来。     

Brick-like texture and radial rays in Triassic pisolites of Lombardy,Italy: A clue to distinguish ancient aragonitic pisolites

Triassic pisolites from the Calcare Rosso, Lombardy, Italy, were formed in a hypersaline vadose environment and now show alternating dolomite and calcite laminae. The calcite consists either of microsparite laminae with a brick-like fabric, or of a mass of mosaic crystals with the external form of square-ended rays.These features suggest that the original laminae and rays were aragonitic, like those of the Holocene supratidal pisolites of the Persian Gulf, which consist of alternating laminae of unoriented nannomicrite with Mg-rich mucilaginous material, and aragonitic fibers with radial orientation separated by mucilaginous films. It is suggested that the transformation to brick-like and ray textures passed through the following diagenetic path: (1) original formation of fibrous aragonite laminae; (2) local aggrading recrystallization of aragonite fibers to large square-ended rays during hypersaline phases; (3) dolomitization of Mg-rich mucilaginous nannomicrite laminae during hyposaline phases; (4) inversion of the aragonite fibers and rays to calcite on a piece-by piece basis that preserved the original textural details, when the pH or Mg/Ca ratio dropped.The brick-like and ray fabrics have not been found in laminae of continental freshwater pisolites because these were deposited as equant and stable crystals of low-Mg calcite. These textures consequently make it possible to establish the chemistry of the depositional and early diagenetic milieu for some ancient pisolitic rocks.     

海相碳酸盐矿物的阴极发光性与其成岩蚀变的关系

引言众所周知,原始的海相碳酸盐矿物为文石、高镁方解石和低镁方解石,但它们对成岩蚀变极度敏感,在大气淡水成岩环境中,文石和高镁方解石都会转变为成岩低镁方解石,同时,原始的低镁方解石也会有不同程度的蚀变。在进行古代碳酸盐岩的沉积环境和古海洋学研究时,往往需要测定岩石中的微量元素及氧、碳、锶等同位素组成,用以判断古海水     

Experimental and natural early diagenetic mobility of Sr and Mg in biogenic carbonates

Ammonium acetate dissolution experiments were performed on shell material of the modern bivalves, Crassostrea virginica (calcite) and Mercenaria mercenaria (aragonite). Their purpose was to determine the order of preferential dissolution of Sr, Mg and Ca; these results subsequently were compared with Sr, Mg and Ca data from other Recent as well as ancient mollusks.Results from these experiments suggest the following relative order of abundance of readily exchangeable Mg and Sr in biogenic carbonates: Mg(arag) > Mg(calc) > Sr(calc) > Sr(arag). It is apparent that incongruent dissolution of minerals with different solubilities cannot entirely explain the observed dissolution patterns for Sr, Mg and Ca in these biogenic carbonates. Secular changes in whole shell Mg and Sr concentrations for Recent and unrecrystallized fossil mollusks suggest an order of “ionic mobility” in diagenesis identical to the order of abundance for readily exchangeable ions found in the NH₄Ac dissolution experiments. It is concluded that this “ionic mobility” is due to a post mortem, early diagenetic (pre-recrystallization) approach to equilibrium with the surrounding chemical environment.     

Stabilities of synthetic magnesian calcites in aqueous solution: Comparison with biogenic materials

Free-drift dissolution data and inverse time plots were used to evaluate the stabilities of synthetic and biogenic magnesian calcites in aqueous solutions at 25°C and 1 atm total pressure. Synthetic phases with MgCO₃ concentrations below 6 mole percent have stoichiometric ion activity products that are less than the value for calcite, whereas the values for phases with higher concentrations are greater than that of calcite. For synthetic phases, stability is a smooth function of composition and all phases (up to 15 mole percent MgCO₃) have values of ion activity products less than that for aragonite. These results agree with those of Mucci and Morse (1984) derived from precipitation of magnesian calcites in aqueous solutions. “Average” seawater at 25° and 1 atm total pressure is supersaturated with respect to all synthetic phases in the compositional range studied.Biogenic samples are less stable than synthetic phases of similar Mg concentrations and stability is not a smooth function of composition. Biogenic materials with compositions greater than 11–13 mole percent MgCO₃ have ion activity products greater than that for aragonite.The difference in stability between biogenic materials and synthetic phases is due to greater variation in chemical and physical heterogeneities found for the biogenic samples. If it is assumed that the results of the dissolution experiments reflect only differences in Gibbs free energies of formation between synthetic phases and biogenic materials of similar Mg concentration, the biogenic materials are 200–850 j/mol less stable than the synthetic phases. Only the results of synthetic dissolution experiments should be used to model the thermodynamic behavior of the magnesian calcite solid solution. The results for the synthetic phases, however, may not be appropriate to use for interpreting diagenetic reaction pathways for magnesian calcites in modern sediments, except as a basis of comparison with the behavior of natural materials.     

Evaluation of bulk carbonate δ13C data from Triassic hemipelagites and the initial composition of carbonate mud

Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ¹³C and δ¹⁸O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ¹³C and δ¹⁸O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ¹³C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ¹³C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.     

Annual trace element cycles in calcite–aragonite speleothems: evidence of drought in the western Mediterranean 1200–1100 yr BP

Each of two calcitic stalagmites from Grotte de Clamouse, Herault, southern France, displays a discrete aragonite layer dated at around 1100 yr BP. The layer of fanning aragonite ray crystals is immediately preceded by calcite with Mg and Sr compositions that are uniquely high for the past 3 kyr. Trace element compositions close to the boundary between original aragonite and calcite are consistent with quasi‐equilibrium partitioning of trace elements between the phases. Study of modern dripwaters demonstrates that pronounced covariation of Mg/Ca and Sr/Ca ratios in dripwater occurs owing to large amounts of calcite precipitation upflow of the drips that fed the stalagmites. Trace element to Ca ratios are enhanced during seasonally dry periods. Ion microprobe data demonstrate a pronounced covariation of trace elements, including Mg and Sr in calcite, and Sr, U and Ba in aragonite. The mean peak spacing is close to the long‐term mean of annual growth rates determined by differences in U‐series ages and so the trace element peaks are interpreted as annual. The trace element chemistry of the stalagmites on annual to inter‐annual scales thus directly reflects the amounts of prior calcite precipitation, interpreted as an index of aridity. The longer‐term context is a multi‐decadal period of aridity (1200–1100 yr BP) possibly correlated with an analogous episode in Central America. The arid period culminated in the nucleation of aragonite, but within a decade was followed by a return to precursor conditions. Copyright © 2005 John Wiley & Sons, Ltd.     

Rock composition and origin of the Duwi Formation calcareous rocks, Upper Egypt

Upper Cretaceous phosphorite beds of the Duwi Formation, Upper Egypt, are intercalated with limestone, sandy limestone, marl, calcareous shales, and calcareous sandstone. Calcareous intercalations were subjected to field and detailed petrographic, mineralogical and geochemical investigations in order to constrain their rock composition and origin. Mineralogically, dolomite, calcite, quartz, francolite and feldspars are the non-clay minerals. Smectite, kaolinite and illite represent the clay minerals. Major and trace elements can be classified as the detrital and carbonate fractions based on their sources. The detrital fraction includes the elements that are derived from detrital sources, mainly clay minerals and quartz, such as Si, Al, Fe, Ti, K, Ba, V, Ni, Co, Cr, Zn, Cu, Zr, and Mo. The carbonate fraction includes the elements that are derived from carbonates, maily calcite and dolomite, such as Ca, Mg and Sr. Dolomite occurs as being dense, uniform, mosaic, very fine-to-fine, non-ferroan, and non-stoichiometrical, suggesting its early diagenetic formation in a near-shore oxidizing shallow marine environment. The close association and positive correlation between dolomite and smectite indicates the role of clay minerals in the formation of dolomite as a source of Mg^2＋ -rich solutions. Calcareous rocks were deposited in marine, oxidizing and weakly alkaline conditions, marking a semi-arid climatic period. The calcareous/argillaceous alternations are due to oscillations in clay/carbonate ratio.     

西藏南部晚白垩世厚壳蛤不同壳层的地球化学特征及其对古海洋信息的保存性

测试了西藏南部岗巴剖面上白垩统宗山组上段地层中厚壳蛤不同壳层的Sr、Mn、Al、Si含量,以及碳、氧、锶同位素组成,测试结果表明:1）厚壳蛤的不同壳层（包括柱状层、珍珠层和二者间的过渡层）的碳酸盐矿物均已新生变形为成岩低镁方解石（DLMC）,各种原始碳酸盐矿物（沉积低镁方解石或文石）都已不同程度地发生重结晶;2）从壳的外层（柱状层）向内层（珍珠层）方向,碳酸盐矿物重结晶作用有逐渐增强的趋势,柱状层仍具有很好的柱状结构,其平均阴极发光强度明显弱于内部的珍珠层;3）厚壳蛤不同壳层的Sr含量和δ18O值都呈现有规律的变化,从靠外部的柱状层到内部的珍珠层,Sr含量和δ18O值都逐渐降低,同样说明内部的珍珠层丢掉的海水信息相对较多;4）厚壳蛤不同壳层和体腔充填物的δ18O值都显著低于前人报道的同期海水值,说明在新生变形过程中,厚壳蛤不同壳层和体腔充填物的氧同位素都与大气水或其它成岩流体发生了交换,但柱状层的氧同位素与大气水（或其它成岩流体）的交换相对较少;5）厚壳蛤不同壳层和体腔充填物的锶、碳同位素组成与同期海水值接近,不同壳层之间也没有表现出有规律性的变化,因而在矿物的新生变形过程中,厚壳蛤的锶、碳同位素与成岩流体间的交换相对较少,同时也说明,在氧、碳、锶三种同位素中,氧很容易与成岩流体发生同位素交换,而碳、锶同位素则更多地代表了海水信息。     

New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy

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.     

海水及淡水养殖珍珠的物质组成

采用偏光显微镜、X射线粉晶衍射仪、傅利叶红外光谱仪、电子顺磁共振谱仪、拉曼光谱仪、原子吸收光谱仪等测定了淡水珍珠、海水珍珠及部分贝壳的矿物及化学组成.研究表明: 珍珠矿物组成单一, 主要由文石构成, 仅含少量球文石或方解石; 淡水珍珠明显富含Mn, 而海水珍珠中明显富含Na、K、Mg、Sr; 这种富集特征与淡水、海水中元素的富集特征相似.此外, 各颜色品种之间化学成分也存在微小的差异, 白色珍珠比有色珍珠更纯净, 所含铁、锰、铬等致色元素更低.紫色珍珠略富含Mg、Mn, 橙色者明显富含Fe, 而黑色海水珍珠的颜色可能与有机组分有关.      

海水化学演化对生物矿化的影响综述

显生宙非骨屑碳酸盐矿物经历了文石海和方解石海的交替,主要造礁生物和沉积物生产者的骨骼矿物与非骨屑碳酸盐矿物具有同步变化的趋势。这种长期的变化趋势可以用海水化学Mg/Ca摩尔比的变化来解释。流体包裹体、同位素和微量元素等证据也证实了海水化学在地质历史中经历过剧烈的变化。虽然生物诱导矿化和生物控制矿化的相对重要性一直存在争议,但古生物地层记录和人工海水养殖实验结果都表明,海水化学演化对生物矿化有重要的影响,体现在造礁生物群落的兴衰、生物起源时对骨骼矿物类型的选择以及微生物碳酸盐岩在地质历史中的分布等。这些为研究前寒武纪海水化学演化、古气候和古环境的重建、同位素地层对比以及碳酸盐的沉积和成岩等问题提供了新的思路。     

Cathodoluminescent bimineralic ooids from the Pleistocene of the Florida continental shelf

A bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite. Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO₃ and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO₃. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization. The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen-depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the sea-floor.     

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones,Central North Sea: an example of cement growth during bioturbation?

Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non‐ferroan, generally unzoned, subhedral to highly anhedral ‘jigsaw piece’ crystals. These are of a similar size to the detrital silicate grains and typically account for ≈5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other significant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has near‐marine δ¹⁸O values and low, positive δ¹³C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near‐surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore fluid of regionally consistent δ¹⁸O. Labile bioclastic debris (e.g. aragonite, Mg‐calcite) supplied isotopically positive carbon to the pore fluids during shallow‐burial dissolution. Such dissolution took place in response to the ambient ‘calcite sea’ conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg²⁺ and Ca²⁺ with the sea‐water reservoir maintained a sufficient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed fine calcitic shell debris, before ankerite precipitation during deep‐burial diagenesis.     

Controls of facies and sediment composition on the diagenetic pathway of shallow-water Heterozoan carbonates: the Oligocene of the Maltese Islands

Relatively few studies have so far addressed diagenetic processes in Heterozoan carbonates and the role that sediment composition and depositional facies exert over diagenetic pathways. This paper presents a study of Oligocene shallow-water, Heterozoan carbonates from the Maltese Islands. We investigate stratigraphic distribution, abundance and timing of diagenetic features and their relationship to sediment composition and depositional facies. The studied carbonate rocks comprise rud- to packstones of the Heterozoan association predominantly containing coralline red algae, bryozoans, echinoids and benthic foraminifers. XRD analyses show that all high-Mg calcite has been transformed to low-Mg calcite and that no aragonite is preserved. Diagenetic processes include dissolution of aragonitic biota, neomorphism of high-Mg calcitic biota to low-Mg calcite and cementation by fibrous, bladed, epitaxial and blocky cements. Stable isotopes on bulk rock integrated with petrographic data suggest that the study interval was not exposed to significant meteoric diagenesis. We interpret early cementation to have taken place in the marine and marine burial environment. The distribution and abundance of early diagenetic features, determining the diagenetic pathway, can be related to the primary sediment composition and depositional texture. Sorting and micrite content are important controls over the abundance of diagenetic features.