Origin of blocky aragonite cement in Cenozoic glaciomarine sediments,McMurdo Sound,Antarctica     

Mingyu Yang  Tracy D. Frank  Christopher R. Fielding  Megan E. Smith  Peter K. Swart 《Sedimentology》2020,67(2):721-741

Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio-chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROS-1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore-filling blocky fabric throughout the core. Oxygen isotope compositions (δ¹⁸O = −30·0 to −8·6‰ Vienna Pee-Dee Belemnite) and clumped isotope temperatures (TΔ₄₇ = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ¹⁸O_w) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluid δ¹⁸O_w values are consistent with those of pore water, previously identified as cryogenic brine in the nearby AND-2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROS-1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROS-1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought.  相似文献   

Aragonite stromatolitic buildups from Santorini (Aegean Sea,Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)     

Pere Anadón  Carles Canet  Walter L. Friedrich 《Sedimentology》2013,60(5):1128-1155

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp ) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ¹³C_PDB (0·16 to 12·97‰) with a narrower variation for δ¹⁸O_PDB (?0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ¹³C = 12·40 ± 0·43‰ and δ¹⁸O = 2·42 ± 0·77‰, n = 21). The δ¹³C and δ¹⁸O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, ⁸⁷Sr/⁸⁶Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO₂ gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.  相似文献   

Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)     

Vincent P. Milesi  Didier Jzquel  Mathieu Debure  Pierre Cadeau  Franois Guyot  Grard Sarazin  Francis Claret  Emmanuelle Vennin  Carine Chaduteau  Aurlien Virgone  Eric C. Gaucher  Magali Ader 《Sedimentology》2019,66(3):983-1001

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l^?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO₂‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.  相似文献   

Formation of low‐magnesium calcite at cold seeps in an aragonite sea     

Dong Feng  Harry H. Roberts  Samantha B. Joye  Ezat Heydari 《地学学报》2014,26(2):150-156

This study investigates the conditions of occurrence and petrographic characteristics of low‐Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The ¹³C‐depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca²⁺‐enriched and Mg²⁺‐depleted (Mg/Ca mole ratio <0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high‐Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H₂S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record.  相似文献   

室温条件下青海湖水中镁离子浓度对沉淀碳酸钙同质多像类型的调控     

杨康  李红  雷云  牛元哲  冯诗海 《岩石矿物学杂志》2021,40(3):571-580

青海湖是我国唯一报道过的现代湖底沉积物中白云石、方解石和文石等多种碳酸盐矿物共存的高原内陆咸水湖泊。以青海湖水和除菌青海湖水作为载体,以CaCl_2和MgCl_2·6 H_2O作为反应原料,在实验室常温条件下采取控制变量法制备出不同浓度Mg~(2+)参与下的钙质沉淀物,探讨Mg~(2+)浓度对沉淀物类型的影响。仅添加CaCl_2时,青海湖水中的沉淀物主要是石膏(Ca SO_4·2 H_2O)和球霰石(CaCO_3);在添加CaCl_2的同时添加MgCl_2·6 H_2O,沉淀物的石膏消失,完全转变成碳酸盐矿物,包括方解石和球霰石;当湖水中Mg~(2+)浓度为0.62 mol/L时,球霰石消失,沉淀物变为方解石和文石;随着Mg~(2+)浓度继续升高,文石含量稳步增加,方解石含量则逐渐减少,当Mg~(2+)浓度达到1.22 mol/L或更高时,方解石全部消失,沉淀物仅剩文石。实验结果表明,青海湖水中较高浓度的SO_4~(2-)对碳酸钙晶体生长有抑制作用,而额外加入的Mg~(2+)可以解除SO_4~(2-)的抑制作用,使得Ca~(2+)与HCO_3~-和CO_3~(2-)结合形成碳酸钙。此外,碳酸钙的同质多像类型也明显受到Mg~(2+)浓度的控制,随着湖水中Mg~(2+)浓度增加,方解石、球霰石不再稳定,而文石逐渐占主导地位,当Mg/Ca值达到6.1时,反应产物中仅有文石稳定存在。  相似文献   

Formation of lacustrine dolomite in the late Miocene marginal lakes of the East Mediterranean (Northern Israel)     

Dotan Shaked Gelband  Yael Edelman‐Furstenberg  Mordechai Stein  Abraham Starinsky 《Sedimentology》2019,66(7):2950-2975

The study focuses on the formation of lacustrine dolomite in late Miocene lakes, located at the East Mediterranean margins (Northern Israel). These lakes deposited the sediments of the Bira (Tortonian) and Gesher (Messinian) formations that comprise sequences of dolostone and limestone. Dolostones are bedded, consist of small‐sized (<7 μm), Ca‐rich (52 to 56 mol %) crystals with relatively low ordering degrees, and present evidence for replacement of CaCO₃ components. Limestones are comprised of a wackestone to mudstone matrix, freshwater macrofossils and intraclasts (mainly in the Bira Formation). Sodium concentrations and isotope compositions differ between limestones and dolostones: Na = ~100 to 150 ppm; ~1000 to 2000 ppm; δ¹⁸O = ?3·8 to ?1·6‰; ?2·0 to +4·3‰; δ¹³C = ?9·0 to ?3·4‰; ?7·8 to 0‰ (VPDB), respectively. These results indicate a climate‐related sedimentation during the Tortonian and early Messinian. Wet conditions and positive freshwater inflow into the carbonate lake led to calcite precipitation due to intense phytoplankton blooms (limestone formation). Dry conditions and enhanced evaporation led to precipitation of evaporitic CaCO₃ in a terminal lake, which caused an increased Mg/Ca ratio in the residual waters and penecontemporaneous dolomitization (dolostone formation). The alternating lithofacies pattern reveals eleven short‐term wet–dry climate‐cycles during the Tortonian and early Messinian. A shift in the environmental conditions under which dolomite formed is indicated by a temporal decrease in δ¹⁸O of dolostones and Na content of dolomite crystals. These variations point to decreasing evaporation degrees and/or an increased mixing with meteoric waters towards the late Messinian. A temporal decrease in δ¹³C of dolostones and limestones and appearance of microbial structures in close association with dolomite suggest that microbial activity had an important role in allowing dolomite formation during the Messinian. Microbial mediation was apparently the main process that enabled local growth of dolomite under wet conditions during the latest Messinian.  相似文献   

A depositional model for hydromagnesite–magnesite playas near Atlin,British Columbia,Canada     

Ian M. Power  Siobhan A. Wilson  Anna L. Harrison  Gregory M. Dipple  Jenine McCutcheon  Gordon Southam  Paul A. Kenward 《Sedimentology》2014,61(6):1701-1733

This study formulates a comprehensive depositional model for hydromagnesite–magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite–magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre‐scale) and amalgamated mounds composed primarily of hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca–Mg‐carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post‐deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg–HCO₃ groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg‐carbonate sediments (hydromagnesite mounds) overlies the Ca–Mg‐carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments ‘emerged’ from the water surface. Capillary action and evaporation draw Mg–HCO₃ water up towards the ground surface, precipitating Mg‐carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO₃·5H₂O] which partially cements pre‐existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca–Mg‐carbonate boundary indicates that there has been ca 8000 years of continuous Mg‐carbonate deposition at a rate of 0·4 mm yr^?1. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.  相似文献   

安徽马鞍山市当涂地区地下水水化学特征及演化机制          下载免费PDF全文

李状  苏晶文  董长春  叶永红  杨洋 《中国地质》2022,49(5):1509-1526

【研究目的】 了解长江中下游平原地区地下水流系统并深入分析其地下水水化学特征及其演化机制。【研究方法】 综合马鞍山市当涂地区的水文地质条件、水动力场等,基于研究区水化学基本特征,运用多元统计分析、水化学图件、离子比值和反向水文地球化学模拟等方法对该地区浅层地下水水化学演化进行分析。【研究结果】 结果表明：（1）研究区地下水主要为低矿化度偏碱性水,地下水组分中阳离子以Ca²⁺和Mg²⁺为主,阴离子以HCO₃^-和SO₄^2-为主。（2）研究区地下水水化学类型主要可分为7类,其中松散岩类孔隙含水岩组和碎屑岩类孔隙裂隙含水岩组的水化学类型主要为HCO₃^-Ca型、HCO₃^-Ca·Na型、HCO₃·Cl-Ca·Na型以及HCO₃^-Ca·Mg型;基岩类裂隙含水岩组的化学类型主要为HCO₃·SO₄-Ca·Mg型和SO₄·HCO₃^-Ca·Mg型。（3）研究区浅层地下水水样超标率为46%,总体水质较差,超标率较高的组分依次为Mn、高锰酸盐指数（COD_Mn）、硝酸盐（以N计）、Fe、As、氨氮（以N计）等。（4）研究区地下水的化学组分主要受到岩石风化作用的控制;此外,还存在Na-Ca的正向阳离子交替吸附作用。反向水文地球化学模拟结果进一步定量论证了水岩相互作用对本区浅层地下水组分的形成和演化起着主导作用。【结论】 研究区地下水主要为低矿化度偏碱性,主要可分为松散岩类孔隙水、碎屑岩类孔隙裂隙水和基岩类裂隙水。主要离子比例和反向水文地球化学模拟揭示了本区浅层地下水化学组分主要是地下水溶滤方解石、白云石等碳酸盐矿物、石英、长石等硅酸盐矿物,高岭土等黏土矿物以及岩盐、石膏等达到过饱和之后形成的。  相似文献   

The stability of CaCO₃·6H₂O (ikaite)     

Gregg Marland 《Geochimica et cosmochimica acta》1975,39(1):83-91

Experimental runs were made in cold-seal pressure vessels using synthetic CaCO₈·6H₂O, calcite and aragonite as starting materials. The reaction CaCO₃·6H₂O (ikaite) ? CaCO₃ (calcite I) + 6H₂O was reversed across its metastable extension into the aragonite stability field and the phase boundary is defined by brackets at 4.14kb, 14.3°C and 2.96 kb, ?3.0°C. An invariant point for CaCO₃·6H₂O, calcite I, aragonite and water thus occurs at about 3.02 kb and ?2.0°C. No other reaction could be reversed. Calculations based on the equilibrium phase boundary between calcite and ikaite and the available thermochemical data for calcite and water yield the stadard free energy of formation, standard enthalpy of formation and third law entropy of CaCO₃·6H₂O at 25°C and 1 bar total pressure; ?607.3 kcal/mole, ?705.8 kcal/mole, and 88.4 cal/deg mole, respectively.  相似文献   

The effect of cobalt ion on nucleation of calcium-carbonate polymorphs     

David M. Barber  Philip G. Malone  Robert J. Larson 《Chemical Geology》1975,16(3):239-241

Cobalt, like Mg, may cause the precipitation of aragonite rather than calcite in aqueous solutions due to the adsorption and crystal poisoning of calcite by a hydrated ion. Solutions containing NaCl and CaCl₂, having the ionic strength and Ca content of seawater (35‰ salinity), were spiked with known amounts of CoCl₂. Calcium carbonate was precipitated by the addition of 0.7 ml of 1 M Na₂CO₃. All experimental runs were made at 25°C, and all products were examined by X-ray diffraction. At low concentrations of Co (< 5·^?4M) calcite and vaterite formed. At concentrations from 5·10^?4 M to 2·10^?3M, the products consisted of combinations of calcite and vaterite; aragonite and calcite; aragonite and vaterite; calcite, vaterite and aragonite. In solutions of 3·10^?3M CoCl₂, most precipitates were aragonite with only one sample containing a small amount of calcite. All precipitates from 5·10^?3M CoCl₂ solutions either contained aragonite or were amorphous. Solutions with concentrations of 1 · 10^?2M CoCl₂ produced only amorphous precipitates. All precipitates contained an amorphous violet phase, assumed to be basic cobaltous carbonate (2CoCO₃·Co(OH)₂·H₂O).  相似文献   

Authigenic replacement of cyanobacterially precipitated calcium carbonate by aluminium‐silicates in giant microbialites of Lake Van (Turkey)     

Barbara Kremer  Jzef Ka mierczak  Stephan Kempe 《Sedimentology》2019,66(1):285-304

Studies of modern cyanobacterial mats and biofilms show that they can precipitate minerals as a consequence of metabolic and degradational activities paired with ambient hydrochemical conditions. This study looked at modern microbial mats forming giant, tower‐like, groundwater‐fed, calcareous microbialites in the world's largest, highly alkaline lake; Van Gölü (Lake Van), East Turkey. Results show that microbial systems play a role not only in carbonate precipitation but also in the formation of siliceous mineral phases. Transmitted light microscopy, scanning electron microscopy and spectral observations revealed that, within the extracellular polymeric substances excreted by the mats abundant minute aragonite grains precipitated first in vivo. These minute grains were quickly succeeded and/or supplemented in the dead biomass of the cyanobacterial mat by authigenic Al–Mg–Fe siliceous phases. Silicon dioxide is available in large concentrations in the highly alkaline water of Lake Van. Divalent cations (Ca and Mg) are delivered to the microbialites mostly by groundwater springs. The precipitation of the fine‐grained siliceous phases is probably mediated by bacteria degrading the cyanobacterial biomass and complexing the excessive cations with their extracellular polymeric envelopes. The bacteria serve as nucleation centres for the subsequent precipitation of siliceous mineral phases. Generally, the biphasic (calcareous and siliceous) mineralization – characterizing Lake Van microbialites – is controlled by their interior highly dynamic hydrogeochemical situation. There, the dramatically different alkaline lake water and the Ca–Mg‐charged groundwater mix at various rates. The early diagenetic replacement of the in vivo aragonite by authigenic siliceous phases significantly increases the fossilization potential of the mat‐forming cyanobacteria. Lake Van and its giant microbialite tufa towers act as a model explaining the transformation of early diagenetic mineral phases observed in many modern and ancient carbonate marine deposits, particularly those influenced by diffusion of silica‐enriched and metal‐enriched pore waters from below the water–sediment interface.  相似文献   

Diagenetic effects on the distribution of uranium in live and Holocene corals from the Gulf of Aqaba     

Boaz Lazar  Rivka Enmar  Miriam Bar-Matthews  Mordechai Stein 《Geochimica et cosmochimica acta》2004,68(22):4583-4593

We investigated the effects of diagenetic alteration (dissolution, secondary aragonite precipitation and pore filling) on the distribution of U in live and Holocene coral skeletons. For this, we drilled into large Porites lutea coral-heads growing in the Nature Reserve Reef (NRR), northern Gulf of Aqaba, a site close to the Marine Biology Laboratory, Elat, Israel, and sampled the core material and porewater from the drill-hole. In addition, we sampled Holocene corals and beachrock aragonite cements from a pit opened in a reef buried under the laboratory grounds. We measured the concentration and isotopic composition of U in the coral skeletal aragonite, aragonite cements, coral porewater and open NRR and Gulf of Aqaba waters.Uranium concentration in secondary aragonite filling the skeletal pores is significantly higher than in primary biogenic aragonite (17.3 ± 0.6 compared to 11.9 ± 0.3 nmol · g⁻¹, respectively). This concentration difference reflects the closed system incorporation of uranyl tri-carbonate into biogenic aragonite with a U/Ca bulk distribution coefficient (K_D) of unity, versus the open system incorporation into secondary aragonite with K_D of 2.4. The implication of this result is that continuous precipitation of secondary aragonite over ∼1000 yr of reef submergence would reduce the coral porosity by 5% and can produce an apparent lowering of the calculated U/Ca - SST by ∼1°C and apparent age rejuvenation effect of 7%, with no measurable effect on the calculated initial U isotopic composition.All modern and some Holocene corals (with and without aragonite cement) from Elat yielded uniform δ²³⁴U = 144 ± 5, similar to the Gulf of Aqaba and modern ocean values. Elevated δ²³⁴U values of ∼180 were measured only in mid-Holocene corals (∼5000 yr) from the buried reef. The values can reflect the interaction of the coral skeleton with ²³⁴U-enriched ground-seawater that washes the adjacent granitic basement rocks.We conclude that pore filling by secondary aragonite during reef submergence can produce small but measurable effects on the U/Ca thermometry and the U-Th ages. This emphasizes the critical importance of using pristine corals where the original mineralogy and porosity are preserved in paleooceanographic tracing and dating.  相似文献   

Nanostructure, composition and mechanisms of bivalve shell growth   总被引：3，自引：0，他引：3  

D.E. Jacob  A.L. Soldati  J. Huth  W. Hofmeister 《Geochimica et cosmochimica acta》2008,72(22):5401-5415

Freshwater and marine cultured pearls form via identical processes to the shells of bivalves and can therefore serve as models for the biomineralization of bivalve shells in general. Their nanostructure consists of membrane-coated granules (vesicles) which contain amorphous calcium carbonate (ACC) at the beginning of the biomineralization sequence, preceding the crystallization of aragonite and vaterite. In contrast to the commonly accepted view, crystallization of ACC occurs rapidly and within the granular nano-compartments mediated by organic molecules much earlier than platelet formation. The interlamellar organic sheets in nacre that form the platelet structure of nacre themselves form by self-organization after the crystallization process of CaCO₃ is completed and, thus, cannot serve as a nucleation template for aragonite. Pores in the organic sheets are postulated to be a result of this process rather than to represent the pathways for CaCO₃ through pre-existing interlamellar sheets. The amorphous phase has the highest concentrations of Mg (5.8 mol%), Mn (6.6 mol%), S (4.7 mol%) and P (1 mol%) of the three CaCO₃-polymorphs. Mg/Ca and Mn/Ca ratios are found to decrease in the order ACC > vaterite > aragonite, corresponding to decreasing organic content in the different phases. This, as well as an observed enrichment of Mg in the organic-rich growth-banding of the pearls, suggests an at least partially organic speciation of Mg and Mn in bivalves and may be responsible for the observed physiological influence on Mg/Ca and Mn/Ca ratios in bivalves as a proxy for environmental parameters.  相似文献   

Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar'a caldera,Anti‐Atlas,Morocco     

J. Javier lvaro  Laura Gonzlez‐Acebrn 《Sedimentology》2019,66(6):2048-2071

This paper presents a case study of the sublacustrine precipitation of hydrothermal silica ± TiO₂ in the Ediacaran Mançour Group of the Saghro inlier, Anti‐Atlas, Morocco. Lacustrine carbonates containing stromatolitic mats and bioherms occur in ephemeral ponds developed within the Oued Da'ra caldera. Its syn‐eruptive infill consists of pyroclastites, ashflow tuffs, and subsidiary lava flows and sills, whereas inter‐eruptive deposition is mainly represented by slope‐related debris‐flow breccias and landslides, alluvial fans and fluvial channels. Carbonate production took place in a mosaic of differentially subsiding, fault‐bounded intra‐caldera blocks controlled by episodic collapse‐induced drowning, pyroclastic blanketing and migration of alluvial/fluvial environments. After microbial carbonate production, the carbonates recorded several early‐diagenetic processes, punctuated by polyphase fissuring (controlling secondary permeability) locally linked to hydrothermal influx. Three generations of carbonate cements are recognisable: (i) fibrous, botryoidal and blocky/drusy mosaics of calcite; (ii) idiotopic mosaics of dolomite caused by flushing of hypersaline Mg‐rich brines; and (iii) euhedral to drusy calcite via dedolomitization. The δ¹³C and δ¹⁸O values from carbonate cements broadly become successively isotopically lighter, as a result of meteoric and hydrothermal influence, and were probably overprinted by the Panafrican‐3 phase that affected the top of the Mançour Group. Two mechanisms of silicification are involved: (i) early‐diagenetic occlusion of interparticle pores at the sediment/water interface of pyroclastic substrates and reefal core and flanks; and (ii) hydrothermal precipitation of silica ± TiO₂ lining fissures and vuggy porosity encased in the host rock. Silica conduits cross‐cutting lacustrine mats and bioherms exhibit high potential of preservation in collapsed volcanic calderas. Primary fluid inclusions of hydrothermal silica contain brine relics with NaCl/CaCl₂ ratios of 2·1 to 4·4, representing minimum entrapment temperatures of about 142 to 204°C, and abiotic hydrocarbons (heavy alkanes) related to serpentinization of the volcanic and volcanosedimentary basement of the Oued Dar'a caldera.  相似文献   

An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures   总被引：1，自引：0，他引：1  

Gen-Tao Zhou 《Geochimica et cosmochimica acta》2003,67(3):387-399

To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO₃)₂ solution at 0 to 50°C in the presence of Mg²⁺ or SO₄²⁻. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO₃ minerals slowly precipitated by the CaCO₃ or NaHCO₃ dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO₂ gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows: