Aragonitic Grains Research Articles

The role of sulfate in the hydrothermal replacement of aragonite single crystals by calcite

Aragonite (CaCO3) is a stable calcium carbonate phase under high pressure conditions. However, its formation in (sub)surface environments, where calcite is the stable polymorph, is widespread. Regardless of its origin, aragonite is expected to undergo transformation into calcite under moderate pressures and temperatures. However, this transformation does not always take place, which results in the presence of abundant aragonitic relics in the geological record. Traditionally, this preservation has been explained by the presence of chemical inhibitors that prevent the conversion of aragonite to calcite. While it is widely accepted that magnesium (Mg) plays a key role in the polymorphic selection of CaCO3, the influence of other ions has also been suggested. This work evaluates the effect that different concentrations of sulfate (SO42−) in the fluid has on the progress of the aragonite-to-calcite transformation at 220 °C. Our results show that, upon reaction with deionized water or sulfate-poor solutions ([SO42−]aq < 0.1 mM), aragonite single crystals are extensively replaced by calcite aggregates (crystal size > 15 µm) through an interface coupled dissolution-precipitation reaction. The replacement starts at the aragonite crystal surfaces and advances inwards thanks to the development of an extensive network of fractures. Contrarily, when the solution bears higher concentrations of sulfate ([SO42−]aq > 0.1 mM), only a thin layer of smaller crystals of calcite (< 10 µm) form on the aragonite substrates, without any further transformation taking place. We interpret that these smaller crystals exert too little crystallization pressure and fail to promote the development of a network of fractures. In the absence of this network, the aragonite-calcite transformation cannot take place. The transformation does not occur neither when the experiments are conducted with deionized water and fragments of gypsum or anhydrite together with the aragonite grains. The results of this study shed light on the influence of dissolved sulfate in the kinetics of the fluid-driven transformation of aragonite into calcite. These results are useful to understand the preservation of aragonite in a variety of current geological settings and provide valuable insights for better understanding the diagenesis of sedimentary carbonates.

Formation and properties of spindle-shaped aragonite mesocrystals from Mg-bearing solutions.

The formation of aragonite under ambient conditions is typically linked to Mg-rich aqueous environments. The grains that form in such environments show peculiar properties such as aggregate-like appearance and mesocrystalline character. We tested the effect of dissolved Mg2+ ions on the formation of aragonite mesocrystals by synthesizing aragonite with an automatic titrator at constant pH and at different dissolved Mg : Ca ratios, and by studying the properties of the precipitated material with various scanning transmission electron microscopy (STEM) techniques. At all studied Mg : Ca ratios the firstly condensed carbonate phase was Mg-bearing amorphous calcium carbonate (Mg-ACC) that transformed into aragonite during the synthesis experiments. The aragonite grains had typically aggregate-like appearance and spindle shapes, with the external morphologies of the spindles unaffected by variation in solution chemistry. The alignment of the nanocrystals within the aggregates was crystallographically highly coherent, the [001] directions of nanocrystals showing only a small misorientation with respect to one another; however, both parallel and twin assembly of neighbouring crystals occurred. An increase in the dissolved Mg concentration decreased the crystallographic coherence between the aragonite nanocrystals, suggesting an important role of Mg2+ ions in the assembly of aragonite spindles. Whereas the mesoscale-ordered arrangement of nanocrystals implies a particle-mediated assembly, the observed differences in particle size and composition between the amorphous precursor and the crystalline end-product suggest that the crystallization includes at least partial dissolution and re-precipitation. These findings provide insight into the formation of aragonite and could contribute to the understanding of important aspects of the formation of mesocrystals and hierarchically structured biogenic minerals.

The incorporation of Mg2+ ions into aragonite during biomineralization: Implications for the dolomitization of aragonite.

Bacteria can facilitate the increase of Mg2+ content in biotic aragonite, but the molecular mechanisms of the incorporation of Mg2+ ion into aragonite facilitated by bacteria are still unclear and the dolomitization of aragonite grains is rarely reported. In our laboratory experiments, the content of Mg2+ ions in biotic aragonite is higher than that in inorganically-precipitated aragonite and we hypothesize that the higher Mg content may enhance the subsequent dolomitization of aragonite. In this study, biotic aragonite was induced by Bacillus licheniformis Y1 at different Mg/Ca molar ratios. XRD data show that only aragonite was precipitated in the media with Mg/Ca molar ratios at 6, 9, and 12 after culturing for 25 days. The EDS and atomic absorption results show that the content of Mg2+ ions in biotic aragonite increased with rising Mg/Ca molar ratios. In addition, our analyses show that the EPS from the bacteria and the organics extracted from the interior of the biotic aragonite contain the same biomolecules, including Ala, Gly, Glu and hexadecanoic acid. The content of Mg2+ ions in the aragonite precipitates mediated by biomolecules is significantly higher than that in inorganically-precipitated aragonite. Additionally, compared with Ala and Gly, the increase of the Mg2+ ions content in aragonite promoted by Glu and hexadecanoic acid is more significant. The DFT (density functional theory) calculations reveal that the energy needed for Mg2+ ion incorporation into aragonite mediated by Glu, hexadecanoic acid, Gly and Ala increased gradually, but was lower than that without acidic biomolecules. The experiments also show that the Mg2+ ion content in the aragonite significantly increased with the increasing concentration of biomolecules. In a medium with high Mg2+ concentration and with bacteria, after 2 months, micron-sized dolomite rhombs were precipitated on the surfaces of the aragonite particles. This study may provide new insights into the important role played by biomolecules in the incorporation of the Mg2+ ions into aragonite. Moreover, these experiments may contribute towards our understanding of the dolomitization of aragonite in the presence of bacteria.

Characterization and origin of micropores in tight gas grainstones of the Lower Triassic Feixianguan Formation in the Jiannan gas field, Sichuan Basin

Economic importance of micropores with diameter less than 10 μm in petroleum reservoirs has attracted more and more attention in hydrocarbon exploration because of a significant influence on evaluation and production of hydrocarbons. The Lower Triassic Feixianguan Formation of the Jiannan gas field in the Sichuan basin has been gas productive over the last 40 years, producing a cumulative total of nearly two billion cubic meters of hydrocarbon gases from tight grainstone reservoirs. However, a complex problem is that there are nearly no visible pores in thin sections with a petrographic microscope, leading to the poorly understanding origin and diagenetic evolution of the pore network in the Feixianguan limestones. The studied reservoirs consist mainly of grainstones and have helium porosity less than 4% with an average of 1.3%. Observation from approximately 410 thin sections demonstrated that all primary interparticle macropores (diameter >10 μm) were totally destroyed by compaction and cementation, and minor amounts of secondary intraparticle macropores (macroporosity ≤0.5%) were visible only in five samples, occurring mainly in the upper parts of depositional cycles. The greater helium porosity than thin-section macroporosity suggests the porosity consists dominantly of micropores. Mercury injection capillary pressure, scanning-electron microscopy, and micro-CT analyses revealed further that micropores are mainly hosted within grains, and contribute more than 95% to total pore systems. The development of micropores in the Feixianguan grainstones is closely associated with the freshwater-involved mineralogical stabilization of aragonitic grains due to subaerial exposure shortly after deposition. This stabilization involves the dissolution of aragonite and re-precipitation of microrhombic calcites, accompanied by the conversion of micropores from between aragonite needles to between calcite microcrystals. The Feixianguan grainstones in the Jiannan gas field are a typical micropore-dominated reservoir for gas storage and production, which provides an excellent analog for other tight marine limestone reservoirs worldwide.

A qualitative-quantitative approach for studying the impact of facies and diagenesis control on the rudist biostrome of the Sarvak formation, Abadan plain, SW Iran

Carbonate reservoir properties and their heterogeneities are of the main issues in reservoir qualitative and quantitative studies. In this paper, the role of facies and diagenesis processes on reservoir quality in the Cenomanian-Early Turonian Sarvak formation of SW of Iran was studied using 1504 samples from the core and a set of full suite logs from three wells. The depositional setting ranges from grain-rich reefal to lagoonal facies with grain-dominated and mud-dominated textures. Several diagenetic realms such as meteoric vadose, phreatic (vadose and marine) and mixing were identified based on petrographic observations. Compared to the mud-dominated facies of the Sarvak formation, the grain-dominated facies were intensely subjected to diagenetic processes. Furthermore, dissolution and neomorphism resulting from the subaerial exposure during meteoric diagenesis enhanced the reservoir quality, while further cementation, compaction and micritization have a negative impact on reservoir characteristics. The quantitative approaches show the eustatic sea-level rise in the Cenomanian-Turonian and the regional Turonian unconformity provide a textural and diagenetic pathway towards enhancing/reducing the reservoir quality. Reservoir properties are highly dependent on the sedimentary processes of the Rudist biostromes where secondary diagenetic agents are probably contributed through the selective leaching of aragonitic grains and matrix under the meteoric water environment.

Marine aragonite evolution in the oxygen-decreasing interval before the Cenomanian-Turonian Ocean anoxic event (OAE2) in the southeastern Neo-Tethys

Environmental scenarios around the Cenomanian-Turonian ocean anoxic event (OAE2) interval are scarcely studied in the southeastern Neo-Tethys and not discussed in terms of variation of primary mineralogies of marine carbonates. Based on petrological and geochemical analyses, this study focuses on the increased aragonite abundance in the oxygen-decreasing interval before the OAE2 and its environmental implications in the southeastern Neo-Tethys. In central Iraq, the upper Cenomanian-lower Turonian Mishrif Formation represents marine carbonate records in the southeastern, tropical Neo-Tethys and includes five members from the lowermost Mi-5 to the uppermost Mi-1. From Mi-5 to Mi-1, dominant carbonate components and primary carbonate mineralogies vary. Mi-5 is dominated by primarily calcitic grains (mainly calcite shells of rudists and echinoids); Mi-4 and Mi-3 dominated by primarily aragonitic grains (mainly green algae and aragonitic bivalves) and planktonic calcitic foraminifera; and Mi-1 dominated by primarily calcitic ooids. Paleoenvironmental interpretations suggest: i) open marine, offshore settings for Mi-5 to Mi-3, and a shallow marine shoal setting for Mi-1; ii) a sea level rise from Mi-5 to Mi-3, followed by a sea level fall to Mi-1. The increasing aragonite abundance in Mi-4 and Mi-3 is exceptional and suggested to most likely reflect enhanced preservation of shallow-marine aragonite transported into open marine offshore settings. This enhanced aragonite preservation is likely related to an oxygen-decreasing level before the OAE2 in the offshore settings. Remarkably, these primarily aragonitic components are commonly preserved now as moldic pores in Mi-4 and Mi-3. The moldic pores are interpreted to be produced via deep burial dissolution of recrystallized aragonitic grains, based on petrological and geochemical analyses. Moreover, the primary carbonate compositional and mineralogical changes from Mi-5 to Mi-1 provide some potential clues for the environmental scenarios around the OAE2 interval in the southeastern, tropical Neo-Tethys: i) from Mi-5 to Mi-4 and Mi-3, the increasing abundance of primarily aragonitic components is accompanied by a sea level rise, increasing seawater temperature and decreasing oxygen level of bottom water; ii) from Mi-3 to Mi-1, the decreasing abundance of primarily aragonitic components is accompanied by a sea level fall, decreasing seawater temperature and enhanced microbial carbonate production.

Quantitative compaction trends of Miocene to Holocene carbonates off the west coast of Australia

In this paper, we describe porosity variations in Miocene to Holocene carbonates off the west coast of Australia and assess their compaction trends. The porosity values were measured using discrete samples of Sites U1459–U1464 obtained by the International Ocean Discovery Program Expedition 356. The carbonate deposits have been influenced by a range of textures and diagenetic conditions throughout a nearly continuous sequence of geological ages from the Miocene to Holocene and at core depths from 0 to 1100 m below the seafloor. The collected samples were mostly grainstone, packstone, wackestone and mudstone textures. Dolostones and dolomitic carbonates were described at the Miocene intervals. Compaction trends were estimated exponentially and linearly based on cored sites, carbonate textures and dominant mineralogies (dolomite, calcite/aragonite). At all six sites, porosity distribution and reduction were generally depth-dependent. The porosity converged to about 30% between 750 and 1100 m, which suggests that the carbonates were close to the densest packing by mechanical compaction at a burial depth of ∼750 m. The porosity deviations are associated with textures and dominant mineralogies. Increasing mud content from grainstone to mudstone is a controlling factor for initial porosity and porosity reduction rate. Dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains cause deviations from the depth-dependent compaction trends. Reflux-related cementation generally decreases porosity in Miocene dolomitic intervals. Higher porosity values of the Quaternary wackestone and mudstone at Site U1461 resulted from the presence of aragonite needle-rich mud hosting abundant micropores and from a high sedimentation rate. The occurrence of non-skeletal grains, such as ooids and peloids, as well as occasional meteoric exposure led to porosity inversion, occluding interparticle permeability and the creation of moldic pores. KEY POINTS Porosity variations of Miocene–Holocene carbonates off the west Australian coast are primarily depth-dependent. Mechanical compaction affected porosity reduction down to a value of ∼30% at a burial depth of ∼750 m. Differences in texture-based compaction trends were affected by increasing mud content. Major porosity deviations are associated with dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains.

Lack of synsedimentary chemical alteration in polar carbonates (Ross Sea, Antarctica): Resolution of a conundrum

ABSTRACTAlthough rare in space and time, skeletal carbonates deposited on polar shelves hold great potential for improving understanding of the oceanography of the high latitudes. Low temperatures, low carbonate saturation states, and strong seasonality govern not only the nature of carbonate communities, but also how their deposits translate into the rock record. To understand the effects of early seafloor processes on preservation, we investigated late Quaternary carbonates recovered in piston cores from the Ross Sea, Antarctica. Sediments are unconsolidated skeletal gravels and sands that mantle areas of the outer shelf swept by strong bottom currents. Deposits are dominated locally by either stylasterine hydrocorals, barnacles, or bryozoans, which comprise assemblages with strong similarities to modern benthic communities. Radiocarbon ages indicate that carbonate factories were most prolific during the lead-up to the Last Glacial Maximum (Tartanian), when sediment input was minimized, and have been mostly dormant since. Results show that synsedimentary alteration is not substantially different in the temperate and polar realms with the significant exception of chemical diagenesis. As is common in temperate deposits, skeletal grains undergo disarticulation, fracturing, abrasion, and intense bioerosion. By contrast, cementation is absent and rare aragonite grains are preserved, indicating that taphonomic loss is not as prevalent as in temperate deposits. Primary skeletal microstructures and stable-isotope compositions are preserved, indicating that chemical alteration of grains is negligible. The preservation of aragonite in polar settings is herein attributed to low rates of organic-matter burial and very low temperatures, which strongly limit microbial activity. These factors allow interstitial waters to remain weakly supersaturated with respect to aragonite. Comparison with Permian analogs indicates that lithification is delayed until deposits reach burial depths at which chemical compaction proceeds. The ultimate end product is limestone with prominent compaction features and a tightly packed fabric. Calcitic skeletal material can retain primary geochemical compositions through the lithification process, although growth of burial cement in intraparticle porosity complicates selective sampling of unaltered material. In providing a cold-water end member for the spectrum of synsedimentary diagenetic processes, results highlight specific differences that should be accounted for when interpreting the deposits of polar, cold-water carbonate systems.

Diagenetic stabilization of the Upper Permian Dalan Formation, Persian Gulf Basin

The Upper Permian Dalan Formation is a supergiant gas reservoir in the Persian Gulf Basin. Reservoir quality is controlled by the original facies type, dolomitization, and anhydrite cementation. This study focuses on the texture (petrographic analysis), bulk stable-isotope values, and trace element composition in order to identify the original mineralogy of carbonate grains and cements, and to assess the synsedimentary and diagenetic processes, including the timing and setting of dolomitization.The limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone, and neomorphism and low-Mg calcite cementation (blocky and drusy crystals, syntaxial overgrowths) in the active freshwater phreatic zone. The relatively pristine δ18O and δ13C values, low Mn and relatively high Sr and Na contents indicate that this meteoric diagenesis was relatively limited. The remaining porous space was cemented by blocky calcite later during burial.Four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites: dolomicrite, dolomicrosparite, dolosparite, and saddle dolomite. Dolomicrite formed syndepositionally from the evaporated Permian seawater or by reflux of dense evaporated brines during shallow burial. The second major episode of dolomitization (dolomicrosparite and dolosparite) was driven by reflux of warmer and more saline fluids at greater depth, which affected both previously non-dolomitized and dolomitized sediments. The final diagenetic stabilization during deep burial is associated with extensive pressure solution, fracturing, vein-filling of calcite and dolomite cements, and crystallization of saddle dolomite.

Architecture of Anoteropora latirostris (Bryozoa, Cheilostomata) and implications for their biomineralization

Cheilostome Bryozoa Anoteropora latirostris, a colonial marine invertebrate, constructs its skeleton from calcite and aragonite. This study presents firstly correlated multi-scale electron microscopy, micro-computed tomography, electron backscatter diffraction and NanoSIMS mapping. We show that all primary, coarse-grained platy calcitic lateral walls are covered by fine-grained fibrous aragonite. Vertical lateral walls separating autozooid chambers have aragonite only on their distal side. This type of asymmetric mineralization of lateral walls results from the vertical arrangement of the zooids at the growth margins of the colony and represents a type of biomineralization previously unknown in cheilostome bryozoans. NanoSIMS mapping across the aragonite-calcite interface indicates an organic layer between both mineral phases, likely representing an organic template for biomineralization of aragonite on the calcite layer. Analysis of crystallographic orientations show a moderately strong crystallographic preferred orientation (CPO) for calcite (7.4 times random orientation) and an overall weaker CPO for aragonite (2.4 times random orientation) with a high degree of twinning (45%) of the aragonite grains. The calculated Young’s modulus for the CPO map shows a weak mechanical direction perpendicular to the colony’s upper surface facilitating this organism’s strategy of clonal reproduction by fragmentation along the vertical zooid walls.

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

AbstractStudies 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.

Collisional and alteration history of the CM parent body

Boriskino is a little studied CM2 chondrite composed of millimeter-sized clasts of different lithologies and degrees of alteration. Boriskino thus offers a good opportunity to better understand the preaccretionary alteration history and collisional evolution that took place on the CM parent body. The least altered lithology displays 16O-poor Type 1a calcite and aragonite grains (δ18O ≈ 30–37‰, δ17O ≈ 15–18‰ and Δ17O ≈ −2 to 0‰, SMOW) that precipitated early, before the establishment of the petrofabric, from a fluid whose isotopic composition was established by isotopic exchange between a 16O-poor water and 16O-rich anhydrous silicates. In contrast, the more altered lithologies exhibit 16O-rich Type 2a and veins of calcite (δ18O ≈ 17–23‰, δ17O ≈ 6–9‰ and Δ17O ≈ −4 to −1‰, SMOW) that precipitated after establishment of the deformation, from transported 16O-rich fluid in preexisting fractures. From our petrographic and X-ray tomographic results, we propose that the more altered lithologies of Boriskino were subjected to high intensity impact(s) (10–30 GPa) that produced a petrofabric, fractures and chondrule flattening. Taking all our results together, we propose a scenario for the deformation and alteration history of Boriskino, in which the petrographic and isotopic differences between the lithologies are explained by their separate locations into a single CM parent body. Based on the δ13C-δ18O values of the Boriskino Type 2a calcite (δ13C ≈ 30–71‰, PDB), we propose an alternative δ13C-δ18O model where the precipitation of Type 2a calcite can occurred in an open system environment with the escape of 13C-depleted CH4 produced from the reduction of C-bearing species by H2 released during serpentinization or kamacite corrosion. Assuming a mean precipitation temperature of 110 °C, the observed δ13C variability in T2a calcite can be reproduced by the escape of ≈15–50% of dissolved carbon into CH4 by Rayleigh distillation.

Microfacies, diagenesis and oil emplacement of the Upper Jurassic Arab-D carbonate reservoir in an oil field in central Saudi Arabia (Khurais Complex)

The Upper Jurassic (Kimmeridgian) Arab-D reservoir of the Arab Formation in the Arabian Peninsula is considered to be the largest hydrocarbon carbonate reservoir in the world. Despite a relative large number of studies dealing with lithofacies and depositional sequences, there are still few studies dealing with diagenesis of this supergiant reservoir. In this study, petrographic, cathodoluminescence and ultra-violet light fluorescence microscopy analyses of the Arab-D reservoir in an oil field of Saudi Arabia (Khurais Complex) have been combined with isotope geochemistry (δ13C, δ18O and 87Sr/86Sr) to decipher the sequence of diagenetic events affecting the reservoir before and during oil emplacements, and the impact of depositional mineralogies and their modifications during the diagenesis on the reservoir quality and heterogeneity. All the analyzed samples correspond to lithofacies that can be ascribed to nine microfacies types (MFT-1 to MFT-9). Three stages of carbonate dissolution (d1 to d3), four stages of calcite cements (C1 to C4) and 5 zones of replacive dolomite and dolomite cement (D1 to D5) have been documented from diagenetic environments ranging from marine to late burial. Abundant oil fluid inclusions have been recognized in calcite C3 and C4 and dolomite D5 from wells located in the flanks of the structure (Group 2 of cements). Cements from wells located in the center of the structure are largely oil-inclusions free (Group 1 of cements). This distribution suggests that Group 2 of cements formed within the oil-water transition zone of the reservoir during the oil infilling of the structure, allowing to constrain the timing of oil emplacement and suggesting at least two phases of oil charging. Porosity enhancement by corrosive dissolution (d3) occurred during late burial and took place just before precipitation of dolomite D5 and calcite C4, and just before the second stage of hydrocarbon arrival, suggesting the presence of aggressive fluids preceding the late phase of hydrocarbon migration. Cross-plot of carbon vs. oxygen stable isotopes shows a general trend toward decreasing δ18O values with increasing burial depth, whereas all δ13C values remain positive except for Group 2 of C3 and C4 calcites, which exhibit negative (up to −4.6‰) δ13C values. The later suggest precipitation from pore waters with increased volumes of 12C as a by-product of hydrocarbon maturation. Except for C4, all the 87Sr/86Sr values are within the range of the Late Jurassic seawater, suggesting precipitation from Jurassic marine or evolved marine pore waters. In contrast, the more radiogenic calcite C4 suggests a source from fluids that interacted with siliciclastics or clay-rich sequences. Depositional mineralogy of the grainy lithofacies is interpreted to have had an impact on the effectiveness of cementation and dissolution, controlling the reservoir heterogeneity. Due to excess of cementation, reservoir quality is worse in grainstone microfacies rich in aragonitic components and better in grainstone and packstone microfacies poorer in aragonitic grains but affected by late dissolution of micrite. The integration of all these data resulted in a more precise understanding of the reservoir quality and heterogeneity.

Ground-Penetrating-Radar Characterization and Porosity Evolution of An Upper Pleistocene Oolite-Capped Depositional Cycle, Red Bays, Northwest Andros Island, Great Bahama Bank

Abstract: Linkages among surface sediments, depositional processes, geomorphic forms, stratigraphic architecture, and pore distribution in Bahamian sand bodies and oolite-capped depositional successions have been characterized in recent studies. An understanding of how these parameters, especially porosity, are preserved or modified in the eogenetic environment is essential in bridging the gap between modern and ancient analogs. Two complementary 3D (200 MHz and 400 MHz) ground-penetrating-radar surveys and three shallow cores reveal that the uppermost part of the upper Pleistocene (Marine Isotope Stage 5e) Lucayan Formation at Red Bays, Andros Island, Great Bahama Bank, comprises a single, 6-m-thick upward-shallowing parasequence. Six radar packages (P1–P6) bounded by radar surfaces (S1–S6) have been identified in the two 3D radar volumes. The stratigraphic succession indicates development of a burrowed backshoal lagoon during early flooding of the bank top followed by deposition of flood-tide-driven, low-angle sheet sands and sigmoidal subaqueous dune deposits as the shelf-margin sand shoal moved bankward as a function of continued sea-level rise. Constructional accommodation on the front of the prograding subaqueous dune in the northwest part of the survey area and erosional accommodation created by downcutting of a shallow tidal channel into the upper part of the subaqueous dune depositional unit in the northeast part of the study area were filled with peloid- and ooid-dominated carbonate sands as sea level stabilized. The uppermost radar package is a thin (0.15 to 0.55 m), partially calichified carbonate sand sheet that mantles the entire survey site and that reflects late highstand and fall of sea level. Exposure of the studied interval to meteoric vadose diagenesis over the past 100,000+ years has resulted in partial stabilization of the largely aragonitic precursor sediment through selective partial dissolution of aragonitic grains (mainly ooid cortical layers, peloids, and mollusk shells) accompanied by precipitation of patchily distributed low-magnesium calcite cement. Core data demonstrate a relationship between stratigraphically controlled sedimentological attributes, diagenesis, and pore distribution. Three distinctive zones of porosity were present upon deposition: a lower zone of relatively high interparticle porosity (21.3% to 33.2%) in the medial part of P1, a low-porosity zone (

Precipitation of halite during calcification of the massive reef-building coral Porites lobata

Detailed observations of mineral phases in the skeleton of a massive reef-building coral were performed at nano- to micrometer scales using an analytical transmission electron microscope. The most notable result is the discovery of a nanosized halite (NaCl) phase coexisting with aragonite (CaCO 3 ) in the coral skeleton. The halite grains are scattered not only along grain boundaries but also inside aragonite grains. The specific relationship of crystallographic orientations and microstructural features between halite and surrounding aragonite suggest that halite precipitated simultaneously with aragonite during calcification. The primary deposition of a mineral phase other than aragonite in a coral skeleton is confirmed for the first time.

IMPACT OF DIAGENESIS ON RESERVOIR QUALITY IN RAMP CARBONATES: GIALO FORMATION (MIDDLE EOCENE), SIRT BASIN, LIBYA

The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin is strongly influenced by depositional facies and various diagenetic processes. Based on data from core samples and well logs from five boreholes in the Assumood and Sahl gasfields in the subsurface of the north‐central Sirt Basin, six major carbonate facies (and fourteen microfacies) are distinguished in the Middle Eocene Gialo Formation (thickness ∼1100 ft/335 m). Wackestones‐packstones dominate the Gialo Formation with abundant grains of larger benthic foraminifera, especially nummulites, in a matrix composed mostly of finely comminuted bioclastic material. Sediments were deposited as a mosaic of facies on a broad carbonate ramp in moderate‐energy nummulite‐packstone banks, with locally restricted back‐bank lagoons and fore‐bank areas of foraminiferal debris passing to offshore lime mudstones.Marine diagenesis was minor with micritization of bioclasts and rare vadose marine fibrous cements. Sparry calcite cements are ubiquitous and were precipitated during shallow to moderate burial from seawater and/or meteoric water, the latter largely unaffected by surface‐pedogenic processes. This is indicated by the fabrics and pre‐ and post‐compaction precipitation of drusy spar and echinoderm overgrowths, and supported by their bright‐to‐dull luminescence and low negative δ18O signatures (‐2.68 to ‐4.16%° PDB). Some early neomorphic alteration of calcitic bioclasts is suggested by bright CL and marine to low negative δ18O values (‐1.06 to –3.93%° PDB). Bioclastic grains have δ13C values ranging between 0.76 and 1.19%° PDB, interpreted as marine signatures. Similar low positive δ13C values of the cements (0.9 to 1.05 %°) indicate a source of carbonate from dissolution of grains and/or seawater/meteoric water, but without any near‐surface/soil effects. Dissolution of grains and matrix, notably originally aragonitic grains but also the originally high‐Mg calcite nummulites, was a major porosity‐enhancing process, and took place from shallow to moderate burial depths. Fractures may locally have increased porosity too.Shallow‐water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality and these are concentrated in the upper parts (top ∼100 ft/30 m) of the wells, with porosity‐permeability decreasing downwards. This trend relates to the broad, large‐scale facies pattern of more mud‐dominated facies giving way upwards to more grainy, nummulite‐dominated facies as a result of an overall shallowing of the depositional environment. In addition, the influx of meteoric waters in the upper part of the Gialo platform, before drowning and deposition of the overlying Augila Shale, increased porosity through dissolution. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Gialo carbonates.

Aragonite loss in a cold‐water coral mound: mechanisms and implications

AbstractSelective dissolution of aragonitic grains is emerging as a volumetrically significant process that affects a broad range of modern carbonate settings. This study explores mechanisms and implications of aragonite loss in Challenger Mound, a giant cold‐water coral (Lophelia pertusa) mound of Pleistocene age, which lies on the continental slope off south‐west Ireland. A comprehensive sampling scheme allowed the integration of petrographic data with geochemical analyses of sediment and pore water. The mound remains virtually unlithified and consists of stacked, fining‐upward cycles of silty coral floatstone–rudstone and bafflestone grading into wackestone. Whereas calcitic grains appear unaltered, aragonitic grains are corroded and fragmented. Aragonite dissolution is attributed to organic matter oxidation at/near the sediment–water interface and, at greater depths, to the initial stages of bacterially mediated sulphate reduction, when alkalinity production is outpaced by the generation of H+. Pore water profiles indicate that undersaturated waters are diffusing towards the mound interior from two centres of sulphate reduction: one located in the upper 10 m of the sediment column and a second that lies below an erosional unconformity which marks the base of the mound. Continued aragonite dissolution is expected to gradually lower the diagenetic potential of the Challenger Mound and delay lithification until deep burial, when solution‐compaction processes come into play. Despite a fundamental role in predestining the final taphonomic and textural characteristics of Challenger Mound, the processes described here are expected to leave little trace in the geological record due to a lack of cementation and calcitization. Assuming that similar processes have been active throughout the Phanerozoic, results imply that the understanding of diagenetic processes in carbonate systems may be incomplete.

Partial high‐pressure aragonitization of micritic limestones in an accretionary complex, Tavşanlı Zone, NW Turkey

AbstractPelagic micritic limestones within an upper Cretaceous accretionary complex in the Tavşanlı Zone, NW Turkey, preserve textures indicating incomplete prograde transformation of micritic calcite to aragonite, representing the only known example of this type. Aragonitization starts at the central parts of the micritic limestone beds and advances towards the lower and upper parts of the layers at the expense of micrite. Micrite is very fine grained (&lt;0.003 mm) and contains radiolaria, foraminifera and thin shell fragments. Aragonite forms large crystals, up to 3 cm across, with straight grain boundaries and c‐axis mostly subparallel to the carbonate beds. Relict micritic portions are devoid of any aragonite grain. Stylolites characterized by the accumulation of clay minerals, Fe‐Mn‐hydroxides and quartz are concentrated in the upper and lower parts of the beds. Stylolite formation precedes aragonitization. Conditions of aragonitization are estimated as 200 ± 50 °C and 0.45–0.65 GPa, based on metamorphic mineral assemblages observed in associated basalts. Several features such as (i) constant composition of micritic calcite (98–99 mol.% CaCO3) throughout individual beds, (ii) enormous grain size difference between micritic calcite and aragonite (up to 1200 times), and (iii) absence of any aragonite grains within the relict micritic portions suggest that kinetic rather than thermodynamic factors controlled selective aragonite formation in the central portions of carbonate layers.

Element partitioning during precipitation of aragonite from seawater: A framework for understanding paleoproxies

This study presents the results from precipitation experiments carried out to investigate the partitioning of the alkaline earth cations Mg 2+, Ca 2+, Sr 2+, and Ba 2+ between abiogenic aragonite and seawater as a function of temperature. Experiments were carried out at 5 to 75 °C, using the protocol of Kinsman and Holland [Kinsman, D.J.J., Holland, H.D., 1969. The coprecipitation of cations with CaCO 3 IV. The coprecipitation of Sr 2+ with aragonite between 16 and 96 °C. Geochim. Cosmochim. Acta 33, 1–17.] The concentrations of Mg Sr and Ba were determined in the fluid from each experiment by inductively coupled plasma-mass spectrometry, and in individual aragonite grains by secondary ion mass spectrometry. The experimentally produced aragonite grains are enriched in trace components (“impurities”) relative to the concentrations expected from crystal-fluid equilibrium, indicating that kinetic processes are controlling element distribution. Our data are not consistent with fractionations produced kinetically in a boundary layer adjacent to the growing crystal because Sr 2+, a compatible element, is enriched rather than depleted in the aragonite. Element compatibilities, and the systematic change in partitioning with temperature, can be explained by the process of surface entrapment proposed by Watson and Liang [Watson, E.B., Liang, Y., 1995. A simple model for sector zoning in slowly grown crystals: implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks. Am. Mineral. 80, 1179–1187] and Watson [Watson, E.B., 1996. Surface enrichment and trace-element uptake during crystal growth. Geochim. Cosmochim. Acta 60, 5013–5020; Watson, E.B., 2004. A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals. Geochim. Cosmochim. Acta 68, 1473–1488]. This process is thought to operate in regimes where the competition between crystal growth rate and diffusivity in the near-surface region limits the extent to which the solid can achieve partitioning equilibrium with the fluid. A comparison of the skeletal composition of Diploria labyrinthiformis (brain coral) collected on Bermuda with results from precipitation calculations carried out using our experimentally determined partition coefficients indicate that the fluid from which coral skeleton precipitates has a Sr/Ca ratio comparable to that of seawater, but is depleted in Mg and Ba, and that there are seasonal fluctuations in the mass fraction of aragonite precipitated from the calcifying fluid (“precipitation efficiency”). The combined effects of surface entrapment during aragonite growth and seasonal fluctuations in “precipitation efficiency” likely forms the basis for the temperature information recorded in the aragonite skeletons of Scleractinian corals.

Present-day temperate carbonate sedimentation on the Balearic Platform, western Mediterranean: compositional and textural variation along a low-energy isolated ramp

Abstract The pre-1980s literature on modern carbonates was biased toward tropical examples because non-tropical carbonates had not been studied extensively. Though non-tropical carbonates have received considerable attention in the past decade, the variety of low-energy, temperate ramp examples in the literature is limited. In contrast, examples of modern and ancient low-energy, tropical ramps are well represented. They are characterized by a gradual change from calcarenites updip to calcilutites downdip, by a photozoan biota, abundant non-skeletal aragonitic grains, such as ooids and peloids, and by widespread and rapid marine cementation. The Balearic Platform in the western Mediterranean is an isolated, low-energy, temperate ramp. This paper describes the Balearic ramp bathymetry, environmental regimes and depositional facies, which include (from the shoreline seaward): coastal lagoons, beach—dune complexes, inner-ramp seagrass meadows with mixed terrigenous—foraminiferal—molluscan muddy calcarenites, middle-ramp bryozoan—rhodalgal facies and outer ramp clastic—carbonate muds. Biotic and textural characteristics vary with depth, with hydrological conditions and with environmental factors, such as temperature, salinity, light and nutrients. Seagrasses extend across the inner and part of the middle ramp, where the grasses offer shelter to a variety organisms, including epibionts, molluscs, bryozoans, echinoderms and red algae. Most of the beach and dune sediments consist of bioclasts derived from the communities that thrive in the seagrass meadows, but the greatest volume of skeletal carbonates is produced as bryozoan, rhodalgal and molluscan gravels that occur as patchy blankets, primarily on the middle ramp. The Balearic Platform is characterized by an oligotrophic, clear water, microtidal environment. The dominant biota of the Balearic ramp — bryozoans, red algae, echinoderms and molluscs — is common in other non-tropical modern environments as well as in ancient temperate settings. This fact, combined with the absence of aragonitic non-skeletal grains, abundant marine cements and photozoans other than red algae, establishes the modern Balearic ramp as a model for comparison with low-energy, non-tropical ramps in the global rock record.