Composition Of Aragonite Research Articles

The origin of calcite in calc‐silicate rocks from the Kokchetav ultrahigh‐pressure metamorphic complex

AbstractUnderstanding calcite genesis in ultrahigh‐pressure crustal rocks is a key to the reconstruction of the evolution of ultrahigh‐pressure metacarbonate rocks. Here, we present new data and a new model on the genesis and the P–T conditions of the formation of calcite found in the ultrahigh‐pressure calc‐silicate rocks from the Kokchetav massif. In the studied sample aragonite inclusions coexist with Type A calcite inclusions (previously interpreted as mineral inclusions) and the inclusions of Type B calcite (previously interpreted as derived from the crystallization of carbonatitic melt) in cores of garnet porphyroblasts. The most Mg‐rich calcite from Type A inclusions coexisting with aragonite inclusions in one garnet growth zone shows XCa = 0.935 implying their crystallization during a retrograde metamorphic stage at P ~ 2.3 GPa and T ~ 870°C along the P–T path. Type A calcite and aragonite inclusions were also found coexisting in one growth zone with K‐bearing clinopyroxene inclusion (ω[K2O] = 0.5 wt.%). Such a high K2O‐content in clinopyroxene testify that the pressure of inclusion capture exceeded 3.5 GPa, which contradicts the P–T conditions estimated by XCa in magnesian calcite. Thus, Type A calcite inclusions were initially captured as an aggregate of aragonite+ magnesian calcite at ultrahigh pressure metamorphic stage (P ≥ 3.5 GPa, T = 900–1,000°C) and then re‐equilibrated at lower conditions (P ≤ 2.3 GPa and T ≤ 870°C). The trace element composition of aragonite and Type A and Type B calcite from inclusions was also studied to clarify calcite genesis in these inclusions. Aragonite shows high LREE (5–57 ppm) and Sr‐content (600–800 ppm). Calcite from Type A inclusions shows low LREE (2.9–19.8 ppm) and Sr‐content (490–670 ppm). Calcite from Type B inclusions forms two groups according to the LREE and Sr content distribution (Type B1 and Type B2). Trace element distribution in Type B1 calcite is identical to that of Type A calcite, while Type B2 calcite shows high LREE (6.8–64.9 ppm) concentrations along with low Sr‐content (180–340 ppm). Type A and Type B1 calcite is interpreted to have been re‐equilibrated. Type B2 calcite inclusions crystallized from the hydrous carbonatitic melt.

Equilibrium Ca isotope fractionation and the rates of isotope exchange in the calcite-fluid and aragonite-fluid systems at 25 °C

The Ca isotope composition of calcite and aragonite can provide insights into their formation conditions. The accurate interpretation of the Ca isotope composition of natural samples, however, requires precise knowledge of the equilibrium Ca isotope fractionation between the solid and fluid phases. In this study, the three-isotope method with 42Ca, 43Ca and 44Ca has been used to estimate the equilibrium isotope fractionation of Ca between the CaCO3 minerals calcite and aragonite and the Ca2+ aquo ion. Reactive fluids were enriched with 43Ca and equilibrated with synthetic calcite and aragonite of natural Ca isotope distribution up to 3745 h at 25°C. The isotopic composition of solids and fluids was measured using MC-ICP-MS and the estimated equilibrium fractionation for calcite-Ca2+(aq) and aragonite-Ca2+(aq) was Δ44/42Casolid-fluid=−0.02±0.13‰ and −0.80±0.10‰, respectively. Textural observations of the reacted solids suggest that isotope equilibration in aragonite experiments occurs via extensive Ostwald ripening, yielding large crystalline needles at the end of the experiments. In contrast, calcite did not exhibit an observable increase in size during the course of the experiments. Isotope exchange rates in the case of calcite are similar to those reported in previous studies and ∼4 orders of magnitude lower than the far-from-equilibrium calcite dissolution rate. Calcium isotope exchange rates for aragonite are more rapid than calcite driven by a greater extent of Ostwald ripening occurring via dissolution/precipitation reactions. The results of this study suggest that the Ca isotope compositions of calcite and aragonite crystals in chemical equilibrium, but isotope disequilibrium, with natural fluids could be significantly altered without overt evidence of diagenesis, especially in the case of calcite. The extent to which Ca isotope compositions are altered, however, would depend strongly on the environmental conditions, such as fluid:solid ratio and permeability of the solid facilitating fluid transport.

Mineralogy of interior jewellery ammonites of the Samara region

Background. Ammonite products are increasingly gaining in popularity. Ammonites are characterised by a variety of colour shades, shapes and sizes, making them suitable for manufacturing a wide range of souvenir and jewellery products. Russia enjoys significant resources of ammonites of interior and jewellery-ornamental quality; however, these materials remain insufficiently studied from the mineralogical point of view. This article presents the mineral composition and gemological characteristics of the Upper Jurassic interior jewellery ammonites of the Samara region for the first time.Aim. To describe the stratigraphic position and mineral composition of interior jewellery ammonites of the Samara region.Materials and methods. Late Jurassic ammonites (45 samples) were collected from a number of their main locations. The characteristic types of ammonites of interior-jewellery quality were distinguished and studied. Experiments included the determination of microhardness (25 determinations), density (12 determinations), luminescence features (10 samples), optical and petrographic analysis (5 thin sections), as well as the quantitative determination of mineral and chemical composition (3 samples), and electron probe studies (2 samples).Results. Ammonites are characterized by the presence of a mother-of-pearl layer with multi-coloured iridescence. The ammonites under study consist mainly of apatite (21—70 wt. %) and calcite (21—87 wt. %) along with quartz, aluminosilicates, gypsum, pyrite, zeolites, goethite, ilmenite and organic matter. The chambers of ammonite shells are made of phosphorite and calcite. The walls and partitions of shells have completely lost their original aragonite composition and consist of apatite and calcite with pyrite inclusions. The increased content of Sr (up to 0.17 wt. %) and Ba (up to 0.01 wt. %) are recorded as trace elements in ammonites. Calcite contains the following trace elements (wt. %): Mg — 0.17, Mn — 0.17, Fe — 1.14, Sr — 0.11, and Y — 0.17.Conclusion. The mineral and chemical (including microinclusions and trace elements) composition of interior-jewellery ammonites was established for the first time. The ammonites of the Samara region are of commercial interest as interior samples. Their shell fragments can be used in jewellery production. The ammonites under study can be collected along the coastal cliffs and beach area of the Volga river, thereby requiring no capital investments and producing no environmental pollution.

LATE JURASSIC INTERIOR-JEWELRY QUALITY AMMONITES FROM THE ULYANOVSK REGION: THE MINERAL COMPOSITION AND GEMOLOGICAL CHARACTERISTICS

Decorative articles made of ammonites are currently in high demand. The mineral composition and gemological characteristics of interior-jewelry ammonites from the Upper Jurassic of the Ulyanovsk Region have first been studied in detail. The ammonites are characterized by a pearl layer showing a bright varycolored iridescence. A complex of ammonite investigations comprised determinations of their microhardness, density, and luminescence; their optical-petrographic analysis; determinations of the mineral and chemical composition; and the electron-probe and electron-microscopic investigations. The ammonites consist mainly of apatite (22-88 wt. % ) and calcite (6-64 wt. %). Quartz, gypsum, pyrite, zeolites, ilmenite, rutile, goethite, zircon, and organic matter are present. Impurity elements recorded in elevated concentrations are (in wt. % ) Sr, 0.237; Ba, 0.011; and Pb, 0.023; and those recorded in low concentrations (less than 0.01 wt. %) are Ni, Cu, Zn, Rb, U, Th, Y, and As. The outer walls and septa of an ammonite shell have completely lost their original aragonite composition and consist of apatite and calcite with inclusions of pyrite and organic matter. The pearl layer iridescence is related to lamellar layers of apatite, that inherited the structure of lamellar layers of aragonite. The iridescence color is determined by the size and order of the lamellars and does not depend on their mineral composition. Ammonites of the Ulyanovsk Region are of commercial interest as interior-decorative specimens. Internal fragments of the shells can be used in jewelry. Ammonites are collected in steep ravine walls, which requires no capital investment and does not violate the environment.

The mineralization and early diagenesis of deep-sea coral Madrepora oculata

The skeletons of aragonitic corals carry essential information about oceanic environmental changes in the past. However, coral-based geochemical proxies are sensitive to vital effect and post-depositional diagenetic processes, which have not been well understood for deep-sea corals. In this study, we have investigated the mineralization and early diagenetic process of the dendroid stem of a dead deep-sea scleractinian coral sample (Madrepora oculata) collected by Remote Operated Vehicle (ROV) from a seamount in the Caroline Ridge, western Pacific. Despite being only ~270 years old, as dated by the U-series disequilibrium method, this sample already has extensive diagenesis including a visible chalky layer. The outermost chalky layer and the adjacent pristine skeleton of the sample were characterized by multiple mineralogical (SEM, XRD, EBSD, Raman Spectroscopy) and geochemical (ICP-OES, LA-ICP-MS, IR-MS, MC-ICP-MS) techniques. Skeletal stable C and O isotopes display strong linearity, with a trend pointing toward the theoretical isotopic composition of inorganically-precipitated aragonite from local seawater. This observation suggests that the epithecal mineralization of M. oculata is far from equilibrium with seawater and can be quantitatively explained by bio-mineralization models developed for solitary scleractinian corals. We further show that chalking of the outer layer, a common feature in sub-fossilized deep-sea corals, is caused by microbial-driven boring and μm-scale dissolution voids at an average rate of 3.5 μmol/cm2/y (scaled to the skeleton surface) without notable secondary infillings or mineralogical alternations. The higher average δ13C and δ18O of the chalky part than the pristine part might be due to the preferential dissolution of carbonate relatively depleted in 13C and 18O during alteration, and/or reduced isotope fractionation from seawater during coral mineralization of the outer layer. The chalky layer shows significantly higher Mn/Ca and Th/Ca ratios than the pristine part, characterized by Th/Mn ratios similar to that of the dissolved component of local seawater. Our study suggests that the incorporation of external Th into the coral skeleton is most likely associated with organic binding rather than detrital contamination, secondary carbonate mineral formation, adsorption onto surfaces of chalky aragonite or ferromanganese oxides, with important implications for coral U-series geochronology.

RESPONSE OF THE OLIGO-MIOCENE BIVALVE FAUNA OF THE KUTCH BASIN (WESTERN INDIA) TO REGIONAL TECTONIC EVENTS

ABSTRACTTectonic changes have influenced the evolution of the marine community by changing the land and seaway configuration through time. Two such events during the Oligo-Miocene—the closure of the Tethyan seaway due to development of the Gomphotherium Landbridge leading to separation of the Arabian Sea from proto-Mediterranean Sea (∼ 19 Ma) and significant uplift of the Tibetan Plateau marking the initiation of the monsoon (∼ 16 Ma)—represent a classic case of tectonic shift influencing the regional environment of the Indian subcontinent. We investigated the taxonomic and body-size related response of the shallow marine fauna to this regional change using bivalves from 11 time-constrained shellbeds of the Kutch Basin (western India) from three formations—Maniyara Fort (Chattian), Khari Nadi (Aquitanian) and Chhasra (Burdigalian-Langian) representing a time span of ∼ 9 Ma (24.4–15 Ma).Our collection of over 2000 individuals represents a total of 15 families and 61 morphospecies. The fossils are predominantly calcitic in nature and families of aragonitic composition are often preserved as molds indicating a potential negative effect of diagenesis. The taphonomic nature, however, does not vary substantially across shellbeds and hence, less likely produced a temporal pattern. The five most abundant species, Ostrea latimarginata, Ostrea angulata, Talochlamys articulata, Anomia primaeva, and Placuna lamellata occur in all the formations. The species composition of the Maniyara Fort Formation is substantially different from those of the younger formations, implying the possible effect of biogeographic separation. The absence of proto-Mediterranean taxa in Oligocene shellbeds supports limited faunal exchange between the Mediterranean-Iranian Province (MIP) and the western Indian Province (WIP) as early as ∼ 24.4 Ma (Chattian). Faunal exchange, however, continued between the WIP and the adjacent Eastern African-Arabian Province (EAAP). Formation-specific evenness shows a monotonic decrease from the Maniyara Fort to the Chhasra Formation. However, shellbed-specific analyses of diversity and body size do not show a strong directional trend through time and supports the stasis model. Although it is difficult to rule out the negative influence of taphonomy on the diversity of the studied fauna, the Oligo-Miocene marine bivalve fauna of the Kutch Basin demonstrates little or no influence of the Tethyan closure and Himalayan upliftment on the diversity through time.

No ion is an island: Multiple ions influence boron incorporation into CaCO3

Boron isotope ratios – as measured in marine calcium carbonate – are an established tracer of past seawater and calcifying fluid pH, and thus a powerful tool for probing marine calcifier physiology and reconstructing past atmospheric CO2 levels. For such applications, understanding the inorganic baseline upon which foraminiferal vital effects or coral pH upregulation are superimposed should be an important prerequisite. Yet, investigations into boron isotope fractionation in synthetic CaCO3 polymorphs have often reported variable and even conflicting results, implying our understanding of the pathways of boron incorporation into calcium carbonate is incomplete. Here we address this topic with experimental data from synthetic calcite and aragonite precipitated across a range of pH in the presence of both Mg and Ca. We observe coherent patterns in B/Ca and Na/Ca ratios that, we suggest, point to paired substitution of Na and B into the carbonate lattice to achieve local charge balance. In addition, we confirm the results of previous studies that the boron isotope composition of inorganic aragonite precipitates closely reflects that of aqueous borate ion, but that inorganic calcites display a higher degree of scatter, and diverge from the boron isotope composition of aqueous borate ion at low pH. With reference to the simultaneous incorporation of other trace and minor elements, we put forward possible explanations for the observed variability in the concentration and isotopic composition of boron in synthetic CaCO3. In particular, we highlight the potential importance of interface electrostatics in driving variability in our own and published synthetic carbonate datasets. Importantly for palaeo-reconstruction, however, these electrostatic effects are unlikely to play as important a role during natural precipitation of biogenic carbonates.

The impact and implications of aragonite-to-calcite transformation on speleothem trace element composition

Trace elements in cave carbonates (both calcite and aragonite) have been considered major proxies in paleo-hydroclimatic research. However, their imprints in authigenic minerals might be altered by neomorphic processes (e.g., aragonite-to-calcite transformation). Therefore, it is crucial to understand the impacts of neomorphism on the trace element compositions of secondary calcite and understand the mechanisms of trace elements redistribution during neomorphic processes. In this study, two aragonite stalagmites (SN3 and SN15) from Shennong Cave, part of which were transformed into calcite, were further inspected and analyzed in terms of petrological observations, trace element geochemistry, and Mg compositional mapping. The results show that (1) the aragonite-to-calcite transformation in SN3 and SN15 occurs by a dissolution-precipitation mechanism and is influenced by several factors, including internal fluid compositions, microstructure (such as porosity and permeability) of the parent material, and crystal defects. (2) The primary aragonite is enriched in Sr but depleted in Mg compared with secondary calcite because of the crystallographic disparity. The non-equilibrium absorption of Mg and slow diffusive element transport may contribute to the extremely high Mg/Ca ratios in secondary calcite near the mineral phase boundary. Incorporation competition among trace elements also plays an important role in affecting the distribution of Sr and Mg in neomorphic calcite. Considering the previous study on the alteration of isotopes (δ18O, δ13C) associated with the aragonite-to-calcite transformation, we propose that intervening fluids play an important role in altering the secondary calcite oxygen isotopic composition and Mg/Ca ratio, while the composition of primary aragonite dictates the secondary calcite carbon isotopic composition and Sr/Ca ratio, provided that the transformation of aragonite to calcite caused by a dissolution–reprecipitation process occurring in trapped pore water. These results highlight the indispensability of petrographic analysis of speleothem specimens before using trace elements and isotopes for paleoclimate research.

ИЗУЧЕНИЕ РАКОВИННОГО ВЕЩЕСТВА СОВРЕМЕННЫХ И ПЛЕЙСТОЦЕНОВЫХ МОЛЛЮСКОВ РОДА DIDACNA

One of the ways to study the reactions of marine invertebrates to the external effects of changes in temperature and salinity is the biogeochemical analysis of skeletal parts, which are consistently built up during ontogenesis and record a variety of information about these changes. The most studied shells of mollusks, sea urchin shells and skeletal parts of corals. Information about the chemical composition of modern and fossil mollusk shells is widely used in solving geological and biological problems, including determining the temperature and salinity of ancient marine basins, studying the diagenesis of carbonate sediments, and the biochemical evolution of invertebrates. X-ray diffraction analysis of the shell matter of didacnae belonging to the Cardiidae showed an aragonite composition. The quantitative determination of elements in mollusk shells by microprobe analysis of spot scanning and spectrometric method is carried out. Samples were taken in successive layers of shell growth within the annual ring, and the seasonal dynamics of strontium changes were detected. For Didacna, strontium is the main element-indicator of seasonal and ontogenetic growth, is included in the crystal lattice of aragonite and forms strong compounds in the process of shell formation during the life of these bivalves. The variability of seasonal, ontogenetic, and taxonomic differences in a number of indicator elements in living and Pleistocene bivalves of the genus Didacna was studied.

Geochemistry and Diagenetic Evidence of Middle-Late Permian Deposits, Northern Iran

This study provides a evaluation of the initial carbonate mineralogy, and diagenetic condition of the Ruteh Formation, a middle-late Permian carbonate sequence, in the Emarat section in Central Alborz, northern Iran. 200 samples were examined for petrographical contents and thirty-four samples were collected for geochemical analysis. The high Sr and Na contents, Sr/Na>1, and the presence of dolomites and dissolution fractures are indicative aragonite early mineralogy. The diagenetic process in the Ruteh Fm. are an indication of the marine, meteoric and burial diagenetic environments. The original carbonate texture in the Ruteh Fm. are Severely affected by diagenetic process. The data obtained from the values of the Major (Ca and Mg), minor (Sr, Na, Fe, and Mn), oxygen and carbone isotopes show the initial aragonite composition of the Ruteh Fm. deposited in a subtropical shallow water. An open diagenetic system was indicated by changes in the values of Sr / Ca and δ18O vs. Mn. The shallow burial diagenetic temperature of the Ruteh carbonate was approximate 28.6°C with the maximum diagenetic temperature of 53.4°C.

Jewelry and ornamental ammonites from the deposits of the Hauterivian stage of the Ulyanovsk Region: Mineral composition, gemological characteristics

Significant volumes of products from ammonites of jewelry and ornamental quality of the Ulyanovsk region are entering the Russian and world markets. Ammonites are associated with the Lower Cretaceous Hauterivian deposits and consist mainly of aragonite, calcite and pyrite. The walls and partitions of the ammonite shells retained the original aragonite composition. Also established inclusions of apatite, algodonite, pyrolusite, hematite, goethite, quartz, ferrihydrite, magnetite, uranium oxide, vernadite, todorokite, monazite, halloysite, plagioclase, kaolinite, glauconite, dolomite, graphite and organic matter. An important role in the formation of new mineral species was played by bacteria. Mineralization of undamaged ammonite chambers was carried out by the sequential formation of calcite and pyrite layers without metasomatic substitutions. The color of calcite layers is determined by the contents of Mn and Fe. Ammonites are characterized by high decorativeness and good technological characteristics; they can be mined in the coastal cliffs and the beach area of the river Volga without mining, without violating the environment.

Mg isotopic systematics during early diagenetic aragonite-calcite transition: Insights from the Key Largo Limestone

The seawater magnesium (Mg) isotopic composition (δ26MgSW) can be applied to quantify marine Mg cycle in Earth's history. However, reconstruction of δ26MgSW by using marine carbonate has been challenged by the large range of variation in Mg isotopic composition of carbonate (δ26Mgcarb). Such variation, on the one hand, reflects, e.g. the mineralogy, temperature, and precipitation rate dependent isotopic fractionation during Ca‑carbonate precipitation. On the other hand, the pristine seawater record could be modified in carbonate diagenesis, during which less stable carbonate minerals would spontaneously convert to more stable mineral phases. In the modern ocean with high Mg/Ca (molar) ratio (~5), aragonite precipitation is favored. However, aragonite is thermodynamically less stable than low-Mg calcite at ambient surface environment, resulting in the spontaneous transition from aragonite to low-Mg calcite in the earliest stage of diagenesis (i.e. the aragonite-calcite transition). As a common process in the modern ocean, Mg isotopic fractionation in aragonite-calcite transition may provide valuable constraints on the variation of δ26Mgcarb. In this study, we measured Mg isotopic compositions of Pleistocene Key Largo Limestone (Florida), which has been experiencing the aragonite-calcite transition during early diagenesis. The Key Largo Limestone samples consist of a mixture of calcite and aragonite. δ26Mg of aragonite (δ26Mgarag) shows higher and more stable values, while calcite has lower yet more variable Mg isotopes (δ26Mgcal). We suggest a model conceptualizing a water-film attached to the mineral surface, in which the aragonite-calcite transition initiates with aragonite dissolution and the Mg exchange with external fluid can be variable, followed by Mg isotopes fractionate during low-Mg calcite precipitation. In this model, δ26Mgcal is controlled by (1) the chemical composition of water-film, which is dependent on the mineralogical composition of aragonite (i.e. Mg content and δ26Mgarag), the composition of external fluid (i.e. fresh water, seawater, or mixed fresh water and seawater), and the mixing ratio between dissolved aragonite and external fluid, and (2) the isotopic fractionation during calcite precipitation. Because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution. Therefore, δ26Mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes, further complicating the interpretation of carbonate data. Thus, the process-controlled Mg isotopic fractionation should be taken into consideration when reconstructing δ26MgSW by using carbonate sediments/rocks.

Calibration of shell δ18O from the common whelk Buccinum undatum highlights potential for palaeoenvironmental reconstruction

The common whelk, Buccinum undatum, is a commercially important gastropod found throughout the North Atlantic. One method of age and life history analysis for gastropod species is the use of oxygen isotope ratio (δ18O) measurements from their shells, which is a well-established technique for the reconstruction of historical seawater temperatures at the time of shell biomineralization. Palaeotemperature calibrations have been developed for different types of calcium carbonate as well as species-specific equations to produce the most accurate seawater temperature reconstructions. Here we investigate the four-layer internal structure of B. undatum shells and confirm an aragonite composition using Micro-Raman Spectroscopy (MRS). We then calibrate a species-specific palaeotemperature equation for this gastropod species. This was achieved through the isotopic analysis of shells from laboratory reared specimens of known provenance reared at specific seawater temperatures to produce the following:t(°C) = 14.96 (± 0.15) – 4.94 (± 0.22) x (δ18Oshell – δ18Owater)The calibrated equation differs significantly from previously published data derived from both aragonite and calcite. An offset of 1.04‰ (± 0.41‰) was discovered between observed δ18OShell values and those expected under equilibrium, suggesting a species-specific vital effect. The calibrated equation was used to reconstruct accurate, high resolution historical seawater temperatures from three sites across the U.K. (Shetland, the Menai Strait and Jersey). With this new accurate calibration, both modern and fossil B. undatum shells now have the potential to be employed as high-resolution archives of recent and historical seawater temperature.

Uranium isotope compositions of biogenic carbonates – Implications for U uptake in shells and the application of the paleo-ocean oxygenation proxy

The application of U isotopes in carbonates as a paleo-ocean oxygenation proxy is based on the critical assumption that the calcareous shell-building organisms incorporate U into their shells without fractionation relative to the U isotopic composition of ambient seawater. Recent studies claim a small, but resolvable, isotopic offset during abiotic and biogenic aragonite precipitation, whereas no isotope fractionation has been recorded during calcite precipitation. Although aragonite is meta-stable and not preserved over geological timescales (>1 Myr) and U precipitates during diagenesis, the U isotope composition of biogenic aragonite is important because aragonite precipitation is an important U sink to carbonate sediments. In contrast, low-magnesium calcite (LMC) is preserved over geological timescales and may provide a reliable fingerprint of ancient ocean chemistry. Therefore, a more general study is needed that compares U isotope compositions of primary marine biogenic carbonate precipitates.We report the U isotope compositions of 32 modern samples from geographically distinct localities in the Atlantic Ocean including corals (Scleractinia, Octocorallia), brachiopods (Articulata), molluscs (Tellina Listeri, Codahia Obicularis) and barnacles as well as one fossil mollusc. These samples reflect variable primary minerals, water temperatures, water depths, pH-values of ambient water, and U concentrations. Several seawater samples have also been measured to compare our methods with those of previously published studies.The analyzed modern corals and brachiopods display U isotopic compositions that are indistinguishable from modern seawater. This suggests that these carbonates have the potential to faithfully record the U isotopic composition of the surrounding seawater in which they form. The analyzed brachiopods are of particular interest as they are composed of the calcium carbonate polymorph LMC that is stable over geological timescales. While this study shows for the first time that LMC phases are robust targets in ancient samples, their low U abundance presents analytical challenges for precise U isotope analyses. We also show that two barnacle shells collected with ambient seawater have U isotopic compositions that are both lighter and heavier than the ambient seawater. The mechanism to explain this offset is not determined, but it demonstrates that at least barnacle shells are not representative of the seawater in which they last lived. Two of three partially fossilized mollusc shells also show resolvable offsets from seawater, likely indicating secondary processes that are known to shift or fractionate U isotopes. Collectively, our new data indicate that: 1) aragonite delivers U with a seawater composition to carbonate sediments, and 2) LMC shells of brachiopods that are stable over geological timescales may be more suitable for reconstructing the U isotope composition of ancient oceans.

Mineral composition and structural features of jewelry ammonites from mine OJSC «Mikhailovcement», Ryazan region

Система комплексной поддержки и сопровождения научного журнала Elpub позволяет запустить двуязычный сайт журнала со встроенной системой электронной редакции в соответствии с лучшими издательскими практиками, а так же предусматривает техническую и методическую поддержку со стороны специалистов НЭИКОН.

Shell beaches to beachrock: Diagenetic stable isotopic trends

Modern shell accumulations on Padre Island, Texas Gulf Coast, are composed of pristine aragonite whereas laterally adjacent Pleistocene beachrock on the mainland shores of Laguna Madre are made up of the same taxa but display a range of mixtures of aragonite and diagenetic low-Mg calcite. Cements and pedogenic crusts associated with the beachrock are 100% low-Mg calcite. Stable isotopic values for shells composed of 0, 15, 20, 45, 75 and 90% calcite display high correlations between the δ18O values and mineralogy (R2 = 0.95) and δ13C values and mineralogy (R2 = 0.87). The highly correlated changes in the stable isotopic compositions and mineralogy of the skeletal material are also evident in the data from the analyses of the pure calcite cements and pedogenic crusts. Changes in stable isotopic composition of the shells are the result of the early diagenetic alteration of the original aragonitic composition to low Mg-calcite.

MICROFACIES AND GEOCHEMICAL STUDY OF THE DALICHAI FORMATION: A CASE STUDY OF CENTRAL ALBORZ IN SEMNAN PROVINCE, NORTHERN IRAN

Top Bajocian progressive deposits are marly accumulation and limestone filled with ammonites and other marine fossils that are called ‘Dalichai formation’ with 1420m in thickness. There are erosion and inconsistent boundary that distinct Dalichai formation from Shemshak formation. The boundary of Dalichai and Lar formations is isocline and graded. The presence of Benthic foraminifera, sponge sparkles, and ammonites in the Sharaf section alludes the existence of shallow to deep open marine. Such diagenetic processes as cementation, micritization, porosity, accumulation, iron oxide and bioturbation are observed in Dalichai formation. The carbonates in this formation could have primary aragonitic composition according to the studies on main and secondary elements, carbon and oxygen isotopes, and drawing these values against each other. In such condition, non-marine diagenesis influenced the carbonates mostly. By comparing Sr/Ca and Mn, a semi-closed to open diagenetic environment can be suggested to have impacts on carbonates. Dalichai formation and Lar formation can identify a boundary in 1200m thickness since there are changes in Sr/Ca, Sr/Na, Sr, and carbon and oxygen isotopes to study. Dalichai formation limestone temperature was calculated 32.9 °C. Regarding lithology and analysis of Sharaf section microfacies, three microfacies are specified that appear as deposits in outer ramp. In this study, the model of Homoclinal ramp is suggested to utilize for such formation. Sharaf section Dalichai Formation entails aragonitic mineralogy composition on which burial diagenesis has substantially influenced.

Mineral composition and gemological characteristics of jewelry-ornamental ammonites of the Republic of Adygea

For the first time, data on the mineral composition and gemological characteristics of the jewelry-ornamental ammonites of the Republic of Adygea have been presented. Ammonites consist mainly of the calcite with inclusions of quartz, glauconite, kaolinite, aragonite, pyrite, apatite and hematite. The walls and partitions of the shells mainly lost the original aragonite composition and consist of calcite with inclusions of pyrite and apatite. Aragonite is preserved fragmentary in the walls and partitions of the shells. Calcite contains some elements-admixture, which average volumes, (wt. %), are: Mg0,62, Mn0,31 and Fe 1,26. The color and transparency of calcite is determined by the structural features of the crystals and mineral inclusions. Ammonites of the Republic of Adygea have a wide range of sizes and are associated with sediments of the lower Cretaceous. According to the decorative and technological characteristics ammonites can be used as a jewelry-ornamental material for the production of a wide range of ware.

Abrasion provides clues on a chiton taphonomic conundrum

Chitons are common marine invertebrates found worldwide that use aragonite to form their shell and are rare in the sedimentary and fossil records. Their rarity in the fossil record is interesting due to how abundant live chitons are. Previous work suggests that chitons resist dissolution, which appears inconsistent with their rarity in the sediment and fossil records and aragonite composition. This result warrants examination of other taphonomic processes on chiton skeletal material since other taphonomic processes may play a larger role in removal than previously thought. As many chitons are intertidal, mechanical abrasion of chiton valves is likely to occur; its extent is largely unknown and yet necessary to provide insight to the taphonomy of chiton material. Eight species of chitons were collected from the Otago Peninsula, South Island, New Zealand. A total of 129 valves were isolated, cleaned, weighed, and tumbled in a Lortone 3A rock tumbler for 96 h. The valves from each species lost an average of between 9 and 44% of their weight; damage varied among species. Leptochiton inquinatus (the sole lepidopleuran) lost the most material while the acanthochitonids (Acanthochitona zelandica, and Notoplax violacea) and the chitonids (Chiton glaucus, Onithochiton neglectus, Sypharochiton pelliserpentis, and S. sinclairi) were more resistant to abrasion. The dorsal layer of the valves (the tegmentum), which is penetrated by aesthete tissue and has a granular crystal microstructure, was the most damaged by abrasion in all species. The ventral layer (the articulamentum), which has a highly-organised crystal structure with fewer organic components than the tegmentum, showed the least signs of abrasion. Since abrasion reduces the tegmentum and changes the valve shape, it is likely that abraded chiton material is present but unrecognisable in the fossil record. The valves are estimated to last about seven years when in the intertidal zone when enduring abrasion alone, which is much less than the hundreds to thousands of years material is exposed.

Evidence for Palaeozoic orthoconic cephalopods with bimineralic shells

AbstractBased on the aragonite composition of extant and exceptionally preserved fossil cephalopods going back to the early Palaeozoic, it is commonly assumed that all externally shelled cephalopods had an aragonitic shell wall. We demonstrate herein that at least two taxa of Siluro‐Devonian orthoconic nautiloids (Dawsonoceras, Spyroceras) had an original bimineralic shell, which developed convergently with gastropods and bivalves.