Incorporation Of Trace Elements Research Articles

Diagenesis of fossil gar fish scales with implications for geochronology and paleoenvironmental applications

Fossil scales from gar fish are made of bioapatite and are common in the continental sedimentary record from the Cretaceous to the present, prompting interest in using these fossil scales as geochemical archives. We investigated the diagenetic characteristics of the two different components of gar scales: ganoine, which has similarities to tooth enamel, and bone. We examined microscopic textural characteristics, spatially resolved trace element patterns, and apparent gar scale ages using the (U-Th)/He and U-Th-Pb systems, in modern scales as well as fossil scales from Cretaceous to Paleogene age strata in the North American Williston and San Juan Basins. Structural and textural characteristics of modern and fossil gar scales suggest that ganoine is more resistant to diagenetic alteration than bone. Trace element concentrations in fossil bone are at least an order of magnitude higher than in ganoine and can be spatially heterogeneous, indicative of a complex and prolonged diagenetic history. Trace element concentrations in fossil ganoine often decrease systematically with distance from the ganoine surface, which may be consistent with a fossilization model in which trace element concentrations are controlled by an inwardly propagating recrystallization front. However, (U-Th)/He dates of ganoine are substantially younger than expected based on basin thermal histories and compared to conventional detrital apatite (U-Th)/He dates from the same localities. The ganoine (U-Th)/He dates are also older than a few Ma for only the lowest effective uranium concentrations. We interpret the young (U-Th)/He dates and the date-effective uranium pattern to be indicative of either protracted or late-stage incorporation of trace elements, which is inconsistent with a recrystallization front model alone. The young (U-Th)/He dates may also reflect microstructurally-controlled low helium retentivity in gar scale ganoine, although this does not explain the observed date- effective uranium systematics. Late-stage incorporation of trace elements is supported by the U-Th-Pb systematics of ganoine, which require addition of U and Th at a time significantly later than the depositional age. Late-stage, or protracted, diagenetic uptake of parent nuclides in gar scale ganoine contrasts with recent studies of bioapatite from conodont elements, which instead appear to exhibit late-stage parent nuclide loss. We conclude that the (U-Th)/He system in gar scale bioapatite is not currently useful as a chronometer and suggest that applications of other trace element proxies to gar scale bioapatite be approached with caution.

The incorporation of chlorine and cosmogenic 36Cl into speleothem carbonate

AbstractCave carbonate mineral deposits (speleothems) contain trace elements that are intensively investigated for their significance as palaeoclimate and environmental proxies. However, chlorine, which is abundant in marine and meteoric waters, has been overlooked as a potential palaeo-proxy, while cosmogenic 36Cl could, in principle, provide a solar irradiance proxy. Here, total Cl concentrations analysed from various speleothems were low (3–14 mg/kg), with variations linked to crystal fabrics. High-resolution synchrotron radiation micro X-ray fluorescence (μ-XRF) trace element mapping showed Cl often associated with Na, Si, and Al. We propose that speleothems incorporate Cl in two fractions: (1) water soluble (e.g., fluid inclusions) and (2) water insoluble and strongly bound (e.g., associated with detrital particulates). However, disparities indicated that alternate unidentified mechanisms for Cl incorporation were present, raising important questions regarding incorporation of many trace elements into speleothems. Our first measurements of 36Cl/Cl ratios in speleothems required large samples due to low Cl concentrations, limiting the potential of 36Cl as a solar irradiance proxy. Critically, our findings highlight a knowledge gap into how Cl and other trace elements are incorporated into speleothems, how the incorporation mechanisms and final elemental concentrations are related to speleothem fabrics, and the significance this may have for how trace elements in speleothems are interpreted as palaeoclimate proxies.

The role of post-mortem alteration in tooth enamel revisited: A combined strontium isotope and geochemical evaluation

Assessing post-mortem alteration of tooth enamel in archaeological burial sites of interest is critical and required in order to accurately interpret the isotope composition of bioavailable Sr for use in determining migration patterns of populations in ancient civilizations. Several statistical approaches have been established to evaluate the degree of post-mortem alteration within tooth enamel, and these are based on the incorporation of trace elements (Mn, V, Fe, REEs, Th, U) typically present within the burial environment. In this study, both new and previously reported chemical data and radiogenic Sr isotope ratios for both modern-day and archaeological tooth enamel samples are presented, which range from pristine to altered in nature. Significant correlations between elevated abundances of mobile elements (V, Fe, Nd, U) and their corresponding Sr isotope signatures are limited and are most prevalent when the latter are compared to their Fe contents. Tooth enamel samples affected by diagenesis based on their outlier chemical composition do not necessarily record perturbed Sr isotope signatures, and therefore should not be precluded precipitously from provenance studies.

In situ trace elements and sulfur isotope analyses of layered sphalerite as a record of ore-forming processes in the world-class Jinding sediment-hosted Zn-Pb Ore Deposit, China

The Jinding deposit is well-known to contain high concentrations of metals Zn, Pb, Cd, Tl, Ag, etc. in the world. The associated critical metals and their substitution mechanisms were wildly concerned. This study investigated and compared sphalerites using a combination of textural, trace element, and sulfur isotopic analyses to give insights into element substitution mechanisms and possible sources of ore-forming fluids and to constrain processes involved in ore genesis of the world-class Jinding sediment-hosted Zn-Pb deposit. LA-ICP-MS analyses reveal chemical zoning in the sphalerites together with the coupling of two main groups of trace elements substitution mechanisms: one is Fe + Ge + Mn + Pb + As + Tl and vacancies to substitute Zn, and the other is Cd + Cu + Ag + Sb to substitute Zn. The incorporation of trace elements is facilitated by the replacement of Zn by Cd and Fe. These two group elements have distinct color oscillatory zonation and are intertwined in the trace element LA-ICP-MS element maps. The sulfur isotopic values show high isotope fractionation in the sphalerite from −28.4 to −0.5 ‰, suggesting a bacteriogenic sulfur origin at Jinding. Sphalerite sample has observed that sulfur isotope is correlated with Fe concentration and anti-correlated with Cd concentration. These variations in isotope fractionation and trace element disequilibrium partitioning were interpreted as the effects of temperature and pH with less influence from sulfur activity. These variability zones of sphalerite alternately enriched in Cd- and Fe-related elements suggested that ore fluids may flow from metal-fertile sedimentary/basement sequences during their ascent, and colloform sphalerite was likely formed by a high degree of supercooling, over-saturation, and rapid mixing between bacteriogenic sulfur and metal-rich in spaces of collapse dome. Based on our observations, we suggested that these variability zones of layered sphalerite alternately enriched in Cd and Fe would likely provide useful insight for a potential economic Zn-Pb and associated critical metals mineral exploration deposit.

Trace elements in bivalve shells: How “vital effects” can bias environmental studies

We report on the concentrations of rare earths (REE), Y, Mg, Mn, Cu, Zn, Sr, Ba, Pb and U in a series of wild scallops (Placopecten magellanicus) collected alive on the same day in Saint Pierre et Miquelon (a French archipelago located south of Newfoundland) and ranging in age from 2 to about 20 years. We analyzed representative fragments from the entire last annual shell growth increment of each animal to assess changes in trace element incorporation with age.No correlation with the age of the animals was obtained for Sr and Mn concentrations, which are quite variable from one individual to another, even of similar age. On the other hand, for REE, Y, Cu, Zn, Ba and U, the concentrations measured in the last annual growth increment formed by animals aged 2 to 7 years are similar. From 8 years of age, the concentrations of these elements increased very strongly (for example for Yb, almost by an order of magnitude between 8 and 20 years of age), and possibly with some variation between individuals of the same age. Two elements show a different behavior: Mg concentrations in the shell margins increase continuously with the age of the animals; Pb concentrations are scattered but seem to decrease with the age of the animals.This turning point in the behavior of the elements from 8 years of age coincides with major metabolic changes, since at this age wild P. magellanicus from Newfoundland reduce their growth, in favor of gamete production. Our results demonstrate that metabolic activity of the animals largely controls the incorporation of many trace elements into the shells. These results demonstrate that the age of the animals must be taken into account when their shells are used to study pollution or (paleo) environmental condition.

Sex influences elemental variation in the mineralized vertebrae cartilage of round stingray (Urobatis halleri)

The elemental composition of mineralized vertebral cartilage has proven useful for reconstructing the movement and environmental history of elasmobranchs. However, the effects of intrinsic and extrinsic factors on vertebral chemistry have not been well explored, making meaningful interpretations of results difficult. This study quantified trace element incorporation along the corpus calcareum of 116 Round Stingray (Urobatis halleri) vertebrae and explored the influence of intrinsic (i.e., sex) and extrinsic (i.e., location and year) factors on vertebral edge chemical variations. We quantified eight element/Ca ratios (Li/Ca, Mg/Ca, Mn/Ca, Co/Ca, Cu/Ca, Zn/Ca, Sr/Ca, Ba/Ca) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Behavioral sexual segregation by Round Stingrays explained differences found in Mn/Ca, Zn/Ca, Sr/Ca, and Ba/Ca ratios. Higher vertebral Mn/Ca and Zn/Ca in females likely originate from the use of salt marsh habitats as a refuge as well as higher reproductive hormone concentrations during gestation, respectively. Female and male Round Stingrays were also found to have an inverse relationship between Sr/Ca and Ba/Ca, likely due to environmental differences (e.g., temperature) between their sexually segregated habitats. This study demonstrates how multielement signatures in vertebral cartilage may improve understanding of habitat use and migration throughout life and advances the development of biomineral usage as an environmental proxy in elasmobranchs.

Relationship between cathodoluminescence response and trace element characterization of cassiterite from the Yunling tin deposit in western Yunnan, China: Implications for substitution mechanism and ore genesis

Cathodoluminescent (CL) imaging, combined quantitative geochemical analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), was employed to study the cassiterite crystals from the Yunling tin deposit in western Yunnan Province, China. This investigation aimed to establish a connection between CL response with trace element compositions. The CL imaging of a well-formed centimeter-scale euhedral single crystal of cassiterite revealed two types of primary growth domains, namely oscillatory and sector zoning. High spatial resolution LA-ICP-MS spot analysis along three centimeter-scale profiles revealed that the oscillatory zoning patterns were primarily defined by the concentrations of Nb and Ta. The CL response associated with the oscillatory zoning was attributed to the interplay between activators such as Ti, Nb, Ta, and V, and quenchers like Fe. On the other hand, sector zoning exhibited significantly high W content, reaching three orders of magnitude greater than that in the oscillatory zoning. Consequently, the sectors displayed a dark CL response. Furthermore, U showed a sectoral preference, with contents within the sector zoning examined in this study reaching up to 10 ppm. The systematic variations in geochemical compositions suggest that the incorporation of trace elements in Yunling cassiterite involves a coupled substitutional mechanism of pentavalent and trivalent elements (e.g., Sc3+ + V5+ ↔ 2Sn4+) with H+ potentially serving as a compensating ion for the Fe3+ excess.It is worth noting that the trace element enrichment and potential substitution in the sector and non-sector regions of the single crystal may differ significantly. Therefore, the use of cassiterite trace elements as indicators of the mineralization environment requires careful consideration, particularly when analyzing randomly selected particles obtained from crushing. In such instances, it is recommended to utilize cassiterite samples that exhibit a preserved crystal texture extending from the core to the rim, such as cassiterite in thin sections or morphology-integral cassiterite separates. Moreover, the contrasting W contents observed between the sector and non-sector zones may provide an explanation for two trends in the Fe–W diagram of cassiterite associated with granite that were previously-unspecified.

Cation inversion in slag magnetite: Energy loss measurements of Fe-L3 edge shift between atom columns

Determination of cation disorder in inverse spinels like magnetite, Fe3O4, is of broad interest for applications in green technologies, storage devices, and nuclear waste management since cation distributions govern magnetic and electrical properties. Magnetite is a main component of slags produced by smelting of copper ores and contains potentially valuable trace elements. We address cation disorder as a factor controlling the behavior of these elements during atmospheric cooling from 1300 °C. To estimate cation disorder, we combine atomic-scale scanning transmission electron microscopy with electron energy loss spectroscopy. The inversion parameter (0.72) indicates minor partial ordering due to fast cooling from high temperature, resulting in skeletal textures. Trace element incorporation into magnetite, instead of exsolution of discrete nanoparticle phases is promoted. Our findings provide insights into the cooling behavior of spinels and facilitate robust thermodynamic modeling that addresses the stability of structures during cooling from melts. Findings carry implications for critical element recovery and prospects for transforming industrial waste into future resources.

Chemical fractionation in benthic fauna from Saya de Malha Bank (Mascarene Plateau, western Indian Ocean): Vital and habitat effects

The temperature, salinity, nutrient availability and dissolved oxygen characterizing ocean water-masses and circulation patterns are essential environmental controlling parameters of marine benthic assemblages. This work presents for the first time the geochemical composition of different taxonomic groups of benthic assemblages from the Saya de Malha Bank, Mascarene Plateau (Indian Ocean). The studied stations, located in water masses at different water depths, are perfect spots for studying biogeochemical proxies of diverse taxonomic groups to elucidate the impact of the vital factor as opposed to environmental or oceanographic factors. Forty-two specimens were studied, including scleractinian corals, bryozoans, brachiopods, bivalves, gastropods, scaphopods, and echinoids. Specimens were taphonomically analyzed in order to discard those with a poor preservation that might affect the geochemical assays. Geochemical analyses of the specimens involved high resolution inductively coupled plasma-mass spectrometry for main and trace elements. According to the results, (1) the content of trace elements in the shells is primarily controlled by their mineralogy: i.e. aragonitic, low-Mg calcite, and high-Mg calcite. (2) Vital effects related to metabolism play a major role in the co-precipitation and adsorption of trace elements in the shells, especially for Ca, Mg, Sr, Ba, and Zn. (3) The environmental parameters (habitat effect) such as oxygenation, primary productivity, salinity, but mainly temperature, at different water depths were found to be related to the growth rate of the specimens and the incorporation of trace elements. In the shallower station, the Li/Mg ratio is lower and the content in U is higher among the selected taxa, which is related to an increased growth rate in warmer waters.

Refined views of ancient ocean chemistry: Tracking trace element incorporation in pyrite framboids using atom probe tomography

The trace element chemistry of pyrite can be used to determine the origin, timing, and conditions of formation of ore deposits; as a vectoring tool for mineral exploration; and to determine the evolution of the Earth’s oceanic and atmospheric chemistry. However, little is known about whether trace elements are held with the pyrite structure or within nano-inclusions of other phases. This distinction is important for two primary reasons. First, trace element incorporation into the pyrite structure can affect the partitioning of other trace elements. Second, if trace elements are held within nano-inclusions, the partition coefficient of the mineral phases that make up the nano-inclusion, rather than pyrite, are the critical consideration in related interpretations. Previous studies addressing this topic have used laser ablation inductively coupled plasma mass spectrometry; however, the resulting data do not provide sufficient spatial resolution to delineate trace element distributions unless the inclusions are large. Further, they use these flat element profiles in time resolved laser ablation output graphs to argue that pyrite trace element content provides a direct relationship between trace element content of seawater and pyrite trace element content. To improve resolution, we have used atom probe tomography to characterize trace element distributions in pyrite framboids from the Cariaco Basin and Demerara Rise in three-dimensions at sub-nanometer resolution. Manganese was found to be concentrated in the pyrite nanocrystal part of the original framboid structure. In contrast, Ni was mostly found along the grain boundary, though it still appeared to be contained within the pyrite structure. Copper was concentrated in later pyrite overgrowths, and As varied in its location. These observations suggest that some important trace elements are incorporated into pyrite during early diagenesis, even in euxinic settings dominated by water-column pyrite formation. Statistical analysis was used to determine whether trace elements were incorporated in the lattice or within nanoscale inclusions (referred to here are nano-inclusions). We found that As, Ni, Cu, and Mn were commonly held within the pyrite structure, but As, Ni, and Cu can also be held as nano-inclusions or within grain interfaces. Incorporation of As is known to enhance the incorporation of other trace elements and in this case appears to correlated to elevated Ni and Cu concentrations in the Cariaco Basin samples. Understanding these relationships strongly impacts our ability to utilize pyrite trace metal concentrations to analyze and quantify early ocean chemistry and its evolution through time.

Surface roughness and overgrowth dynamics: The effect of substrate micro-topography on calcite growth and Sr uptake

Substrate roughness influences epitaxial nucleation, growth, and trace element incorporation. We qualitatively and quantitatively examine the role of calcite surface roughness on overgrowth morphology and Sr concentration. Surfaces were made by cleaving optically clear Iceland spar, then abrading calcite cleavage surfaces with sandpaper (physical weathering analogues), or by attacking the cleavage surface with dilute HCl (chemical weathering analogues). Calcite precipitated on “weathered” surfaces have morphologies that vary systematically from one substrate type to the next. Growth on the abraded substrate was shingled, didn't form a continuous growth layer, and was poorly adhered to the seed crystal. Growth on the acid etched substrate mostly formed a continuous growth layer in optical continuity with the seed crystal. Differences in overgrowth morphology are interpreted to be due to differences in island coalescence dynamics on the different substrates. Rough substrates have a high density of island nuclei per unit surface area, which inhibits island coalescence due to interfacial misfit between individual islands. In contrast, overgrowths on smooth substrates coalesced into a single crystalline overgrowth layer. There is a weak positive correlation between the initial fractal dimension of the substrate and average Sr concentration in the overgrowth.

Incorporation of trace elements in aragonite skeletons of South East Mediterranean vermetids

The Mediterranean Sea is a unique archive of past climatology, as this region is sensitive to both global climate fluctuations and manmade disturbances. Our understanding of these processes is however, limited, due to the absence of high-resolution multi-element proxies in this region. Here, I present the first high-resolution record of trace-elements (TE) in Mediterranean aragonite veremtid reefs, spanning the last millennium. Modern vermetid TE contents are mostly in agreement with other marine biogenic aragonites, reinforcing the potential use of veremtid reefs as paleo-environmental proxies. The down-core U/Ca and Sr/Ca fluctuations resemble the main climatic events of the last millennium, potentially linked to changes in sea surface temperature (SST). Pb and Cd records are associated with anthropogenic pollution and demonstrate trends related to growing industrial activity in the Anthropocene. Carbonate records of Al, Fe and Rb are attested as potential proxies to infer terrestrial inputs. Fe/Ca is used to decouple dust from riverine sources, while Rb/Ca is proposed for differentiation between sources of dust. The sub-millennial multi-element study enables the observation of independent environmental patterns within a single archive, and the decoupling of anthropogenic imprint from natural variability.

Hide and Seek—Trace Element Incorporation and Diffusion in Olivine

Olivine, once overlooked as a host of trace elements, is becoming increasingly important for our understanding of the kinetic and equilibrium behaviour of these elements. Much of our understanding of trace element substitution and diffusion in geological materials comes as a result of experimental and petrological studies of olivine. Here, we consider trace element concentrations and incorporation mechanisms, and how these relate to diffusive behaviour. If we understand trace element behaviour in olivine, we have a powerful tool kit that can be directly applied to address many problems in petrology and volcanology. Perhaps more importantly, what we have learned from olivine can be applied to other minerals and aid us in addressing other far-reaching questions from across the Earth sciences.

Metasomatic alteration of zircon at lower crustal P-T conditions utilizing alkali- and F-bearing fluids: Trace element incorporation, depletion, and resetting the zircon geochronometer

Natural alteration of zircon takes place in melts or fluids either via dissolution coupled with overgrowth or via a coupled dissolution-reprecipitation process. The latter results in the zircon being partially or totally replaced by new, compositionally re-equilibrated zircon or a new mineral phase or both. In this study, fragments (50–300 μm) from a large, inclusion-free, clear, 520–530 Ma euhedral zircon with light radiation damage from a nepheline syenite pegmatite, Seiland Igneous Province, northern Norway, were experimentally reacted in 20 mg batches with 5 mg of ThO2 + ThSiO2 + SiO2 and a series of alkali-bearing fluids in sealed Pt capsules at 900 °C and 1000 MPa for 6–11 days in the piston cylinder press using a CaF2 setup with a cylindrical graphite oven. ThO2 + ThSiO2 + SiO2 was present at the end of the experiment. In experiments involving H2O, H2O + NaCl, H2O + KCl, and 2 N KOH, no reaction textures formed other than a slight dissolution of the zircon grain fragments. Experiments involving 2 N NaOH, Na2Si2O5 + H2O, and NaF + H2O resulted in zircon reaction textures with varying degrees of intensity, which took the form of partial replacement by compositionally modified zircon via a coupled dissolution-reprecipitation process. In the NaF + H2O experiment some overgrowth also occurred. Altered zircon is separated by sharp compositional boundaries from unaltered zircon. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis indicates that, relative to the unaltered zircon, the altered zircon is strongly enriched in Th, and heavily to moderately depleted in U and (Y + REE). In all the experiments, 206Pb (3–5 ppm in unaltered zircon) is depleted in the altered zircon to below the SIMS detection limit and to at or below the LA-ICP-MS detection limit. Hafnium and Ti concentrations in the altered zircon retained the same approximate value (within error) as the original zircon. The results from these experiments demonstrate that zircon can be compositionally modified by alkali-bearing and alkali-F-bearing fluids via a coupled dissolution-reprecipitation process. Near to total loss of radiogenic Pb via such processes under high-grade conditions resets the internal zircon geochronometer. Although the end result is the same as with zircon overgrowth, i.e. the production of new generation zircon at the time of a metamorphic/metasomatic event, such replacement processes can explain incomplete isotopic ‘resetting’; inclusion production through unmixing of solid solutions in metastable zircon compositions; and ‘ghost’ textures that preserve initial growth features but with isotopic disturbance. Diagnostic replacement features produced in experiments, such as interface geometries between altered and unaltered zircon, provide markers of the mechanism and aid in zircon interpretation. A major implication from this study is that if zircon with low radiation damage can be metasomatically altered under high-grade conditions, this would have important consequences with respect to zircons presumed role as an impregnable container for mineral inclusions. Namely the mineral inclusions contained within zircon could also be altered, reset as a geochronometer, or even replaced by another mineral.

Nanoscale Distribution of Elements in Gold: Examples from Contrasting Deposit Types

Abstract Quantifying the distribution and nature of trace elements in native gold is a novel approach to understanding the genesis of gold deposits and has implications for metallogenic processes, mineral exploration, and metallurgical processing. Most previous research on the formation of gold deposits has utilized proxies for gold, rather than gold itself, to infer its source, transport, and depositional mechanisms, despite the difficulty in establishing unequivocal relationships among the proxies and gold. This approach has left many questions regarding the formation of high-grade coarse gold. This contribution presents results on the composition and distribution of trace elements in native gold using electron probe microanalysis and, for the first time, atom probe tomography. Gold grains selected are from a deformed and metamorphosed Archean orogenic gold deposit (Preston Mine) and a young, undisturbed epithermal gold deposit (McLaughlin mine) which are endmembers in terms of deposit type and geologic time. Results show that Cu, Ag, and Hg are homogenously distributed in gold while Sb is not, which raises questions regarding the substitution mechanisms of large radii metallic elements in gold. These data provide new insights into the incorporation of trace elements in gold, as well as primary and secondary processes related to the formation of high-grade gold deposits through time.

Earthworm granules: A model of non-classical biogenic calcium carbonate phase transformations.

Different non-classical crystallization mechanisms have been invoked to explain structural and compositional properties of biocrystals. The identification of precursor amorphous nanoparticle aggregation as an onset process in the formation of numerous biominerals (crystallization via particle attachment) constituted a most important breakthrough for understanding biologically mediated mineralization. A comprehensive understanding about how the attached amorphous particles transform into more stable, crystalline grains has yet to be elucidated. Here, we document structural, biogeochemical, and crystallographic aspects of the formation as well as the further phase transformations of the amorphous calcium carbonate particles formed by cultured specimens of earthworm Lumbricus terrestris. In-situ observations evidence the formation of proto-vaterite after dehydration of earthworm-produced ACC, which is subsequently followed by proto-vaterite transformation into calcite through nanoparticle attachment within the organic framework. In culture medium spiked with trace amounts of Mn2+, the cauliflower-like proto-vaterite structures become longer-lived than in the absence of Mn2+. We propose that the formation of calcite crystals takes place through a non-classical recrystallization path that involves migration of proto-vaterite nanoparticles to the crystallization site, and then, their transformation into calcite via a dissolution-recrystallization reaction. The latter is complemented by ion-by-ion crystal growth and associated with impurity release. These observations are integrated into a new model of the biocrystallization of earthworm-produced carbonate granules which highlights the sensibility of this process to environmental chemical changes, its potential impact on the bioavailability of contaminants as well as the threat that chemical pollution poses to the normal development of its early stages. STATEMENT OF SIGNIFICANCE: Understanding the mechanisms of nucleation, stabilization and aggregation of amorphous calcium carbonate (ACC) and factors controlling its further transformation into crystalline phases is fundamental for elucidation of biogenic mineralization. Some species of earthworms are natural workbench to understand the biogenic ACC, stabilization and the transformation mechanisms, because they create millimeter-sized calcareous granules from amorphous calcium carbonate, which crystallize to a more stable mineral phase (mostly calcite). This study undergoes into the mechanisms of ACC stabilization by the incorporation of trace elements, as manganese, and the ulterior precipitation of calcareous granules by a coupled process of amorphous particle attachment and ion-by-ion growth. The study points to sensibility of this process to environmental chemical changes.

Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting

Abstract Global volcanic and plutonic olivines record the compositional characteristics and physicochemical conditions of the parental magmas. Thus, they have significant potential for use as petrogenetic discriminators of the olivine formation environment and prospecting indicators for potential host rocks of magmatic sulfide deposits. Several data visualization approaches have been proposed by researchers to determine olivine origins. However, they can only discriminate specific olivine populations and require the incorporation of trace elements for which data are lacking globally. In this study, a machine-learning method consisting of the random forest algorithm and the synthetic minority oversampling technique (SMOTE) is used to discriminate the crystallization environments of olivine and predict the sulfide potential of olivine-bearing mafic-ultramafic intrusions. We employ a global data set of 24 341 olivine samples from 12 environments to determine the contents of MgO, FeO, Ni, Ca, Mn, and Cr and the Fo number [100 × Mg/(Mg+Fe)]. The results indicate that the proposed method can classify olivine into genetically distinct populations and distinguish olivine derived from mineralized intrusions from that derived from sulfide-barren intrusions with high accuracies (higher than 99% on average). We develop a dimensionality reduction algorithm to visualize the olivine classifications using low-dimensional vectors and an olivine classifier (accessible at http://101.33.204.62:8080/olivine_web/main.html, China University of Geosciences, Beijing). The model is used successfully to identify the contributions of distinct sources to regional magmatism using olivines from the late Permian picrite and basalt along the western margin of the Yangtze block (SW China) and to predict the sulfide potential of the newly discovered Qixin mafic-ultramafic complex in the southern Central Asian Orogenic Belt (NW China). The findings suggest that the proposed approach enables the accurate identification of olivine origins in different formation environments and is a reliable indicator suitable for global Ni-Cu-platinum group element (PGE) exploration.

Controls on potassium incorporation in foraminifera and other marine calcifying organisms

Seawater chemistry exerts an important control on the incorporation of trace elements into the shells of marine calcifying organisms. Variability in the major ion chemistry of seawater is a tracer of past geological processes, and the influence of seawater chemistry on trace element incorporation in calcium carbonate can be harnessed to determine changes in the composition of seawater through time. Here, we investigate whether key oceanographic parameters (temperature, salinity, and the carbonate system) affect the incorporation of potassium (K) into foraminiferal calcite, and explore the utility of K/Ca ratios in foraminifera as an indicator of past variability in the seawater Ca2+ concentration. We analysed both low-Mg and high-Mg modern foraminifera, including planktonic (Globigerinoides ruber) and shallow-dwelling larger benthic (Operculina ammonoides) species, using laser-ablation sector-field inductively-coupled plasma mass spectrometry (LA-SF-ICPMS). Both species show no resolvable influence of temperature, salinity, pH, or [CO32−] on K incorporation across the range that these vary at our samples sites. In order to determine the effect of the seawater Ca concentration ([Ca2+]sw) on K incorporation, we analysed laboratory-cultured O. ammonoides, the close living relative of the abundant Eocene Nummulites, grown at four different [Ca2+]sw. We find a significant relationship between seawater and shell K/Ca, albeit with a shallower slope compared to most other trace elements which we suggest is driven by a crystal growth rate effect on K incorporation, constrained using culture experiments of O. ammonoides grown at different pH. If the K+ concentration has remained relatively constant throughout the Phanerozoic Eon, our data may pave the way forward for the use of K/Ca as a direct proxy for past [Ca2+]sw variability. Alternatively, coupling K/Ca with the similar Na/Ca proxy would allow more accurate reconstruction of [Ca2+]sw or verification of whether [K+]sw and [Na+]sw have indeed remained within narrow bounds.

Growth dynamics and amorphous-to-crystalline phase transformation in natural nacre

Biominerals, such as nacreous bivalve shells, are important archives of environmental information. Most marine calcifiers form their shells from amorphous calcium carbonate, hypothesised to occur via particle attachment and stepwise crystallisation of metastable precursor phases. However, the mechanism of this transformation, including the incorporation of trace elements used for environmental reconstructions, are poorly constrained. Here, using shells of the Mediterranean mussel, we explore the formation of nacre from the meso- to the atomic scale. We use a combination of strontium pulse-chase labelling experiments in aquaculture and correlated micro- to sub-nanoscale analysis to show that nacre grows in a dynamic two-step process with extensional and space-filling growth components. Furthermore, we show that nacre crystallizes via localised dissolution and reprecipitation within nanogranules. Our findings elucidate how stepwise crystallization pathways affect trace element incorporation in natural biominerals, while preserving their intricate hierarchical ultrastructure.

Gas-mediated trace element incorporation into rhyolite-hosted topaz: A synchrotron microbeam XAS study

Abstract Magmatic gas exsolving during late-stage cooling of shallow magmas has been considered an important facilitator of low-pressure alteration and metal transport. However, the chemical properties of such gas, particularly its metal transport mechanisms and capacity, remain elusive. Trace elements in minerals produced by gas-mediated surface reaction or precipitation from gas capture details of gas composition and reaction pathways. However, interpretation of mineral trace element contents is dependent on understanding crystallographic controls on gas/mineral partitioning. This work investigates the structural accommodation of As, Mn, Ga, Ge, Fe, and Ti in vapor-deposited topaz of vesicular topaz rhyolite from the Thomas Range, Utah, through single-crystal synchrotron microbeam X-ray techniques on picogram quantities of those trace elements. X-ray absorption near edge structure (XANES) data indicates that these elements are incorporated into topaz as As5+, Fe3+, Mn3+, Ti4+, Ga3+, and Ge4+. Extended X-ray absorption fine structure (EXAFS) analysis for these trace elements, compared to EXAFS of structural Al and Si, reveals that As5+ and Ge4+ are incorporated directly into the tetrahedral site of the topaz structure, with the octahedral site accommodating Mn3+, Fe3+, Ga3+, and Ti4+. For As5+ and Fe3+, the structural impact of substitution extends to at least second neighbors (other elements were only resolvable to first neighbors). Further interpretation of the EXAFS results suggests that the substitution of Ti4+ results in increased distortion of the octahedral site, while the other trace elements induce more uniform expansion correlating in magnitude to their ionic radius. Comparison of quantified X-ray fluorescence (XRF) data for two topaz crystals from this rhyolite reveals variable trace element concentrations for As5+, Fe3+, Ga3+, and Ti4+, reflective of a source gas undersaturated in these trace elements changing in concentration over the period of topaz deposition. The identical Ge4+ content of the two topaz crystals suggests that Ge4+ in the gas was buffered by the growth of another Ge4+-bearing phase, such as quartz. The very low Mn3+ content in the topaz crystals does not reflect the abundance of Mn3+ in the gas (saturation of Mn is evidenced by coexisting bixbyite). Instead, it suggests a strong Jahn-Teller inhibitory effect to the substitution of Mn3+ for Al3+ in the distorted octahedral site of topaz. It is proposed that exsolution of an HF-enriched gas from cooling rhyolitic magma led to local scouring of Al, Si, and trace metals from the magma. Once topaz crystals nucleated, self-catalyzed reactions that recycle HF led to continued growth of topaz.