Search

A newly located exposure of the Niveau Thomel, an organic-rich level at the Cenomanian-Tur-onian boundary, provides a highly expanded record of Oceanic Anoxic Event (OAE) 2, except for the lower relatively condensed glauconite-rich part of the section. The new locality, close to Barreme in the Vocontian Basin, SE France, is developed in deep-water hemi-pelagic facies (shales, marls, marly limestones, variably enriched in organic matter) and provides an improved understanding of palaeoceanographic events associated with OAE 2. Investigation of the biostratigraphy (nannofossils and planktonic foraminifera), organic and inorganic geochemistry (bulk carbonate δ 18 O, total organic carbon (TOC), bulk organic, biomarker-specific and carbonate δ13C, major and trace elements, and Rock-Eval data) has allowed characterization of the sediments in great detail. The combined study further constrains the detailed relationship between bio-and chemostratigraphy (particularly with respect to the details of the well-displayed positive carbon-isotope excursion) for this interval. The section also provides new evidence, in the form of a positive oxygen-isotope excursion and an offset between carbonate and organic-carbon carbon-isotope records, which confirms the importance of cooling accompanied by a drop in dissolved CO 2 in near-surface waters during the Plenus Cold Event that characterized the early part of OAE 2. Evidence for increased oxygenation of bottom waters, together with elevated concentrations of redox-sensitive and chalcophilic elements registered elsewhere through the level of the Plenus Cold Event, may be reflected in enhanced concentrations of iron (in glauconite) and nickel in coeval strata from the Clot Chevalier section. © 2018 Gebrüder Borntraeger, Stuttgart, Germany.

Allanite-bearing black coastal sands of Kavala (N. Greece) were studied using a combination of single-crystal XRD, EMPA, μ-XRF, bulk ICP-MS, LA-ICP-MS, μ-XANES and γ-ray spectrometry. The sands are rich in REE (ΣREE + Y: 4010 to 10,810 mg/kg), Th (236–1205 mg/kg) and other critical metals such as Nb, Ta and Co. The main REE- and Th-hosts are allanite and titanite. The allanite unit cell parameters were calculated whereas its formula was found to be (REE0.470Ca1.499Th0.031) (Ti0.031Fe+3 1.089Al1.880) (Si2.906Al0.094O12) (OH). The μ-XANES spectra showed that LREE are present in trivalent oxidation state. Analyses of the non-magnetic sand fractions showed higher LREE (12,470 mg/kg) due to accumulation of allanite. The materials showed elevated radioactivity ranging from 885 ± 13 to 3467 ± 20 Bq/kg. The obtained results provide new insights on the provenance of the sands, the abundance of REE, Th, and other immobile elements, and offer new clues for potential exploration and exploitation. © 2019 Elsevier GmbH

Rare earth element (REE) abundances in iron formations (IFs) provide key evidence to track the redox evolution of the Precambrian oceans. Deriving information about processes and sources from IFs is complicated by the variable roles of diagenetic dissolution and precipitation that modify primary depositional signals in the rocks. Iron formations are typically mineralogically heterogeneous with varying amounts of carbonate (ankerite + siderite), oxide (predominantly magnetite) and silicate (chert and Fe-silicates). To date, little is known about the respective contributions of these fractions to the bulk-rock REE signal, and which fraction most accurately records primary seawater signals. Here we evaluate an optimized sequential extraction scheme for REE analysis in the ca. 2.46 Ga Asbestos Hills IFs from the Griqualand-West Basin in South Africa. These rocks were deposited immediately before the Great Oxidation Event and are potentially an important archive for large-scale redox changes in the (near) surface environment. To fully evaluate our method, we measured lanthanide elements plus ytrrium (REY) in individual minerals, in bulk rock samples and in three separate fractions using acetate, oxalate and total digestion sequential extractions. The extraction techniques were then verified using a mass balance approach, the first time that this has been comprehensively applied to IFs. Similarities between the carbonate fraction data and modern seawater, coupled with strong measurement reproducibility and systematic stratigraphic variability suggests that carbonate most closely tracks the primary seawater composition. The Asbestos Hills IFs show distinctive REY signatures with strongly variable (Yb/Pr)SN ratios, positive La anomalies, positive and constant Y/Ho, weakly positive Eu anomalies and a lack of Ce anomalies in their carbonate fraction. Stratigraphic variations in these parameters suggest deposition in a stratified fully anoxic ocean, which became shallower over time. The silicate fraction was influenced by various amounts of allochthonous material (detrital and volcanic ash), although some minerals including greenalite, minnesotaite and stilpnomelane may have tracked seawater REE variability. The oxide fraction was found to be the most REY-depleted and therefore susceptible to allochthonous influences as well as diagenetic controls. It shows a systematic MREE enrichment that has not been described in IFs before, but has been seen in sub-oxic, ferrous environments. An increase in the ratio of carbonate to oxide minerals ongoing upwards through the Asbestos Hills IF, from deep to shallow water conditions, most likely reflected an increase in dissimilatory iron reduction close to the margin of the anoxic basin.

Iron formations (IFs) are important geochemical repositories that provide constraints on atmospheric and ocean chemistry, prior to and during the onset of the Great Oxidation Event. Trace metal abundances and their Mo-Cr-U isotopic ratios have been widely used for investigating ocean redox processes through the Archean and Paleoproterozoic. Mineralogically, IFs consist of three main Fe-bearing fractions: (1) Fe-Ca-Mg-Mn carbonates, (2) magnetite and/or hematite and (3) Fe-silicates. These fractions are typically fine-grained on a sub-μm scale and their co-occurrence in varying amounts means that bulk-rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy. Fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on IF mineralogy. Here we adapt an existing sequential extraction scheme for Fe-phases (Poulton and Canfield, 2005) to the high Fe-content in IF and the specific three-fraction mineralogy. We optimized the scheme for magnetite-dominated Archean IFs using samples from the hematite-poor Asbestos Hills Subgroup IF, Transvaal Supergroup, South Africa. Previously commonly-used hydroxylamine-HCl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these IF samples. The acetate leach was tested at variable temperatures, reaction times and under different atmospheres in order to ensure that all micro-crystalline Fe-carbonates were effectively dissolved, resulting in an optimum extraction for 48 h at 50 °C under anoxic conditions. The dissolution of magnetite by NH4-oxalate was also tested, resulting in an optimum extraction for 24 h under an ambient atmosphere. Finally, a HF-HClO4-HNO3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in IF. Accuracy of the extraction technique was generally excellent, as verified using 1) elemental recoveries, 2) comparison of major and trace element distributions against mineralogy and 3) comparison to results from microanalytical techniques. This study focuses on the distribution of three frequently used geochemical proxies in IF; U, Mo and Cr. Molybdenum abundances in the Kuruman and Griquatown IF are low and show an apparent correlation with mineralogical variability, as determined by the sequential extraction. This suggests that changes in bulk-rock mineralogy, rather than redox chemistry might significantly affect Mo stable isotopes. For Cr, a minor bulk-rock stratigraphic increase can be related to the oxide and silicate fraction. However, a positive relationship with Zr indicates that this was also controlled by detrital or volcanic ash input. Uranium is predominantly bound to the silicate fraction and shows clear correlations with Zr and Sc implying detrital reworking under anoxic conditions. The discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk-rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry.

We presented new C and O isotope data of rockforming calcite in terrestrial carbonate deposits from Neogene basins of Attica (Greece), coupled with standard mineralogical and bulk geochemical results. Whereas both isotope datasets [δ18O from -8.99 to -3.20‰(VPDB); δ13C from -8.17 to +1.40‰(VPDB)] could be interpreted in principle as indicative of a meteoric origin, the clear lack of a statistical correlation between them suggests diverse sources for the isotopic variation of the two elements. On the basis of broad correlations between lower carbon isotope data with increasing Fe and bulk organic carbon, we interpreted the light carbon isotope signatures and As enrichments as both derived mainly from a depositional process involving increased supply of metals and organic carbon to the original basins. Periodically augmented biological production and aerobic cycling of organic matter in the ambient lake waters, would have led to the precipitation of isotopically light calcite in concert with elevated fluxes of As-bearing iron oxy-hydroxide and organic matter to the initial terrestrial carbonate sediment. The terrestrial carbonate deposits of Attica therefore represented effective secondary storage reservoirs of elevated As from the adjacent mineralized hinterland; hence these and similar deposits in the region ought to be regarded as key geological candidates for anomalous supply of As to local soils, groundwater and related human activities. © 2017 E. E. Kampouroglou et al.