Mn Enrichment Research Articles

Using trace elements of magnetite to constrain the origin of the Pingchuan hydrothermal low-Ti magnetite deposit in the Panxi area, SW China

The Pingchuan iron deposit, located in the Yanyuan region of Sichuan Province, SW China, has an ore reserve of 40 Mt with ~ 60 wt% Fe. Its genesis is still poorly understood. The Pingchuan iron deposit has a paragenetic sequence of an early Fe-oxide–Pyrite stage (I) and a late Fe-oxide–pyrrhotite stage (II). Stage I magnetite grains are generally fragmented, euhedral–subhedral, large-sized crystals accompanying with slightly postdated pyrite. Stage II magnetite grains are mostly unfragmented, anhedral, relatively small-sized grains that co-exist with pyrrhotite. Combined with micro-textural features and previously-obtained geochronological data, we consider that these two stages of iron mineralization in the Pingchuan deposit correspond to the Permian ELIP magmatism and Cenozoic fault activity event. Both the Stage I and II magnetites are characterized with overall lower contents of trace elements (including Cr, Ti, V, and Ni) than the ELIP magmatic magnetite, which suggests a hydrothermal origin for them. “Skarn-like” enrichment in Sn, Mn, and Zn in the Stage I magnetite grains indicate significant material contributions from carbonate wall-rocks due to water–rock interaction in ore-forming processes. Stage II magnetite grains contain higher Mn concentrations than Stage I magnetite grains, which possibly implies more contribution from carbonate rocks. In multiple-element diagrams, the Stage I magnetite shows systematic similarities to Kiruna-type magnetite rather than those from other types of deposits. Combined with geological features and previous studies on oxygen isotopes, we conclude that hydrothermal fluids have played a key role in the generation of the Pingchuan low-Ti iron deposit.

Microstructure evolution and strain behavior of a medium Mn TRIP/TWIP steel for excellent combination of strength and ductility

The structure-property relationship in a low carbon medium Mn steel subjected to annealing at temperatures from 700 to 850 °C was elucidated. The phase transformation kinetics, thermal stability, and strain behavior of reverted austenite were studied to obtain excellent combination of strength and ductility. The recovery and recrystallization of deformed bainite were observed. Reverse transformation from bainite to austenite occurred during the intercritical annealing process. With an increasing annealing temperature, the volume fraction of austenite first increased and reached the maximum of 65% at 775 °C, it then decreased sharply to 21% at 850 °C due to the weak Mn enrichment and coarsened grain size. Mn enrichment played a significant role in enhancing the thermal stability of reverted austenite. The maximum product of strength and ductility of 44.1 GPa% was obtained at 725–750 °C by adjusting the volume fraction and mechanical stability of austenite. The cooperative TRIP and TWIP effects contributed to the continuous strain hardening and local stress relaxation of this steel. In this way, the uniform elongation was greatly improved and resulted in a ductile fracture mode.

Transport of carbon dioxide and heavy metals from hydrothermal vents to shallow water by hydrate-coated gas bubbles

Deep-sea hydrothermal plumes are of major importance in the biogeochemical ocean cycles and in this study we focus on plumes emitted from the Jan Mayen vent fields in the Norwegian-Greenland Sea. These vent fields are of interest because of the high CO2 concentrations and also due to the different styles of venting occurring here. Venting at these sites occurs between 550 and 700 m depth and is characterized by the release of hydrate coated bubbles as well as focused flow venting. This study aims to enhance our current understanding of the impact of CO2 rich hydrate coated bubbles on the water column as well as the interaction between hydrothermally derived gases and metals in the water column. Three water column surveys were conducted in this area in between 2012 and 2014, in which the non-buoyant plume (NBP) produced by focused flow venting from both the Troll Wall and the Perle & Bruse vent sites was identified by primordial helium (3He), methane (CH4), carbon dioxide (CO2) and dissolved manganese (Mn) enrichments close to 500 m water depth. Our results show that venting of hydrate coated CO2 rich bubbles increases bubble rise height, which results in shallow acidification locally above the vent sites. A polymetallic anomaly in the mid-depth water column above the NBP is also hypothesized to be a result of the hydrate coated bubbles. We argue that nanoparticles get sequestered to the hydrate lattice and travel with the bubbles until the lattice becomes unstable due to gas expansion upon depressurization during ascent. This process could fuel the primary production in the pelagic water column.

Effects of Heating Rate on Formation of Globular and Acicular Austenite during Reversion from Martensite

The effects of heating rate on the formation of acicular and globular austenite during reversion from martensite in Fe–2Mn–1.5Si–0.3C alloy have been investigated. It was found that a low heating rate enhanced the formation of acicular austenite, while a high heating rate favored the formation of globular austenite. The growth of acicular γ was accompanied by the partitioning of Mn and Si, while the growth of globular γ was partitionless. DICTRA simulation revealed that there was a transition in growth mode from partitioning to partitionless for the globular austenite with an increase in temperature at high heating rate. High heating rates promoted a reversion that occurred at high temperatures, which made the partitionless growth of globular austenite occur more easily. On the other hand, the severer Mn enrichment into austenite at low heating rate caused Mn depletion in the martensite matrix, which decelerated the reversion kinetics in the later stage and suppressed the formation of globular austenite.

The formation mechanism of reverted austenite in Mn-based maraging steels

Based on the microstructural evolution in Mn-based maraging steels aged at 460–540 °C, the formation mechanism of reverted austenite is proposed. Reverted austenite formed at grain boundaries at the onset of aging. During further aging, the accumulated Mn enrichment at newly formed alpha^{{prime }} /gamma interfaces led to thin austenite layers growing into lath-like austenite grains. The lateral growth of lath-like reverted austenite was sluggish, reflecting the low diffusivity of Mn. We show that reverted austenite formed by a shear-dominated mechanism assisted by a small amount of Mn diffusion, whereas the nucleation of reverted austenite within martensite laths was kinetically slower.

Dynamics of Manganese and Cerium Enrichments in Arctic Ocean Sediments: A Case Study From the Alpha Ridge

Manganese (Mn) and cerium (Ce) are known as reactive metals sensitive to marine redox conditions, and can therefore serve as useful proxies for paleoceanographic environments. Quaternary sedimentary records in the Arctic Ocean show a consistent cyclicity of Mn enrichments, but Mn sources, transportation and deposition patterns, and relationship to paleoclimatic conditions are not well understood. Sediment core ARC3-B85D from the Alpha Ridge with the estimated stratigraphy covering ~350 kyr is used to investigate a coupled distribution of Mn and Ce in Quaternary Arctic Ocean sediments. By analyzing Mn and Ce distribution patterns in the core and surface sediments from the western Arctic Ocean and adjacent shelves, we investigate the conditions and dynamics of concurrent metal enrichments. Stratigraphic Ce and Mn patterns follow inferred glacial-interglacial cycles, with enrichments generally occurring during interglacial-type conditions with high sea levels. However, the relationships involved are not straightforward as highest Mn and Ce enrichments seem to occur closer to the end of interglacial/major interstadial periods, when sea levels were lowering from their highest positions. We conclude that the enrichment patterns are primarily defined by sediment dynamics controlling resuspension and transportation of reactive metals and their deposition in the central Arctic Ocean after diagenetic preconditioning on the shelves. We further infer that major transportation agents are sea-level affected cross-shelf and mid-depth ocean currents rather than sea ice as has been proposed earlier. Comprehending this coupled geochemical and sedimentary system is important for improving the chronostratigraphic framework for Quaternary deposits in the Arctic Ocean.

Relative effect of C and Mn on strength-toughness of medium Mn steels

ABSTRACTIn order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.

Sedimentological and geochemical evidence for seismoturbidite generation in the Kumburgaz Basin, Sea of Marmara: Implications for earthquake recurrence along the Central High Segment of the North Anatolian Fault

Holocene earthquake history of the Central High Segment of the North Anatolian Fault is examined here for the first time based on analysis of seismoturbidites within a 21-m-long piston core recovered from the Kumburgaz Basin in the Sea of Marmara. The visual lithological description combined with detailed grainsize analyses indicate that the deep basin hemipelagic sediments are interrupted by 28 turbidite units during the last 6.1 cal kyrs BP. The turbidites show strong segregation and a sharp boundary between a coarse basal part and overlying homogenite as inferred from detailed sedimentological and geochemical data. Several amalgamated turbidites are recognized by repeated fining upward sequences with no intervening homogenite indicating multiple episodes of traction and deposition as a result of various slope failures and turbidity currents. Each amalgamated unit was possibly triggered by the same earthquake event rupturing in the Sea of Marmara. The most common sedimentary feature is the continuous parallel lamination that was presumably introduced by long lasting water oscillations on suspended sediments due to the seiche effect. The establishment of geochemical criteria and exclusive sedimentary processes distinguish earthquake triggered turbidites (seismoturbidites) from other trigger factors. Moreover, such distinction allows us to evaluate hydrodynamic sedimentary conditions and processes in the Kumburgaz Basin. The base of most seismoturbidites are associated with a sharp increase in Mn concentration that can be explained by a diagenetic enrichment of Mn at the oxic/anoxic interface of the sediments near the seafloor prior to the deposition of the turbidite. An age-depth model of the studied core based on seven AMS 14C ages allows correlation between historical earthquakes and seismoturbidites in the Kumburgaz Basin. At least the latest nine of them fit well with the previously recorded major earthquake events between ca. ~500 cal yrs BP and 2.5 cal kyrs BP.

Impact of natural re-oxygenation on the sediment dynamics of manganese, iron and phosphorus in a euxinic Baltic Sea basin

The Baltic Sea is characterized by the largest area of hypoxic (oxygen (O2) < 2 mg L−1) bottom waters in the world’s ocean induced by human activities. Natural ventilation of these O2-depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in O2 and decline in phosphate (HPO42−) in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese (Mn), iron (Fe) and phosphorus (P) in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur (S)-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total Mn and to a lesser extent Fe oxides and P were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of P-bearing rhodochrosite and Mn(II) phosphates. In contrast to what is typically assumed, the formation of Fe oxides in the surface sediment was limited. We attribute this lack of Fe oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows Fe(II) in the form of FeS to be preserved and restricts the penetration of O2 into the sediment. We estimate that enhanced P sequestration in surface sediments accounts for only ∼5% of water column HPO42− removal in the Eastern Gotland Basin linked to the recent inflows. The remaining HPO42− was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic O2 demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, P sequestration in the sediment in association with Fe oxides is limited. This implies that artificial ventilation projects that aim at removing water column HPO42− and thereby improving water quality in the Baltic Sea will likely not have the desired effect.

Effect of Cu addition on hardness and microstructural features of low alloy white cast iron

The aim of this work is to investigate in details the effect of Cu on hardening variation and microstructural feature of low alloy white cast iron. For this purpose, a wide variety of iron melts consisting of different weight percent Cu were prepared with step by step addition of about 0.5%Cu to a constant base chemical composition of low alloy white iron. Hardness measurements were supplemented with x-ray diffraction, colored light metallography and electron microscopy to follow the relationship between hardness and microstructure of as-cast white irons. The experimental results show that as-cast hardness has been sequentially increased from 49HRC to the values of 52, 57 and 61HRC with step by step addition of only about 0.5%Cu to the chemical composition of low alloy base melt iron, respectively. The remarkable improvement of as-cast hardness has been rationalized to the formation of more and more displacive martensitic phase transformation products developed as a consequence of alloying partitioning within the associated austenite and carbide during solidification. The M3C eutectic carbide is just rich in Cr while the austenite phase, apart from a significant enrichment of Mn, Ni and Cu, has been surprisingly enriched from Mo as well.

The metallogenic environment of the Dounan manganese deposit, Southeast Yunnan, China: evidence from geochemistry and Mössbauer spectroscopic

The Dounan manganese deposit is a typical large-scale marine sedimentary manganese deposit of the Middle Triassic in China. The metallogenic environment and change process directly dictate the migration, enrichment, and precipitation of Mn. To better understand its metallogenetic environment, a detailed study was undertaken involving field observation, mineralogical and geochemical and Mossbauer spectroscopic analyses. The major findings are as follows: (1) Lithofacies paleogeography, sedimentary structural characteristics, and geochemical indexes indicate that the deposits were formed in an epicontinental marine sedimentary basin environment of normal salinity; (2) there were three ore phases including Mn oxides, Mn carbonates, and mixed Mn ores. The ore minerals found were braunite, manganite, Ca-rhodochrosite, manganocalcite, and kutnahorite. Petrographic and mineralogical information indicates that the metallogenic environment was a weakly alkaline and weakly oxidized to weakly reduced environment, and the mineralization occurred near the redox interface; (3) the V/(V + Ni) ratios, δCe and Fe2+/Fe3+ found in profiles of Baigu and Gake ore sections show that the redox conditions of the ore-forming environment were continuously changing; and (4) three Fe species, α-Fe2O3, para-Fe3+, and para-Fe2+, were found in hematite and clay mineral samples using Mossbauer spectrum analysis. The presence and distribution of these Fe species indicate that the deposit was formed in a typical sedimentary environment during the mineralization process. In summary, our study showed that redox was a key factor controlling the mineralization of the Dounan manganese deposit. Our results have led us to the conclusion that transgression and regression caused fluctuations in sea level, which in turn caused the change of the redox environment. Mossbauer spectroscopy is an effective tool for studying the redox conditions of the paleoenvironment in which sedimentary manganese deposits were formed.

On the reactive wetting of a medium-Mn advanced high-strength steel during continuous galvanizing

The effects of process atmosphere oxygen partial pressure (pO2) and Sn-addition on the reactive wetting of a prototype medium manganese third generation advanced high strength steel by a 0.20 wt% Al (dissolved) zinc bath was determined through galvanizing simulations. External MnO layers with thicknesses ranging from 19 nm to 45 nm were formed after annealing for 120 s at 690 °C under a variety of experimental process atmospheres. Despite the presence of surface oxides prior to immersion, an integral zinc coating and good coating adherence was obtained on all steel substrates, with the exception of the reference steel annealed under the lowest pO2−50 °C dew point process atmosphere, which exhibited poor reactive wetting. Excellent reactive wetting was demonstrated by the Sn-containing steels annealed under the −30 °C and +5 °C atmospheres. It was determined that the Al uptake as well as the population of the Fe-Al intermetallics at the coating/steel interface increased as a result of annealing under the higher pO2 atmospheres and adding Sn to the steel. The significant improvement in reactive wetting of the Sn-added steel was attributed to the decreased surface enrichment of Mn and the discrete distribution of the fine external oxide particles formed on the surface of these steel prior to dipping; which allowed for progression of reactive wetting through mechanisms such as the aluminothermic reduction of MnO, oxide bridging and oxide lift-off by the molten galvanizing bath.

Tectono-magmatic events and genesis of Fe-Pb-Zn resources in Nefza area, NW Tunisia

Fe-ore and Pb-Zn (Ag) resources of Nefza region occur within hydrothermally altered sericite-schists and breccias, with Neogene molasse and lacustrine sediments and with a sequence of tertiary granodiorite-rhyodacite and basalts. Intrusions of intermediate igneous rocks, whose correlate with the successive Alpine tectonic events, started at 15 Ma and continued to near 5 Ma. Four main stages of mineralization are identified: (1) the first stage started during or soon after the maximum alpine compression (Langhian-Lower Serravallian) and magmatism. Silicification and carbonatization prevailed and sericite-bearing sediments formed by K-sericite alteration. (2) During the second stage, a mixture of marine and meteoric ore-forming fluids (H2O, H2CO3, F and Cl; temperature range, 315–500 °C) has deposited mineralizations. The sericite schist assemblages and breccias were extremely altered and four types of minerals were formed: (i) silica, feldspar, Mg-micas, chlorite-vermiculite, Mg-clays, and kaolinite-halloysite; (ii) fluorite and sorbed U; (iii) Fe-(Mn) oxyhydroxides; (iv) Zn, Pb, Mn, and Fe, deposited in the carbonates, in coronadite and in minor sulfides. (3) The third stage was related to the extensional tectonic phase (Upper Serravallian-Lower Tortonian) and leads to the subsidence of Miocene grabens and deposition of molasse with interbeded pyroclastites. The strata-bound iron ores were reworked from pre-existing Fe-mineralization hosted in folded structures. (4) During the fourth stage (8 to 5 Ma), magmatic activity changed from calc-alkaline granitoid to alkaline basalt. This correlates with a moderate Atlasic compression, thrust faulting, and Miocene graben folding. During this stage, Pb (Ag)-Zn sulfides were deposited in veins that cut across the predating iron deposits, with neoformed Mg-rich clays and carbonates (calcite, siderite, ankerite) with noticeable Zn and Mn enrichments.

Mn-based mixed oxides for low temperature NOx adsorber applications

Pd-promoted ternary oxides of the type Mn-Ce-Zr and Mn-Pr-Zr were characterized and evaluated for low temperature NOx storage applications such as diesel vehicle cold starts. While X-ray diffraction data were in all cases consistent with the formation of solid solutions, Mn/Zr and Mn/Ce(Pr) ratios found by XPS were consistently higher than the bulk values, indicative of an enrichment of Mn at the surface of the solids. Both sets of Pd-promoted mixed oxides showed remarkably high NOx storage efficiency in the range 80–160 °C, while a 1.8% Pd/Mn(27)-Ce(7)-Zr catalyst showed excellent NOx storage in simulated cold start experiments. Moreover, ramping the temperature to 370 °C in these experiments, simulating higher speed operation, resulted in near complete purging of stored NOx from the catalyst. NOx storage efficiency in isothermal storage experiments was found to improve with increasing Mn content for the 1%Pd/Mn(x)-Ce(7)-Zr series (x = 9, 18, 27 wt.%), DRIFTS measurements showing that relative to Ce-Zr mixed oxides, Mn incorporation favored NOx storage as nitrate. During temperature programmed desorption (TPD) two main desorption events were observed, corresponding to decomposition of nitrites (up to 200 °C), followed by loss of nitrates (⁓200–400 °C). Nitrates stored on Pd/Mn-Ce(Pr)-Zr mixed oxides desorbed during TPD at lower temperatures than for CeO2-ZrO2 mixed oxides, a finding attributed to the lower basicity of Mn compared to Ce. Hydrothermal aging of 1.8%Pd/Mn(27)-Ce(7)-Zr at 700 °C reduced NOx storage efficiency, although the catalyst was still able to store significant amounts of NOx. However, catalyst sulfation led to a large decrease in NOx storage efficiency and the efficiency could not be completely recovered with lean or rich desulfations at high temperatures.

New evidence and interpretation of facies, provenance and geochemistry of late Triassic-early Cretaceous Tethyan deep-water passive margin-related sedimentary rocks (Ayios Photios Group), SW Cyprus in the context of eastern Mediterranean geodynamics

The late Triassic-early Cretaceous deep-sea sedimentary succession exposed in western Cyprus is used here to help understand the wider context of continental break-up and seafloor spreading in the easternmost Mediterranean region. The restored sequence begins with late Triassic, base-of-slope sandstone turbidites and mudrocks, and fines upwards into mudrocks, distal calciturbidites and metalliferous sediments. After a hiatus, late Jurassic-early Cretaceous non-calcareous mudstones and radiolarites accumulated beneath the carbonate compensation depth, together with redeposited shallow-water-derived carbonates, including calcirudites. The succession culminates in late Jurassic-early Cretaceous very thickly bedded and highly mature quartzarenites that accumulated by mass-flow processes. The fine-grained siliciclastic sediments are terrigenous in origin, as indicated by whole-rock, trace element and REE analysis. Fe, Mn and trace metal enrichment in the late Triassic represents hydrothermal input related to submarine volcanism that was probably triggered by the nearby onset of seafloor spreading. The late Triassic sandstones were derived from a low to intermediate-grade metamorphic source terrane. The late Jurassic-early Cretaceous sandstones reflect extreme physical and chemical maturity in the source area, followed by transport and mass flow to a deep-water base-of-slope setting. Regional facies variation and locally available palaeocurrent data suggest sediment supply, originally from a continental block, no more than tens of km to the west or north. Similar successions are exposed in W Turkey, represented by the Antalya Complex. In the light of alternatives, the sedimentary succession is restored as the base-of-slope apron of a rifted microcontinental block, adjacent to the northern margin of the S Neotethys. The overall succession indicates (unstable) passive margin development after regional latest Triassic-early Jurassic continental break-up, with implications for plate reconstructions and hydrocarbon exploration.

Geochemical Characteristics of Limestone of Wonosari-Punung Formation, Gunungkidul Regency, Yogyakarta, Indonesia

DOI: 10.17014/ijog.5.2.179-197A geochemical study was carried out to determine the geochemical characteristics of limestone in Wonosari-Punung Formation, and to suggest its depositional conditions and the source of rare earth elements. The study was conducted at Ponjong Area, Gunungkidul Regency, Daerah Istimewa Yogyakarta, Indonesia. White limestone which contains more abundant calcite is characterized by the highest CaO concentration. Red limestone which is associated with impure minerals such as siderite, rodochrosite, hematite, and titanite is typified by depleted CaO but higher SiO2, Fe2O3, MnO, and TiO2 concentrations. Grey limestone characterized by depleted CaO and higher MnO contents is considered to be influenced by the presence of impure minerals of manganite (Mn2O3.H2O) and/or pyrolusite (MnO2). Depletion of Sr concentration in the coloured limestone indicates that the diagenetic process increases from the white limestone to the coloured limestone. The process possibly occurred post-deposition of the limestones. The limestones show positive Mn* values (2.46 - 2.95) and authigenic U values (0.89 - 3.38) that suggest an oxidative environment. The Ce/Ce* values (0.57 - 0.80), Eu/Eu* values (1.04 - 1.88), high Y/Ho ratio, and low LaN/YbN ratio are indications that the rare earth elements in the limestones were derived from terrigenous materials. The positive Eu anomaly and enrichment of Cr and Mn of the limestones are indications of hydrothermal fluid activity taking place in the studied area. Based on the geochemical characteristics of the Punung-Wonosari limestones, the depositional environment and the source of rare earth elements of the limestones were therefore influenced by a combination of hydrothermal fluid activity with small amount of terrigenous material input and post depositional diagenetic process.

From unweathered core to regolith in a single weathering andesitic clast: Rates and trends of in situ chemical weathering on a tropical volcanic island (Basse Terre Island, French Guadeloupe)

We conduct X-ray microprobe, chemical and U-series isotope analyses on an oriented weathering clast collected from the regolith of a weathered Quaternary volcanoclastic debris flow on Basse Terre Island, French Guadeloupe. The sample consists of an unweathered basaltic andesite core surrounded by a weathering rind, and an indurated crust that separates the rind from the overlying soil matrix. U/Th disequilibria dating indicates that rind age increases away from the core-rind boundary to a maximum of 66 ka. This translates to a rind-advance rate of ~0.2 mm kyr−1, broadly consistent with rind advance rates calculated elsewhere on Basse Terre Island. The overlying indurated crust is 72 ka, indicating a possible minimum duration of the rind formation. Elemental variations are constrained by a bulk chemical analysis along a vertical transects from the core to the overlying soil matrix and parallel electron microprobe analyses. The hierarchy of elemental loss across the core-rind boundary varies in the order Ca > Na ≈ Mg > K > Mn > Si > Al > Ti = 0 > P > Fe, consistent with the relative loss of phases in the clast from plagioclase ≈ glass ≈ pyroxene > apatite > ilmenite. The abrupt, <900 μm wide, Ca, Na and porosity reaction fronts at the core-rind boundary approximately equal the length of the long dimension of plagioclase phenocrysts observed in the unweathered core. The <1000 μm wide reaction front at the rind-soil interface is marked by an indurated horizon with Fe and Mn enrichment that spans into enrichment of Mn, Ba, Al, Mg and K in the soil matrix. Unlike previously studied clasts, the preservation of the rind-soil interface permits characterization of weathering reactions and material exchanges between the weathering core, the rind, and the surrounding soil matrix, shedding insights into communication between the enveloping weathering rind and host regolith. The lack of an enrichment signal of Mn within the weathered rind suggests that weathering processes active within clasts are distinct from surrounding soil formation processes.

Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea

Redox conditions in the marine realm prior to the Great Oxidation Event (GOE; ∼2.46–2.32 Ga ago), during which the atmospheric oxygen level rose dramatically for the first time, are still debated. Here, we present C, O, Fe, and Mo stable isotope systematics of Fe-, Mn-, and carbonate-rich shales, deposited at different water depths in association with iron formations (IFs) of the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. δ13C values between −22.3 and −13.5‰ VPDB, and δ18O values between −21.1 and −8.6‰ VPDB for Fe–Mn-rich carbonate minerals indicate their precipitation out of equilibrium with seawater. Instead, early diagenetic reduction of Fe–Mn-oxyhydroxide precursor minerals, along with microbially induced oxidation of organic matter (OM), formed these carbonates. δ56FeIRMM-014 values between −1.27 and 0.14‰ and δ98MoNIST3134+0.25 values between −0.46 and 0.56‰ co-vary with Mn concentrations and inferred water depth of deposition. This suggests that, despite the diagenetic origin of the Fe–Mn carbonates, the primary light Fe and Mo isotopic signature of Fe–Mn-oxyhydroxides that originally precipitated from seawater is still preserved. While isotopically light Mo implies that Mn(II) was oxidized to Mn(IV) due to the availability of free, photosynthetically produced O2, Mn enrichment suggests that the water column was redox stratified with a Mn-redoxcline situated at a depth below the storm wave base. A trend to highly negative δ56Fe values with increasing Mn/Fe ratios and decreasing depositional depth suggests progressive oxidation of Fe(II) as deep-waters upwelled across a redoxcline towards shallow, locally oxygenated waters where Mn(IV) oxyhydroxides precipitated. Combined δ56Fe and δ98Mo data indicate pervasive oxygenation of seawater with the O2 content in the photic zone likely reaching levels higher than the maximum value of 10 μM proposed for Archean oxygen oases. Since abiotic Mn(II) oxidation is kinetically very slow in marine environments, it is likely that Mn-oxidizing microorganisms catalyzed Mn-oxidation in the oxygenated Pongola surface waters during deposition of IFs. This implies that aerobic metabolism had evolved before the GOE in shallow, aquatic habitats, where it exerted a first-order control on the deposition of shallow-marine, Mn-rich iron formations.

A multi-elemental study to establish the natural background and geochemical anomalies in rocks from the Sonora river upper basin, NW Mexico

Growing industrialization, including mining activity, dramatically increases environmental pollution. Exploitation of natural resources triggers landscape alteration, inputs of potentially toxic chemical elements to the environment and health diseases. In the case of mining activity, the measure of any associated environmental disturbance requires a geochemical background as a reference frame. Since in Mexico, there is a lack of this type of references in mining districts, the present study took a recent mining spill as an opportunity to establish the geochemical background of the upper Sonora river basin, which includes active and historical mining areas. The weighted average of elemental values was calculated using GIS tools. The data show that rocks have a similar geochemical behavior, which is characterized by pristine enrichment of Mn, Ca, P, Ba, Zn, As, Ag, Sb, W, Pb and Bi relative to the upper continental crust. Coefficients of variation values reveal that Sc, Y, Zr, Ce, Yb, Fe, Ti and Al display a conservative behavior and, therefore, they are recommended to be used as reference elements in environmental studies in the basin. The findings of this work highlight the need of determining the geochemical background in mining regions to reach more realistic environmental assessments.

Metal concentrations in the beach sediments of Bahia Solano and Nuquí along the Pacific coast of Chocó, Colombia: A baseline study

Thirty sediment samples from four different beaches along Bahia Solano and Nuquí (Department of Chocó) of eastern Colombia, with tourism and gold mining activities, were analysed to estimate the concentrations of fourteen different acid leachable metals. Metal distribution patterns showed elevated concentrations of Co, Cr, Cu, Pb and Zn compared with the upper continental crust values. Calculation of geochemical indices confirmed that the enrichment is due to periodic gold mining activities (severe to extremely severe enrichment of Cu, Zn, V, Co, Cr and Pb) along with natural (geological) contributions (minor and moderate enrichment of Ca, Mg, Fe, Ti, Mn and Li). Potential ecological risk index revealed that Pb posed the highest risk. Our results together with a global comparison suggest that the observed metal enrichments are mainly caused by mining and to a lesser extent by tourism in this region, thus instigating continuous monitoring of metal concentrations in this region.