Weathering Of Ultramafic Rocks Research Articles

Occurrence, Origin and Transformation Processes of Geogenic Chromium in Soils and Sediments

Weathering of ultramafic rocks has been linked to the occurrence of elevated concentrations of hexavalent chromium (Cr(VI)) in soils, sediments, and groundwater. Ultramafic rocks and the derived serpentine soils and sediments are encountered in populated areas around the world and present high Cr concentrations, with an average of 2200 and 2650 mg/kg for rocks and soils, respectively. Groundwater concentrations between 0.2 and 180 μg/L have been reported for Cr(VI) in ultramafic areas, exceeding occasionally the most prevalent drinking water limit of 50 μg/L Crtot, the 5 μg/L Cr(VI) limit established in Italy, and the 10 μg/L Cr(VI) limit proposed in California. Cr release in groundwater occurs through the dissolution of trivalent chromium (Cr(III)) from its mineral hosts, followed by sorption of Cr(III) onto high-valence Mn oxides and oxidation to Cr(VI), which desorbs and is mobile at alkaline pH. Recent findings indicate that hydrogen peroxide and birnessite produced on the surface of Cr(OH)3 by heterogeneous oxidation are two additional potential mechanisms. Thus, groundwater concentrations are controlled by a variety of geoenvironmental factors, including climate, soil mineralogy, pH, organic matter, and others. To provide a basis for the evaluation of Cr mobility in ultramafic environments, this paper presents an overview of the mineralogy and geochemistry of Cr-rich rocks, sediments, and soils, along with the weathering and geochemical processes that control the fate and transport in the subsurface.

Potentially toxic metal contamination and enzyme activities in soil around chromite mines at Sukinda Ultramafic Complex, India

Mining and processing of chromite ore resulting in degradation of soil quality. Sukinda ultramafic complex in Odisha hosts the biggest chromite deposit of India. Therefore, this study carried out to assess the level of various metal concentration, their source and impact of metals on soil enzyme activities. The soil is mostly characterized as acidic to neutral with low organic carbon. Along with the dominance of quartz in soil, common mineral phases identified are clinochlore, lizardite, talc, actinolite, illite, goethite, chromite and maghemite. Average concentrations of Cr, Co, Cu, Mn, Ni, Pb and Zn in soil are 7646mg/kg, 124mg/kg, 47mg/kg, 1791mg/kg, 780mg/kg, 18mg/kg and 77mg/kg respectively. Concentration of total Cr(VI) in soil varies from 1.45 to 16.7mg/kg with an average of 7.3mg/kg. High level of Cr, Ni, Co and Mn in soil is attributed to the weathering of ultramafic rocks. However, Cr(VI) is expected to be derived due to the oxidation of Cr(III) by Mn-oxides as higher concentration of Mn is available in soil. Based on the pollution and contamination index, soil in the area nearby mines is severely contaminated by metals. Enzyme activities are significantly negatively correlated with all metals except Pb. The order of the decrease in the activities is as follows: urease>acid phosphatase≥dehydrogenase>β-glucosidase≥alkaline phosphatase.

Chromium fluxes and speciation in ultramafic catchments and global rivers

The role of ultramafic rock weathering in global chromium (Cr) budgets and how these may be linked to changes in Earth's climate over geological time was investigated by compiling dissolved Cr speciation and fluxes in (i) 73 of the worlds large rivers, representing ~35% of global river discharge to oceans, and in (ii) an ultramafic catchment (Putah Creek watershed) in the California Coast Range Mountains, USA. Weathering of ultramafic rocks creates ideal conditions for Cr release and redox cycling. Alkaline river water draining ultramafic catchments is naturally enriched in Cr (up to 582nmol/L). Chromium concentrations increase with the extent of rock–water interaction (as indicated by correlations with Mg and HCO3− concentrations and pH), whereas Cr cycling depends on the availability of electron donors and acceptors (e.g. Fe(II), organics, Mn(III/IV)-oxides). Thus, dissolved Cr is exported from ultramafic watersheds as both the soluble hexavalent Cr(VI) species (MgCrO4, CrO42−), and also as trivalent Cr(III) species (CrOH(CO3)22−), Cr(OH)3). The latter Cr(III) species were previously thought to have low solubility. Ultramafic catchments have higher area normalized Cr and major ion fluxes for a given runoff when compared to global rivers and may thus have a disproportional impact on global Cr-budgets, long-term carbon cycling and moderation of the Earth's climate over million-year timescales. Riverine export fluxes of Cr are linearly correlated with fluxes of Ca+Mg and alkalinity in both ultramafic catchments and global rivers. This suggests that silicate weathering is a key control on Cr fluxes and that major element weathering fluxes may be a useful proxy for estimating Cr and other trace element fluxes. Globally, present-day riverine dissolved Cr fluxes to oceans are spatially variable and estimated to be 1.7×109mol/yr, three times higher than previously reported. However, when geochemical reactions in estuaries are considered riverine Cr fluxes may be lower. Throughout Earth history, Cr weathering fluxes also may have varied with changes in the global distribution of ultramafic rocks, Cr concentrations in continental rocks, or climactic conditions (e.g. atmospheric CO2 levels or runoff). Such variation should be considered when interpreting both modern and past Cr seawater isotopic compositions and residence times.

Miocene to Pleistocene osmium isotopic records of the Mediterranean sediments

In the late Miocene the Mediterranean Sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins. In this study we report Os isotopic records from Deep Sea Drilling Project and Ocean Drilling Project cores in the Mediterranean: the Balearic Sea (Site 372), the Tyrrhenian Sea (Site 654), the Ionian Basin (Site 374), and the Florence Rise (Sites 375–376), as well as Integrated Ocean Drilling Project Site U1387 in Gulf of Cadiz, North Atlantic. Pliocene-Pleistocene sediments at all sites show 187Os/188Os values close to that of the coeval ocean water, indicating that the Mediterranean was connected to the North Atlantic. Evaporitic sediments deposited during the latest Miocene, however, have 187Os/188Os values significantly lower than coeval ocean water values. The offset of the Mediterranean evaporite 187Os/188Os is attributed to limited exchange with the North Atlantic during the Messinian salinity crisis. The source of unradiogenic Os is likely to be weathering of ultramafic rocks (ophiolites) cropping out in the Mediterranean's drainage basins. Based on a box model we estimated the amount of unradiogenic Os and the Atlantic-Mediterranean exchange rate to explain this offset. Os isotopic ratios of the pre-evaporite sediments in the western Mediterranean are almost identical to that of the coeval ocean water. In contrast, equivalent sediments from the Florence Rise have significantly lower 187Os/188Os values. The offset in the Os isotopic ratio on the Florence Rise is attributed either to limited water exchange between eastern and western Mediterranean or to local effects associated with exhumation of the Troodos ophiolites (Cyprus).

Nickel isotope fractionation during laterite Ni ore smelting and refining: Implications for tracing the sources of Ni in smelter-affected soils

Nickel isotope ratios were measured in ores, fly ash, slags and FeNi samples from two metallurgical plants located in the Goiás State, Brazil (Barro Alto, Niquelândia). This allowed investigating the mass-dependent fractionation of Ni isotopes during the Ni-laterite ore smelting and refining. Feeding material exhibits a large range of δ60Ni values (from 0.02 ± 0.10‰ to 0.20 ± 0.05‰, n = 7), explained by the diversity of Ni-bearing phases, and the average of δ60Nifeeding materials was found equal to 0.08 ± 0.08‰ (2SD, n = 7). Both δ60Ni values of fly ash (δ60Ni = 0.07 ± 0.07‰, n = 10) and final FeNi produced (0.05 ± 0.02‰, n = 2) were not significantly different from the feeding materials ones. These values are consistent with the very high production yield of the factories. However, smelting slags present the heaviest δ60Ni values of all the smelter samples, with δ60Ni ranging from 0.11 ± 0.05‰ to 0.27 ± 0.05‰ (n = 8). Soils were also collected near and far from the Niquelândia metallurgical plant, to evaluate the potential of Ni isotopes for tracing the natural vs anthropogenic Ni in soils. The Ni isotopic composition of the non-impacted topsoils developed on ultramafic rocks ranges from −0.26 ± 0.09‰ to −0.04 ± 0.05‰ (n = 20). On the contrary, the Ni isotopic composition of the non-ultramafic topsoils, collected close to the plant, exhibit a large variation of δ60Ni, ranging from −0.19 ± 0.13‰ up to 0.10 ± 0.05‰ (n = 4). This slight but significant enrichment in heavy isotopes highlights the potential impact of smelting activity in the surrounding area, as well as the potential of Ni isotopes for discerning anthropogenic samples (heavier δ60Ni values) from natural ones (lighter δ60Ni values). However, given the global range of published δ60Ni values (from −1.03 to 2.5‰) and more particularly those associated to natural weathering of ultramafic rocks (from −0.61 to 0.32‰), the use of Ni isotopes for tracing environmental contamination from smelters will remain challenging.

Synchrotron micro-spectroscopic examination of Indonesian nickel laterites

Nickel is typically distributed across several fine-grained minerals in nickel laterites, formed by intense tropical weathering of ultramafic rocks. Indonesia accounts for approximately 16% of the world’s lateritic nickel reserves, which play an increasing role in global nickel production. However, relatively few geochemical studies of Indonesian laterites have been undertaken and quantification of Ni speciation is unclear. In this study the Ni geochemistry of an Indonesian laterite composed of limonite and saprolite has been examined using synchrotron microprobe analysis (microprobe X-ray fluorescence microscopy, μ-XFM; microprobe X-ray diffraction, μ-XRD; microprobe X-ray absorption spectroscopy; μ-XAS) and bulk XAS. This approach provides semi-quantitative species specific information not readily obtainable using traditional laboratory methods that are hampered by the fine-grained heterogeneous nature of laterites. In the limonite 16 ± 4% of the Ni was found to be substituted for Al in lithiophorite with 26 ± 7% being associated with lizardite (a serpentine) substituted for Mg. A minor proportion of the Ni is adsorbed onto the Mn layers of phyllomanganate (e.g., lithiophorite). However the majority of the Ni, 58 ± 15% is substituted for Fe in goethite. The majority of the Ni (85 ± 21%) within the saprolite is found to be associated with lizardite, the predominant mineral. A few relatively large Ni asbolane grains are also observed, which account for 14 ± 3% of the Ni with the remaining 1 ± 0.3% of the Ni replacing Fe within goethite.

Nickel isotope fractionation during tropical weathering of ultramafic rocks

Although Ni isotopes have been shown to be significantly fractionated in terrestrial samples, their use in continental environmental studies has not yet been evaluated. The present study focuses on an ultramafic (UM) massif (Barro Alto, Goiás, Brazil) because such areas are naturally rich in Ni. We present developed lateritic weathering profiles. The goal of the study is to evaluate the potential of using Ni isotopes in environmental continental studies by combining its isotopic signature with mineralogy, in order to better understand the geochemical cycling of Ni in UM settings during weathering. As such, Ni isotope values were measured in samples from the Barro Alto UM complex in the main stages of the lateritic weathering profile of UM rocks, including bedrock, ores (saprolitic and lateritic samples) and soil. The mineralogical composition of the samples, with a focus on the different Ni-bearing minerals, was also determined to decipher the potential links between isotopic fractionation and weathering dynamics. Isotopic signatures (δ60Ni) from the natural Ni geochemical cycle include: bedrock samples (δ60Ni=0.28±0.08‰), ore samples (saprolitic and lateritic, δ60Ni from −0.60 to 0.30‰) and soil samples (δ60Ni from −0.19 to −0.02‰). An overall trend of heavier isotope depletion was observed in the solid phase during weathering (Δ60NiSoil–Bedrock=−0.47‰). The mineralogical results were consistent with the literature and showed that the mineralogy of the lateritic part and soil was dominated by Fe-oxides, whereas clay minerals were the primary Ni phase scavengers in the saprolitic part of the profile. Thus, the formation of Ni-bearing clay minerals and Fe-oxides appeared to lead to depletion in heavier isotopes, which indicates preferential export of heavy isotopes in the dissolved phase. This result is consistent with isotopic signatures measured in the exchangeable pool of the solid phase (Δ60Niexch-total up to 0.47‰), and Ni isotopes appear to be a promising tracer to better understand the biogeochemical Ni cycling on the Earth's surface.

In situ sequestration of atmospheric CO2 at low temperature and surface cracking of serpentinized peridotite in mine shafts

The investigation of carbonate formation at low temperature during weathering of ultramafic rocks has become increasingly important as it represents an analog study for a cost-efficient carbon disposal strategy. Here we present new insight into carbonate formation under subarctic surface conditions obtained from extensively carbonated chromite mine shafts in the Feragen ultramafic body, E Norway. Carbonation proceeded by formation of mm- to cm-thick carbonate surface coatings on the serpentinized peridotite host rock. The surface coatings consist in most cases of pure lansfordite (MgCO3·5H2O) and sometimes nesquehonite (MgCO3·3H2O). Heavy O- and C-isotopes are strongly enriched in the carbonate coatings relative to drip water samples from the mines and water samples from subaerial ponds. The major element composition of the water samples reveal that infiltrating rainwater reacts rapidly with the peridotite mainly due to brucite dissolution. Within the mines, discharging alkaline water forms thin films on walls and ceilings that evaporate upon circulation of cold, dry air. In mines with only one opening the carbonation is limited to the entrance area. In mines with a more efficient air circulation due to multiple entrances carbonation is found throughout the mine. Evaporation of the mine water results in lansfordite saturation and precipitation and is accompanied by a Rayleigh-type distillation effect causing enrichment of the heavy C- and O-isotopes in the liquid from which the carbonates precipitate. Formation of the carbonate coatings involved fragmentation of the underlying serpentinite substrate. Fragmentation proceeded by subcritical cracking, i.e. by a combination of substrate weakening due to brucite dissolution and stress build-up due to lansfordite precipitation in fractures close to the surface. Reaction-induced fracturing represents a positive feedback mechanism as it creates additional reactive surface area during the carbonation. Mining operations in the area ceased in the mid 1920s indicating that the described carbonation occurs on a time-scale relevant for the disposal of anthropogenic CO2.

Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation

Changes in Ni speciation in a 64m vertical profile of a New Caledonian saprolitic–lateritic regolith developed over ultramafic rocks under tropical weathering conditions were investigated by EXAFS spectroscopy. Quantitative analysis of the EXAFS spectra by linear combination-least squares fitting (LC-LSF) using a large set of model compound spectra showed that Ni hosted in primary silicate minerals (olivine and serpentine) in the bedrock is incorporated in secondary phyllosilicates (serpentine) and Fe-oxides (goethite) in the saprolite unit and mainly in goethite in the laterite unit. A significant concentration of Ni (up to 30% of total Ni) is also hosted by Mn-oxides in the transition laterite (i.e. the lowest part of the laterite unit which contains large amounts of Mn-oxides). However, the amount of Ni associated with Mn-oxides does not exceed 20% of the total Ni in the overlying laterite unit. This sequence of Ni species from bedrock to laterite yields information about the behavior of Ni during tropical weathering of ultramafic rocks. The different Ni distributions in phyllosilicates in the bedrock (randomly distributed) and in the saprolite unit (clustered) indicate two generations of Ni-bearing phyllosilicates. The first, which formed at higher temperature, is related to serpentinization of oceanic crust, whereas the second one, which formed at lower temperature, is associated with post-obduction weathering of ultramafic rocks. In addition, the observed decrease in the proportion of Ni hosted by Mn-oxides from the transition laterite to the upper lateritic horizons indicates dissolution of Mn-oxides during the last stages of differentiation of the lateritic regolith (i.e. lateritization). Finally, the ubiquitous occurrence of Ni-bearing goethite emphasizes the major role of this phase in Ni speciation at the different weathering stages and suggests that goethite represents the major host for Ni in the final tropical weathering stages of New Caledonian ultramafic rocks.

Synchrotron-based speciation of chromium in an Oxisol from New Caledonia: Importance of secondary Fe-oxyhydroxides

In New Caledonia, the weathering of ultamafic rocks under a tropical climate has led to the residual accumulation of trace elements in lateritic soils widely dominated by Fe-oxyhydroxides. The speciation of trace elements, such as Cr, Ni, and Co, in these Oxisols remains a major subject of interest regarding mining and environmental issues. We have assessed the speciation of chromium in the upper part of an Oxisol, by combining bulk and spatially resolved chemical analyses (EPMA and SEM-EDS) with synchrotron-based spectroscopic data (EXAFS and XANES). EPMA indicates that the main hosts for chromium in the bedrock sample are the silicates forsterite, enstatite, and lizardite. Hosting of chromium in these easily weatherable mineral species could lead to a significant loss of this element upon weathering. However, total analyses of major elements indicate only a slight depletion of Cr, together with an immobility of Fe and Al and drastic losses of Si and Mg, after the weathering of the bedrock. Such a low mobility of chromium is likely related to its significant incorporation in goethite and hematite formed after the weathering of Fe 2+ -bearing primary silicates. This efficiency of secondary Fe-oxyhydroxides at immobilizing chromium is demonstrated by quantitative analysis of EXAFS data that indicates that these mineral species host between 67 and 75 wt% of total Cr (compared to the 18 to 22 wt% of total Cr hosted by chromite). In addition, SEM observation and SEM-EDS analyses performed on the Oxisol samples also show some evidence for chemical weathering of chromite. Chromite could then represent a past and/or present source of chromium upon extended tropical weathering of the studied Oxisol, rather than a stable host. These results emphasize the importance of secondary Fe-oxyhydroxides, compared to Cr-spinels, on chromium hosting in Oxisols developed upon tropical weathering of ultramafic rocks. Although the trapping mechanism of chromium mainly corresponds to incorporation within the structural network of goethite and hematite, sorption reactions at the surface of these mineral species could also be involved in such a process. In addition, considering their potential oxidative reactivity that can generate Cr 6+ or enhance the chemical weathering of chromite, the occurrence of Mn oxides could significantly modify the behavior of chromium upon weathering. These considerations indicate that further studies are needed to assess the actual potential of chromium release from Oxisols developed upon weathering of ultramafic rocks under a tropical climate.

Growing up green on serpentine soils: Biogeochemistry of serpentine vegetation in the Central Coast Range of California

Serpentine soils derived from the weathering of ultramafic rocks and their metamorphic derivatives (serpentinites) are chemically prohibitive for vegetative growth. Evaluating how serpentine vegetation is able to persist under these chemical conditions is difficult to ascertain due to the numerous factors (climate, relief, time, water availability, etc.) controlling and affecting plant growth. Here, the uptake, incorporation, and distribution of a wide variety of elements into the biomass of serpentine vegetation has been investigated relative to vegetation growing on an adjacent chert-derived soil. Soil pH, electrical conductivity, organic C, total N, soil extractable elements, total soil elemental compositions and plant digestions in conjunction with spider diagrams are utilized to determine the chemical relationships of these soil and plant systems. Plant available Mg and Ca in serpentine soils exceed values assessed in chert soils. Magnesium is nearly 3 times more abundant than Ca in the serpentine soils; however, the serpentine soils are not Ca deficient with Ca concentrations as high as 2235 mg kg−1. Calcium to Mg ratios (Ca:Mg) in both serpentine and chert vegetation are greater than one in both below and above ground tissues. Soil and plant chemistry analyses support that Ca is not a limiting factor for plant growth and that serpentine vegetation is actively moderating Mg uptake as well as tolerating elevated concentrations of bioavailable Mg. Additionally, results demonstrate that serpentine vegetation suppresses the uptake of Fe, Cr, Ni, Mn and Co into its biomass. The suppressed uptake of these metals mainly occurs in the plants’ roots as evident by the comparatively lower metal concentrations present in above ground tissues (twigs, leaves and shoots). This research supports earlier studies that have suggested that ion uptake discrimination and ion suppression in the roots are major mechanisms for serpentine vegetation to tolerate the chemistry of serpentine soils.

Chemical and physical transfers in an ultramafic rock weathering profile: Part 2. Dissolution vs. accumulation of platinum group minerals

The chemical weathering of ultramafic rocks has resulted in eluvial concentration of Pt-group minerals (PGM) in lateritic weathering profiles of southern New Caledonia. The Pt mineralization interpreted as being primary consists of Pt-group minerals included within chromite crystals. The oc- currence of PGM as free particles in the weathering profile results from the supergene dissolution of Pt-bearing chromite (Traore et al. 2008). Following their release in the profile, supergene dissolution processes variably affect the PGM particles. The behavior of Pt-group elements in the weathering profile is characterized by significant loss of Pd and relative enrichment of Pt indicating that Pd is more mobile than Pt in the exogenous cycle. Unstable Pt-Fe-Cu-Pd alloys and PGE oxides undergo chemical and mineralogical changes to acquire the chemical configuration of isoferroplatinum (Pt3Fe), which is the most stable Pt phase in a lateritic environment. The isoferroplatinum phase may also be dispersed throughout the weathering mantle and/or accumulated in the lower parts of profiles ac- cording to a translocation mechanism of residual Pt-rich fine particles driven by percolation of water through the connected pore spaces.

Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemi- cal analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Ultramafic rock weathering and slope erosion processes in a South West Pacific tropical environment

Weathering and erosion processes are investigated using electrical resistivity tomography (ERT) imaging and the quantification of geomorphic patterns at the edges of a lateritic plateau overlying ultramafic rocks in the north western region of the main island of New Caledonia (Southwest Pacific). The obtained ERT images document the structure and long-term evolution of the regolith, while source area parameters such as area ( A) local slope above channel head (tan θ) and longitudinal river profiles allow the characterization of contrasting geomorphic patterns around the plateau. The geo-electrical profiles show a succession of hard rock protrusions and weathering troughs, whose depth varies greatly. The area–slope relationship allows the distinction between saprolite- and ferricrete-mantled source areas. The former could result from a regolith erosion process by shallow landslides; the latter from a secondary ferruginization process of reworked lateritic debris. The deepest troughs underlie saprolite-mantled source areas above channel heads, which are characterized by a low permeability saprolite, relatively high slope gradient, and lower area/slope ratios. Such source areas generate fairly high runoff, sustaining rivers and creeks with relatively high erosion power. The ferricrete-mantled source areas are characterized by higher permeability and area/slope ratios, leading to lower runoff and less erosion but further chemical rock weathering. The ferricrete of those source areas acts as a protective hardcover against mechanical erosion of the underlying regolith. This ferricrete reworks, at least partly, allochtonous lateritic materials inherited from a previous disaggregated ferricrete that suggests past erosion processes driven by hydro-climatic condition changes.

A mid miocene flood basalt event observed in the osmium isotope record of seawater

Two Fe–Mn crusts from the central Pacific and one from the central Atlantic were dated by osmium (Os) isotope stratigraphy. All three crusts show a pronounced Os isotope minimum around 12 Ma. The Os/Os decreased from 0.8 at 15 Ma to 0.7 at 12 Ma and reached a value of 0.8 again at 9 Ma indicating symmetrical shoulders of this excursion. In the modern ocean, the concentration of dissolved Os as well as the Os/Os of 1.06 are essentially homogenous, which is accurately reflected by recent marine sediments in several depositional settings. The observed negative Os isotope excursion at 12 Ma, which has not been observed in earlier studies of pelagic and metalliferous sediments, thus must have been caused by an event that changed the osmium isotope signature of global seawater. Possible interpretations for the Mid-Miocene decrease include changes in continental weathering intensity i.e. a reduction of the radiogenic Os flux, or an increase in the unradiogenic input fluxes, such as intensified weathering of ultramafic rocks, exhalations derived from large igneous flood basalts or their weathering, a meteorite impact, or a combination of several of these reasons. In the Mid-Miocene there was at least one larger meteorite impact, the Nordlinger Ries Crater and at the same time the large Columbia River igneous flood basalt province formed. Application of a simple mass balance model RSW = Rr (fr) + Ru (1 fr) where fr is the ratio of riverine flux of Os to the total flux of Os, and were subscripts sw, r and u refer to seawater, riverine and unradiogenic, respectively, shows that exhalations from a large igneous flood basalt are the most likely cause of the observed minimum. Therefore the most viable explanation of the unradiogenic Os is the eruption of the Columbia River flood basalts. Such an interpretation is also possible for the two other more pronounced minima in the Cenozoic Os isotope record of seawater that have previously been reported for the Cretaceous/Tertiary and Eocene/Oligocene boundaries. The exhalations of the Deccan Traps and the Ethiopian Flood Basalts were large enough to release sufficient amounts of unradiogenic Os into the ocean to cause the observed minima.

Chromium Geochemistry of Serpentine Soils

Serpentine soils derived from the weathering of ultramafic rocks, mainly ophiolitic serpentinites, are typically characterized by Cr concentrations in excess of 200 mg kg-1, comparatively higher than non-serpentine soils. We review the chemistry of Cr in serpentine soils and their protoliths, focusing on serpentine soils collected from New Caledonia, Oregon, and California. Overall, serpentine soils are slightly acidic (average pH of ∼6), contain a variety Fe(III) oxides (magnetite and hematite), Fe(III) (oxy)hydroxides, phyllosilicates (serpentine and chlorite), and clays (smectites and vermiculites), and contain concentrations of Cr (> 200 mg kg-1), Ni (> 1,000 mg kg-1), and Mn (> 200 mg kg-1) exceeding values of non-serpentine soils. Although Cr concentrations in serpentine soils have been reported as high as 6 wt% in New Caledonia, Cr values in New Caledonia, Oregon, and California serpentine soils evaluated in this study range from 827 to 9,528 mg kg-1. Chromium(III) is the only valence state observed in the serpentine soil solids; however, Cr(VI) has been identified in New Caledonia and California serpentine soil solutions at concentrations below 30 μM. The enrichment and range of Cr concentrations in serpentine soils are directly related to the presence of Cr-spinels, specifically chromite and Cr-magnetite. These phases are resistant to weathering and are preserved in the soil environment; however, oxidation of Cr(III) from Cr-spinels by high-valent Mn oxides, or other strong oxidants, is a potential source of Cr(VI) identified in serpentine soil solutions. Due to the weathering resistant nature of the Cr-spinels, Cr-bearing silicates including clay minerals, Cr-chlorite, Cr-garnet, Cr-mica, and Cr-epidote are more viable sources of Cr identified in vegetation, soil extractions, soil solutions, and related waters.

Weathering of ultramafic rocks and element mobility at Mt. Prinzera, Northern Apennines, Italy

AbstractThe weathering of the Mt. Prinzera serpentinites (Parma province, Northern Apennines, Italy) produced dominant smectite and minor Fe-hydroxides. The mobility of the elements during weathering has been estimated using the ratio Ki = MiS/MiR, where Mi indicates the mass of a generic component i before (R) and after (S) the weathering, and using TiO2 as a practically immobile component. For prevalently to tendentially mobile elements, the degree of mobility, which in our case increases as Ki decreases, is in the order Mn ≈ Cr = Fe < V ≈ Zn ≈ Co < Ni < Si < Mg < Ca. The elements Ti, Ga, Al and Zr are prevalently immobile. The mass chemically removed by the circulating waters during weathering may reach very high values (on average 52% of the original mass of serpentinite) and the main contribution to the mass loss is due to Si and Mg. The composition of the perennial and ephemeral springs in the area is in agreement with the degree of the element mobility at least for Cr, Mg and Ca.

Ground Water Pollution With Chromium in Leon Valley, Mexico

Abstract Groundwater contamination by chromium has been detected in the Leon valley central Mexico. In order to determine the contaminant concentration levels and to find its source, a sampling and analysis program was developed. The analytical determinations of chromium and physico-chemical parameters, in a known hydrogeological framework, allowed the identification of three sources of chromium, two of them anthropogenic and one natural. The anthropogenic sources are: the inadequate solid wastes disposal by a chromate factory, which produces high localized concentrations (up to 50mg/l) in groundwaters to the southwest of the valley; and residual ashes produced in brick manufacturing which generates much lower concentrations (lower than 0.05 mg/l of hexavalent chromium) over a much wider area (about 180 km2) south of Leon city. The natural source is weathering of ultramafic rocks which produces detectable amounts (between 0.004 and 0.015 mg/l) of Cr (VI) in groundwaters to the Northeast of the valley. Alt...

O DEPÓSITO DE NÍQUEL DE JACUPIRANGA (SP): EVOLUÇÃO MINERALÓGICA E GEOQUÍMICA

The nickel deposit of Jacupiranga was formed by lateritic weathering of ultramafic rocks. The estimated reserves are 13 m. ton, 1.4% Ni. The alteration profile is very thick and consists from bottom to top: a) fresh rock: serpentinized dunite; b) altered rock and argillaceous saprolite: serpentine and smectite, 30 to 40 m thick; c) silicified ferruginous saprolite: quartz and goethite, up to 25 m thick; d) upper horizon: kaolinite and goethite, up to 10 m thick. The weathering evolution of the serpentinized dunite has led to a loss of Si and Mg and a relative accumulation of Fe and Al. In the silicified horizon absolute accumulation has occurred. The upper horizon seems to be only partially derived from the evolution of the dunite (too high content of Al). Nickel is mainly concentrated in the altered rock and in the argillaceous saprolite associated with serpentine and smectite. Furthermore it occurs as pimelite in the garnieritic veins that cut the altered rock. The content of Ni in these horizons ranges between 0.5% and 2% (up to 9% locally). In the ferruginous saprolite there is always less Ni (< 1%), probably associated with geothite. Three factors make difficult the exploitation of the ore deposit: a) the thick overburden; b) the irregularity of the Ni distribution; c) the high silica content of the ore.

Lateritic weathering of ultramafic rocks and the concentration of nickel in the western Ivory Coast

Weathering of ultramafic rocks from the Sipilou and Moyango areas of the western Ivory Coast was studied from a series of profiles distributed along a topographic slope from upper plateau zones to lower slopes (catena profiles). The distribution, thickness, and composition of the different weathering layers vary according to their location, i.e. whether they are on the plateau, on the hillside, or in the lower areas. Nickel distribution follows these vertical and horizontal variations. The highest nickel contents are linked to the distribution of secondary phyllosilicates (principally talc and smectites) which are generated by the weathering of olivines, pyroxenes, and serpentines. In general, ferruginous layers are the most developed in the plateau areas, where extensive silicification exists on the hill slope. The accumulation of carbonates takes place downslope and at the bottom of the weathering profiles.