Lateritic Weathering Profiles Research Articles

Micro- to macroscale controls on REE distribution in the Ambohimirahavavy ion adsorption deposit, Madagascar

Ion adsorption deposits, in which the rare earth elements (REE) occur in easily leachable form with lateritic weathering profiles, are currently the worlds primary source of the HREE. These are min...

Application of HVSR to estimating thickness of laterite weathering profiles in basalt

AbstractLateritic weathering profiles (LWPs) are widespread in the tropics and comprise an important component of the Critical Zone (CZ). The Hawaiian Islands make an excellent natural laboratory for examining the tropical CZ, where the bedrock composition (basalt) is nearly uniform and rainfall varies greatly. This natural laboratory is employed to assess the utility of the HVSR (horizontal/vertical spectral ratio) method to characterize the shear‐wave velocity (Vs) structure of LWPs, particularly the depth to the contact between saprolite and basalt bedrock.LWP thicknesses determined from HVSR provide good agreement with multi‐channel analysis of surface waves (MASW) profiles, well logs and outcrop. LWP thicknesses may be estimated from the fundamental mode equation or through forward models. Prior knowledge about the subsurface from well, outcrop, and MASW profiles may greatly aid modeling in some cases. For the 3.2 to 1.8 Ma Koolau Volcano on Oahu, the downward rate of advance of the weathering front varies from 0.004 to 0.041 m/ka. For the 0.44 to 0.10 Ma Kohala Volcano (Big Island of Hawaii) rates vary from 0.013 to 0.047 m/ka.Simple H/V spectra develop in areas where the combined effects of time and elevated rainfall produce thick LWPs with a flat base and a general absence of core stones with an ideal layered geometry. Abundant buried core stones violate the assumption of simple layered geometries and scatter acoustic energy, leading to uninterpretable results. This is common where low rainfall and a young basaltic substrate leave abundant core stones as well as an undulating contact between saprolite and bedrock. Velocity inversions (high Vs intervals within low Vs saprolite) may also be present and originate from relatively intact bedrock horizons or mineralogical changes within saprolite. At Kohala, a gibbsite‐rich horizon produces such a velocity inversion due to enhanced weathering and subsequent collapse of saprolite in a discrete horizon. © 2019 John Wiley & Sons, Ltd.

Rare-earth element and stable isotope signatures of kaolin from a Pliocene lateritic weathering profile at mid-latitude region (Andalusia, Spain): Implications for paleoweathering and paleoclimatic reconstructions

This study reports the impact of weathering on behavior of rare earth elements (REE) in a coastal lateritic profile developed on Pliocene sediments at mid-latitude location (~37° N), in the Guadalquivir Basin (Spain). It also explores the stable isotope signature of kaolinite providing constraints on paleotemperature and isotopic composition of the ancient meteoric water. The paleoweathering profile (~20 m thick) comprises a white sandy clayey saprolite overlain by a red-mottled clay zone, which in turn is capped by a pisolitic ferricrete. The kaolinitic regolith was practically reduced to a mixture of kaolinite and quartz by intense chemical weathering under warm and seasonally humid climate.The kaolinized material is markedly depleted in total REE (mean value of 77.30 mg kg−1 in the white saprolite and 72.70 mg kg−1 in the mottled zone) relative to the parent rock (168.96 mg kg−1). Development of acidic and oxidizing conditions promoted a suitable soil environment for REE release and leaching with percolating rainwater, leading to an apparent loss of REE from the profile (up to 87%). The parent rock-normalized REE patterns display concave-upward shapes tracing a remarkable depletion in middle REE (MREE), with no significant anomalies. The REE concentrations normalized against the North American Shale Composite (NASC) showed consistently similar fractionation patterns. The MREE-depleted signature (NASC-normalized ratios of La/Sm up to 3.32 and Gd/Lu as low as 0.45) probably arises from reductive dissolution of iron oxyhydroxide phases due to seasonal water saturation, as indicated from redoximorphic features.The <2 μm kaolinite separates showed δ18O values ranging from 17.3‰ to 20.0‰, and δD values between −71‰ and −60‰. The δ18O and δD mean values indicated a crystallization temperature of about 24 °C, which is higher than the local present-day annual mean air temperature (~18 °C). For the calculated temperature, the oxygen isotope fractionation factor between kaolinite and water (αk-w = 1.0248) implies that kaolinite formed in equilibrium with meteoric waters (δ18O = −5.4‰ and δD = −33‰) that closely reflect the regional meteoric precipitation.The lateritic weathering profiles evolved during the mid-Pliocene warm period in the southwestern Iberian margin could be useful for predicting potential future environmental effects of increased atmospheric CO2 on the Earth's critical zone.

Mineralogical and geochemical trends in lateritic weathering profiles on basement rocks in Awa-Oru Ijebu and its environ, southwestern Nigeria

Three soil profiles on the basement rocks in Awa-Oru-Ijebu Igbo area of southwestern Nigeria were investigated for the formation secondary minerals and the comparison of the mineralogical and geochemical patterns in the weathering profiles overlying the rocks during the humid tropical weathering. X-ray data showed that the primary mineral constituents of the rocks such as feldspar, biotite, hornblende have altered to kaolinite and hematite. In soil horizon, kaolinite and quartz are the dominant minerals. The concentration of hematite in the laterite indicated that most of the iron oxides generated in the soil horizon have been leached into the underlying laterite layer. Kaolinite is the dominant mineral in the clayey horizon and its presence suggests that some of the iron bearing ferromagnesian minerals have altered into the kaolinite. Geochemical data further revealed significant enrichment of Al2O3 and Fe2O3 in the laterite profile compared with the parent rock, while, CaO, Na2O, K2O, P2O5 and MnO show strong depletion up the profiles. The removal of mobile elements by meteoric water and subsequent concentration of stable weathering products results in lateritization. The absence of bauxite minerals like gibbsite and diaspora in the profiles shows that the trend of weathering is towards iron enrichment (ferralitization) and not aluminum accumulation (bauxitization). The CIA values of the lateritic layers shows that the weathering process is matured and has reached advanced stage.Keywords: Basement rocks, weathering, lateritization, kaolinite, Hematite.

Goethite (U–Th)/He geochronology and precipitation mechanisms during weathering of basalts

(U–Th)/He geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the Paraná flood basalt in the Guarapuava region, Paraná, Brazil, reveals ages ranging from 3.6±0.4 to 0.4±0.1Ma. Thirty-one grains from detrital fragments of ferruginous duricrust yield ages in the 6.2±0.6 to 0.7±0.1Ma range. The results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the Guarapuava sites are relatively young (≤6.2±0.6Ma). Goethites show variable U and Th contents depending on the mode of precipitation. Goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in Th, U-poor, contain various mineral inclusions (e.g., kaolinite, rutile, ilmenite) and display a distinct “checkered” appearance. Colloform goethite precipitated within cavities and desiccation cracks contain higher U, show low Th/U values, and are free of mineral contaminants, suggesting direct precipitation from solution. The geochronological results reveal a dynamic evolution for the soils overlying the Paraná basalt, where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles. The modes of formation and U, Th, and He contents of goethite at Guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on Mars.

The Yatela gold deposit: 2 billion years in the making

Gold mineralisation in the Yatela Main gold mine is hosted in a saprolitic residuum situated above Birimian supracrustal rocks, and at depth. The supracrustal rocks comprise metamorphosed calcitic and dolomitic marbles that were intruded by diorite (2106 ± 10 Ma, 207Pb/206Pb), and sandstone-siltstone-shale sequences (youngest detrital zircon population dated at 2139 ± 6 Ma). In-situ gold-sulphide mineralisation is associated with hydrothermal activity synchronous to emplacement of the diorite and forms a sub-economic resource; however, the overlying saprolitic residuum hosts economic gold mineralisation in friable lateritized palaeosols and aeolian sands (loess).Samples of saprolitic residuum were studied to investigate the morphology and composition of gold grains as a proxy for distance from source (and possible exploration vector) because the deposit hosts both angular and detrital gold suggesting both proximal and distal sources. U–Pb geochronology of detrital zircons also indicated a proximal and distal source, with the age spectra giving Archaean (2.83–3.28 Ga), and Palaeoproterozoic (1.95–2.20 Ga) to Neoproterozoic (1.1–1.8 Ga) zircons in the Yatela depocentre. The 1.1–1.8 Ga age spectrum restricts the maximum age for the first deposition of the sedimentary units in the Neoproterozoic, or during early deposition in the Taoudeni Basin. Models for formation of the residuum include distal and proximal sources for detritus into the depocentre, however, it is more likely that material was sourced locally and included recycled material. The creation of a deep laterite weathering profile and supergene enrichment of the residuum probably took place during the mid-Cretaceous-early Tertiary.

China: Tosaf/Colloids – masterbatch

Ion adsorption deposits, in which the rare earth elements (REE) occur adsorbed onto clay mineral surfaces, currently provide the world’s dominant supply of heavy REE (Gd-Lu). Concentration of REE within ion adsorption deposits has been proposed to be a dominantly supergene process, where easily degradable REE-minerals (e.g. REE-fluorcarbonates) break down and release REE that are then adsorbed onto clay mineral surfaces in the weathered material. Here we present data from the Cenozoic Ambohimirahavavy alkaline complex in Madagascar, with the aim of further constraining controls on the formation and HREE enrichment processes in ion adsorption deposits.The laterite weathering profiles described here are developed on alkaline igneous rocks, including both SiO2-undersaturated and oversaturated lithologies. The latter group includes REE mineralised peralkaline granitic pegmatites and granitic dykelets. The weathering mineralogy includes gibbsite, clay minerals and minor Fe and Mn oxyhydroxides. X-ray diffraction and infrared spectroscopy show that the clay fraction in all sites is dominated by kaolinite and halloysite (7 Å and 10 Å). Extraction with ammonium sulfate (0.5 M (NH4)2SO4, pH = 4; removes weakly adsorbed metals retained on the solid surface and dissolves carbonates) indicates that, in the different profiles and along the same profile, the leachable REE content is heterogeneous, ranging from 5 to 2300 mg/kg total REE, with 1 to 32% heavy REE. Leaching with magnesium chloride (0.5 M MgCl2, pH = 6; removes only weakly adsorbed REE) releases similar total REE concentrations, suggesting that most of the leachable REE are adsorbed onto mineral surfaces. In most of the laterite profiles, the amount of leachable REE continuously increases with depth up to the saprock. Recovery rates depend on the REE atomic number, generally decreasing from La to Lu except for Ce which is invariably low.In the Ambohimirahavavy complex, the nature of the protolith is the main factor controlling the amount of easily leachable REE in the laterite weathering profile. Hydrology and topography are secondary factors. The most favourable protoliths include SiO2-undersaturated volcanic lithologies and altered mudstone with granitic dykelets. The main primary REE minerals include agpaitic minerals (eudialyte) and allanite-(Ce). Locally in granitic pegmatite dykes, autometasomatism by late magmatic fluids inhibits formation of ion adsorption ore by transforming easily weathered agpaitic minerals into unweatherable zircon.

A geophysical strategy for measuring the thickness of the critical zone developed over basalt lavas

Estimates of the thickness variation in lateritic weathering profiles (LWPs) are important in tropical areas underlain by young basalt lavas like those found in Hawaii. Seismic shear-wave velocity data were obtained by a new application of multichannel analysis of surface waves (MASW) to map variations in the LWP and to derive the downward rate of advance of the weathering front in basaltic lavas. The MASW technique proved highly capable of imaging the internal structure and base of the critical zone, as confirmed by borehole data and direct field measurements. Profile thickness thus obtained, rapidly and without drilling, has applications to engineering and geochemical studies. The rate of advance of the weathering front derived from MASW in Oahu ranged from 0.010 m/ka to 0.026 m/ka in mesic zones (∼1500 mm/a rainfall), whereas an area with ∼800 mm/a revealed rates from 0.005 m/ka to 0.011 m/ka. These rates are comparable to those derived from recent solute-based mass balance studies of ground and surface water. Conventional P-wave seismic reflection did not perform as well for detecting boundaries due to a gradational seismic velocity structure within the weathering profile. Shear-wave velocity models showed internal variations that may be caused by textural differences in parental lava flows. Limitations in imaging depth were overcome by innovative experiment designs. Increasing source-receiver offsets and merging surface-wave dispersion curves allowed for a more objective derivation of velocity-frequency relations. Further improvements were made from a recently developed form of the combined active and passive source technique. These advances allowed for more detailed and deeper imaging of the subsurface with greater confidence. Velocity models derived from MASW can thus describe the LWP in terms of depth and variability in stiffness.

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.

Paleosol occurrences along the Archean–Proterozoic contact in the Aravalli craton, NW India

The Archean–Proterozoic contact (APC) comprises a large scale folded surface around Udaipur in NW India. The APC is exposed on both eastern and western flanks of a synclinorium formed during thick skinned thrusting of the Archean basement and the Paleoproterozoic cover units. Paleosol occurrences on both the limbs of this structure imply an originally continuous paleoweathering horizon and not individual pockets of paleosol as seen today. Preservation of paleosol sections along the Archean–Proterozoic interface in the Aravalli craton is strongly controlled by the structural grain. Complete paleosol sections are recognized in the eastern flank of the Udaipur synclinorium in low strain regions with respect to the regional F1 and F2 folding. In this sector the Tulsi Namla paleosol section represents an ancient lateritic weathering profile with an iron depleted pallid lower zone (lithomarge) and an iron enriched upper zone (mottled zone to ferricrete). Original kaolinite in the saprolite and pallid zone has been transformed into kyanite during metamorphism and subsequently to white mica during retrograde overprint, while Fe rich clay and goethite were probably the primary minerals for the chloritoid and hematite which now constitute the upper zone. At the western flank of the synclinorium the contact is heavily sheared and Archean basement and Proterozoic units show interfingering on a small scale. Here, the paleosol sections are incomplete.The weathering interval can be inferred by the stabilization of the craton at ∼2.5Ga and deposition of rift related sequences of the Aravalli basin (Delwara formation) at ∼2.15Ga. The Udaipur paleosols, especially in the eastern flank, represent the most complete Archean–Paleoproterozoic paleosol sections in India.

The formation and evolution of the Bitincke nickel laterite deposit, Albania

The Bitincke nickel laterite deposit, located in the south east of Albania, contains an estimated ore resource of 35.6 Mt of nickel ore with a grade of 1.2 % Ni. The deposit developed on peridotites within Late Jurassic ophiolites which were obducted in the Early Cretaceous and which form part of the Albanian Mirdita ophiolite zone. Limestones and conglomerates overlying the deposit delimit the minimum age of lateritization to the mid-Eocene. The laterite is composed of two distinct ore zones characterized by silicate nickel and iron oxide phases. Within the silicate zone, Ni concentrations reach a maximum of 1.5 wt% and although laterally and vertically variable this zone is typically characterized by olivine and serpentine at the base of the horizon which are gradually replaced by secondary silicates and iron oxides up section. The boundary between the silicate and the oxide zone above is normally sharp and characterized by an increase in Fe2O3 (from ~10 to 80 wt%), decreases in SiO2 (from ~30 to 5 wt%) and a dramatic reduction in MgO content (from ~10 to 0.1 wt%). The oxide horizon is dominated by goethite and displays relatively little variation in textural morphology or geochemistry. Nickel concentrations are greatest near the base of this zone, reaching a maximum of 2.3 wt%. Weathering profile formation and variation in the thickness of the deposit was controlled by the interaction between topography, faulting and protolith fracture density. The oxide zone formed on topographic highs was subject to increased rates of erosion, whereas the laterite profile within topographic lows, and in areas of relatively high fracture density, tends to be thicker due to increased permeability. The most substantive sections of the Ni laterite weathering profile developed in small fault controlled basins and were preserved by the deposition of a sequence of limestones and mudstones.

Ground air of the bauxite-bearing lateritic weathering crust (Republic of Guinea)

The ground air of the bauxite deposit in West Africa was tested for its chemical composition. The obtained data provide the first insight into the air effect on the mineralogical-geochemical zoning of the lateritic weathering profile. The air was tested in areas with different geomorphologic features, which allowed us to study the effect of relief and landscape on the ground-air formation.

Opening of the Neo-Tethys Ocean and the Pangea B to Pangea A transformation during the Permian

ABSTRACTWe studied the stratigraphy, composition, and paleomagnetic properties of lateritic weathering profiles of Permian age from northern Iran and western Karakoram, Pakistan. A limited set of samples deemed representative yielded stable low-inclination paleomagnetic components carried essentially by hematite of chemical origin isolated in massive, fine-grained, and homogeneous ferricrete facies. These laterites originated at equatorial paleolatitudes characterized by intense weathering processes under warm and humid climatic conditions. Paleomagnetic estimates of paleolatitude from Iran, Karakoram, and north Tibet from this study and the literature, albeit sparse, provide testable constraints on the motion of the Cimmerian terranes as the result of the opening of the Neo-Tethys Ocean along the eastern margin of Gondwana during the Permian. We confirm and help refine previous suggestions that the Cimmerian terranes migrated from southern Gondwanan paleolatitudes in the Early Permian to subequatorial paleolatitudes by the Middle Permian – Early Triassic. As a novel conclusion, we find that timing, rates, and geometry of Cimmerian tectonics are broadly compatible with the transformation of Pangea from an Irvingian B to a Wegenerian A-type configuration with Neo-Tethyan opening taking place contemporaneously essentially in the Permian.

Ferricrete biochemical degradation on the rainforest–savannas boundary of Central African Republic

In southeastern Central African Republic the lateritic weathering mantles are capped by 2 to 5 m thick ferricretes, which previously formed under a seasonal tropical climate. The actual humid tropical climatic conditions result in the biophysical disaggregation of the ferricretes everywhere the forest develop that lead to a soil formation composed of ferricrete relicts and ferruginous nodules embedded in a soft bioturbated micro-aggregated clay-ferruginous matrix. The potential effect of the biological activity (e.g., termites) on the soil and vegetation dynamics and therefore on the evolution of the previous consolidated ferricrete is discussed. Following its mechanical disaggregation by the forest tree roots, the ferricrete underwent a chemical degradation under the combined effect of hydration and redox conditions, which result from the biodegradation and oxidation of the organic matter. The transformation of the ferricrete into a soft bioturbated micro-aggregated clay-ferruginous soil matrix implies the hematite dissolution, and the kaolinite transformation into gibbsite, which may characterize a late “bauxitization” (secondary gibbsitization of kaolinite) of previous ferricrete profiles linked to a late Quaternary environmental (climate and vegetation) change. However, the geochemical and mineralogical patterns of the matrix not only reflect the geochemistry of the parental ferricrete but also depend on the physical transfers of quartz and heavy minerals from the lower horizons of the profile. Hence, the use of geochemical indexes such as Ti and/or Zr for mass balance calculations in the lateritic weathering profiles is precluded because these supposedly inert chemical elements are not simply accumulated by in situ chemical weathering process.

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.

Morphology and composition of gold in a lateritic profile, Fazenda Pison “Garimpo”, Amazon, Brazil

This study describes the morphological evolution of gold grains in a lateritic weathering profile in an equatorial rainforest climate. Primary sources of gold are quartz veins associated with shallow granophyric intrusion. Gold grains were found in fresh ore, saprolite, transition zones, ferruginous duricrust, red latosol, and yellow latosol. Irregularly shaped grains predominate, with smaller proportions of dendritic and prismatic forms. Gold grains are weathered in the uppermost 10 m of the regolith. Mean gold grain size is maximum in the duricrust (>125 μm) and decreases progressively upward into the yellow latosol (<90 μm). Voids and corrosion pits appear on grain surfaces, and progressive rounding is observed from the bottom of the profile to the top. Gold grains can be classified as either homogeneous or zoned with respect to their chemical composition. Homogeneous grains contain 2–15% Ag (mean 8.3%). Zoned grains have more variable Ag contents; grain cores have means of approximately 10% or 23% Ag, with Ag-poor zones of approximately 3.7% Ag along internal discontinuities and/or outer rims. Formation of Ag-poor rims is due to preferential depletion of silver. Processes responsible for duricrust formation may preserve some grains as large aggregates, but subsequent transformation into latosol further modifies them.

Isotopic evidence for iron mobilization during Paleoproterozoic lateritization of the Hekpoort paleosol profile from Gaborone, Botswana

The isotopic composition and elemental abundance of Fe in a lateritic weathering profile (drillcore Strata-1) formed on the ∼ 2.2 Ga Hekpoort basalt of the Pretoria Group in Gaborone, Botswana, document open-system behavior for Fe during paleosol formation. The δ 56Fe values of the profile increase from ∼ 0‰ at the bottom (parental basalt) to + 1‰ in the mid-depth mottled (Fe-depleted) zone, then decrease to + 0.3‰ at the top of the laterite (Fe-enriched) zone. The reworked ferricrete and red beds that overlie the laterite have high δ 56Fe values of + 0.2 to + 0.4‰, which are comparable to those of the underlying laterite. Because of the absence of a complementary low- δ 56Fe zone, these results are not compatible with a model of simple internal redistribution (closed-system) of Fe within the paleosol column. Instead, these results are well explained through fluid–rock interaction involving transport of aqueous Fe 2+ through the system during paleosol evolution. Production of positive δ 56Fe values for Fe 3+ oxides were simulated through oxidation of low- δ 56Fe Fe 2+ aq using experimentally determined Fe 3+ oxide–Fe 2+ aq fractionation factors. The increased quantities of Fe 2+ involved in formation of the Hekpoort paleosol relative to previous estimates based on a closed-system model require a commensurate increase in the abundance of an oxidant, such as atmospheric O 2, to explain the Fe 3+/Fe 2+ ratios in the paleosol. Therefore, previous estimates of atmospheric O 2 levels of 2.5 × 10 − 4 to 9 × 10 − 3 atm based on this section [W. Yang, H.D. Holland, The Hekpoort paleosol profile in Strata 1 at Gaborone, Botswana: Soil formation during the great oxidation event, American Journal of Science 303(2003) 187–220] are too low by an order of magnitude or more. These results demonstrate that Fe isotopes can provide important constraints on open- versus closed-system behavior of Fe in ancient weathering profiles, a distinction that is critical when calculating Fe mobility as a means for inferring surface redox conditions in the past.

Mineralogy and geochemistry of lateritic weathering profiles on ultramafic rock bodies around Mokuro in Ile-Ife area, southwestern Nigeria

Mineralogy and geochemistry of lateritic weathering profiles on ultramafic rock bodies around Mokuro in Ile-Ife area, southwestern Nigeria

The accurate crystal chemistry of ferric smectites from the lateritic nickel ore of Murrin Murrin (Western Australia). II. Spectroscopic (IR and EXAFS) approaches

AbstractFe-rich smectites from lateritic weathering profiles have previously been studied by XRD, ICP-AES, SEM-EDX and TEM-EDX analyses (Gaudinet al., 2004). These smectites exhibit intermediate chemistries between five end-members: Al-Fe beidellites, Al-Fe montmorillonites and Mg+Ni-saponite. The spectroscopic study by FTIR and XAS of these smectites reveals that: (1) tetrahedral Fe3+is near or below the detection limit (0.05 cation for 4Si); (2) the large chemical variability is due to substitution of the three major cations (Fe, Al, Mg) within adjacent octahedra; (3) Ni is not concentrated in another clay phase such as Ni-kerolite and is located in the octahedral sheets of smectite; (4) octahedral cations are not randomly distributed but ordered in separated Fe, Al, Mg, Ni clusters; (5) the Mg-Ni saponite end-member actually appears as small trioctahedral clusters of Mg and Ni distributed within the dioctahedral smectite.