Nordic Center for Earth Evolution

Copenhagen, Denmark

Nordic Center for Earth Evolution

Copenhagen, Denmark
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Polat A.,University of Windsor | Frei R.,Copenhagen University | Frei R.,Nordic Center for Earth Evolution | Schersten A.,Lund University | Appel P.W.U.,Geological Survey of Denmark
Chemical Geology | Year: 2010

The Archean Fiskenæsset Complex, SW Greenland, consists of an association of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, and ultramafic rocks (peridotite, pyroxenite, dunite, hornblendite). The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. The trace element systematics of the Fiskenæsset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting.The Fiskenæsset anorthosites, leucogabbros, gabbros and ultramafic rocks collectively yield an Sm-Nd errorchron age of 2973±28Ma (MSWD=33), with an average initial εNd=+3.3±0.7, consistent with a long-term depleted mantle source. Regression of Pb isotope data define an age of 2945±36Ma (MSWD=44); and the regression line intersects the average growth curve at 3036Ma. Slightly lower Pb-Pb errorchron age is interpreted as reflecting partial disturbance of the U-Pb system in gabbros, leucogabbros and ultramafic rocks during intrusion of TTGs.Complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca. 3200 and 2650. Ma. Zircon ages peak at about 3200, 3100, 3000, 2950, 2820, and 2750. Ma. The 3200-3000. Ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks. 2950. Ma is considered as an approximate intrusion age of sampled TTGs. The 2940-2650. Ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the Fiskenæsset region.On the basis of recently published trace element data, and new Nd and Pb isotope and U-Pb zircon age data, a three-stage geodynamic model is proposed to explain the evolution of the Fiskenæsset Complex. Stage 1 represents the formation of depleted shallow mantle source>3000Ma (εNd=+3.3±0.7) for the complex. Stage 2 corresponds to the development of an intra-oceanic island arc between 3000-2950Ma. Stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950-2940Ma. © 2010 Elsevier B.V.


Naeraa T.,Lund University | Naeraa T.,Geological Survey of Denmark | Kemp A.I.S.,University of Western Australia | Schersten A.,Lund University | And 3 more authors.
Lithos | Year: 2014

The late Neoarchaean QÔrqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The QÔrqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547±4Ma (MSWD=0.63). Initial εHf values range from -12 to -18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon δ18O values range from 6.1±0.2‰ to 6.5±0.3/0.7‰ best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a 176Lu/176Hf around 0.015-0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to >ca. 13-18kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750-850°C. The obtained P-T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting. © 2014 Elsevier B.V.


Polat A.,University of Windsor | Fryer B.J.,University of Windsor | Samson I.M.,University of Windsor | Weisener C.,University of Windsor | And 4 more authors.
Precambrian Research | Year: 2012

The Fiskenæsset Complex, SW Greenland, contains the world's best preserved Archean (~2970 Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new whole-rock major and trace element, petrographic, and SEM (Scanning Electron Microscope)-BSE (backscatter electron) image data for a 45-60 m-thick ultramafic sill and cross-cutting hornblendite veins. The ultramafic sill is composed of ~75% olivine-pyroxene hornblendite, ~15% pyroxene hornblendite, and ~10% hornblende pyroxenite. Despite granulite facies metamorphism and multiple phases of deformation, the Fiskenæsset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and Alaskan-type ultramafic complexes. Hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism. Formation of orthopyroxene-magnetite symplectitic (vermicular) intergrowths, mainly at the expense of olivine, is attributed to chemical reactions between late stage, residual hydrous melts and olivine.Field, geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion. High MgO (16-31. wt.%) contents suggest that the sill was emplaced as a crystal mush, rather than as melt only. Petrographic observations and BSE images indicate that olivine, orthopyroxene, and clinopyroxene were the major crystal phases, whereas hornblende represents the main interstitial liquid phase in the crystal mush. The sill exhibits three different REE patterns, corresponding to three cryptic layers. Although the origin of these REE patterns is not fully understood, they may reflect the intrusion of three batches of magma, containing different hornblende-forming melt/crystal ratios. Geochemical data indicate that the cross-cutting hornblendite veins likely originated from evolved, late-stage melts but were not related to the melts of the ultramafic sill.Large negative Nb anomalies (Nb/Nb* = 0.04-0.66) suggest that the Fiskenæsset hornblendites, hornblende pyroxenites, and hornblendite veins were derived from a hydrous sub-arc mantle peridotite. Phanerozoic hornblendites are typically associated with supra-subduction zone ophiolites and magmatic arcs. Accordingly, it is suggested that water was recycled to the source of the Fiskenæsset hornblendites through subduction of altered oceanic crust. Recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites, but also played an important role in the formation of TTG-dominated Archean continental crust. Therefore, the field, petrographic and geochemical data presented in this study suggest that the origin of the Mesoarchean to Neoarchean terrane in the Fiskenæsset region, SW Greenland, is consistent with subduction zone geodynamic processes, rather than non-uniformitarian, density-driven, vertical, crustal overturn, and diapiric processes. © 2011 Elsevier B.V.


Frei R.,Copenhagen University | Frei R.,Nordic Center for Earth Evolution | Polat A.,University of Windsor
Precambrian Research | Year: 2013

This study reports detailed stable Cr, Sm-Nd and Rb-Sr isotope data for a ca. 1.9Ga old subaerial weathering profile at Schreiber Beach, Ontario, Canada, from which detailed major and trace element signatures and δ18O values were previously reported. The weathering profile developed on Neoarchean (∼2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (∼1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation.The aim was to investigate the behaviour of chromium during ancient weathering processes and to contribute to the question whether or not stable isotope fractionation accompanied the release of chromium from the weathering rocks at times when atmospheric oxygen was supposedly high enough (in a period following the Great Oxygenation Event (GOE) ∼2.45. Ga ago and prior to the deposition of the Gunflint cherts at ∼1.82. Ga). These iron-rich cherts belong to one of the last regionally extensive banded iron formations (BIFs) deposited before the ∼1 billion year-long Mesoproterozoic period during which a drastic change in ocean chemistry prevented deposition of BIFs.The gradual textural, mineralogical, and geochemical changes from unweathered basalts to strongly weathered hematite-bearing basalts with increasing stratigraphic height are associated with shifts in the chromium isotope compositions. The δ53Cr value of unweathered pillow basalt cores (-0.19±0.02‰, 2σ) is within the range of mantle inventory values, whereas weathered brown to green basalts (soils), exhibiting up to 30% lower Cr concentrations compared to unweathered pillow cores, are isotopically lighter (δ53Cr=-0.35±0.11‰). In contrast, red, hematite-rich basalts and hyaloclastites underlying the brown to green basaltic soils, with the highest δ18O enrichment in the profile, are isotopically heavier (δ53Cr=+0.05±0.15‰). Rb-Sr isotope data of weathered basalts define a correlation line with a slope corresponding to an age of 1574±24Ma (MSWD=13) which we interpret to reflect a diagenetic event accompanied by alkaline metasomatism during subsidence/burial of the Gunflint basin. The non-correlation of K enrichment factors with δ53Cr, δ18O and other major and trace elements in the weathered rocks indicates that alkaline metasomatism did not affect the geochemical signatures produced during in situ subaerial weathering. Instead, correlations between δ53Cr and δ18O, and between δ53Cr, Ce/Ce*, U and V, indicate coupled mobilization of the redox sensitive elements during oxidative processes and their partial re-deposition at depth from high δ18O freshwaters. Partial re-precipitation of heavy Cr at depth can be explained on the basis of the weak positive correlation between δ53Cr and Fe enrichment factors; it is seen as a consequence of the reduction of mobile Cr(VI) and co-precipitation as mixed Fe(III)-Cr(III) oxhydroxides during contemporaneous oxidation of the Fe2+-bearing groundwaters at depth, a process similar to the one governing the deposition of most Precambrian BIFs. Redistribution of certain elements during the weathering process, in this case REEs, is furthermore indicated by a correlation line in a Sm-Nd isochron diagram defined by the Schreiber Beach data with a slope corresponding to an age of 1.93±0.19Ga (MSWD=26), an age which is compatible with the direct geological constraints for the timing of the paleosol formation at this locality.An oxidative atmosphere at ∼1.9Ga, as implied by the results from the Schreiber profile, is furthermore supported by positively fractioned Cr isotopes (δ53Cr from +0.06 to +0.39‰) recorded in the iron-rich Gunflint Cherts directly above the palaeo-weathered horizons at Schreiber Beach. These values are interpreted to stem from a positively fractionated shallow seawater chromium composition at ∼1.88Ga, potentially reflecting a continental run-off characterized by positive δ53Cr, and are in accordance with Cr isotope signatures in worldwide BIFs (including the Gunflint Iron Formation).Our results show the potential of Cr isotope studies on ancient paleosols to untangle the presence of oxidative weathering processes, making this isotope system a viable and important tracer for the reconstruction of surface oxygenation in Earth's history. © 2012 Elsevier B.V.


Berger A.,University of Bern | Janots E.,ISTerre | Gnos E.,Natural History Museum Geneva | Frei R.,Copenhagen University | And 2 more authors.
Applied Geochemistry | Year: 2014

In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE+Y concentrations are typical of granitoids (299-363ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9wt% SO3 and 7wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite-jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE+Y reach 2194ppm with high Ce concentrations (1638ppm; 9*Cebedrock) compared to other REE (3-4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing. © 2013 Elsevier Ltd.


Hoffmann J.E.,University of Bonn | Hoffmann J.E.,University of Cologne | Munker C.,University of Cologne | Naeraa T.,Geological Survey of Denmark | And 5 more authors.
Geochimica et Cosmochimica Acta | Year: 2011

It has been proposed that Archean tonalitic-trondhjemitic-granodioritic magmas (TTGs) formed by melting of mafic crust at high pressures. The residual mineralogy of the TTGs (either (garnet)-amphibolite or rutile-bearing eclogite) is believed to control the trace element budget of TTGs. In particular, ratios of high-field-strength elements (HFSE) can help to discriminate between the different residual lithologies. In order to place constraints on the source mineralogy of TTGs, we performed high-precision HFSE measurements by isotope dilution (Nb, Ta, Zr, Hf) together with Lu-Hf and Sm-Nd measurements on representative, ca. 3.85-2.8. Ga TTGs and related rock types from southern West Greenland, W-India and from the Superior Province. These measurements are complemented by major and trace element data for the TTGs. Texturally homogeneous early Archean (3.85-3.60. Ga old) and Mesoarchean (ca. 3.1-2.8. Ga old) TTGs have both low Ni (<11. ppm) and Cr contents (<20. ppm), indicating that there was little or no interaction with mantle peridotite during ascent. Ratios of Nb/Ta in juvenile Eoarchean TTGs range from ca. 7 to ca. 24, and in juvenile Mesoarchean TTGs from ca. 14 to ca. 27. Even higher Nb/Ta (14-42) were obtained for migmatitic TTGs and intra-crustal differentiates, most likely mirroring further fractionation of Nb from Ta as a consequence of partial melting, fluid infiltration and migmatisation. In the juvenile TTGs, positive correlations between Nb/Ta and Gd/Yb, La/Yb, Sr/Y, Zr/Sm and Zr/Nb are observed. These compositional arrays are best explained by melting of typical Isua tholeiites in both, the rutile-bearing eclogite stability field (>15. kbar, high Nb/Ta) and the garnet-amphibolite stability field (10-15. kbar, low Nb/Ta). With respect to the low end of Nb/Ta found for TTGs, there is currently some uncertainty between the available experimental datasets for amphibole. Independent of these uncertainties, the TTG compositions found here still require the presence of both endmember residues. A successful geological model for the TTGs therefore has to account for the co-occurrence of both low- and high-Nb/Ta TTGs within the same geologic terrane. An additional feature observed in the Eoarchean samples from Greenland is a systematic co-variation between Nb/Ta and initial εHf(t), which is best explained by a model where TTG-melting occured at progressively increasing pressures in a pile of tectonically thickened mafic crust. The elevated Nb/Ta in migmatitic TTGs and intra-crustal differentiates can shed further light on the role of intra-crustal differentiation processes in the global Nb/Ta cycle. Lower crustal melting processes at granulite facies conditions may generate high-Nb/Ta domains in the middle crust, whereas mid-crustal melting at amphibolite facies conditions may account for the low Nb/Ta generally observed in upper crustal rocks. © 2011 Elsevier Ltd.


Hoffmann J.E.,University of Cologne | Hoffmann J.E.,University of Bonn | Hoffmann J.E.,Free University of Berlin | Nagel T.J.,University of Bonn | And 5 more authors.
Earth and Planetary Science Letters | Year: 2014

We present new major and trace element, high-precision high-field-strength-element, hafnium and neodymium isotope data for well preserved Eoarchean TTGs within the Itsaq Gneiss Complex (IGC) of southern West Greenland. These data are combined with thermodynamic model predictions of partial melting and fractional crystallization to gain new insights into continental crust formation in the Archean. Our results show that the observed compositional range of Eoarchean TTGs can be explained by a combination two processes: (1) 5-25% partial melting of amphibolite within thickened mafic crust and (2) subsequent fractional crystallization processes. The Eoarchean TTG suite of SW Greenland probably formed through mixing of melt batches that originally formed at different source depths between 10 and 14 kbar and ponded as plutons at mid-crustal levels. The trace element compositions of some TTGs point to subsequent fractional crystallization processes involving plagioclase, clinopyroxene, amphibole and garnet. Our model is consistent with recent studies proposing that the Eoarchean Itsaq Gneiss Complex TTGs from the IGC formed by re-working of mafic protocrust that stabilized as accreted juvenile crustal terranes in the Eoarchean. The model is also in good agreement with field observations from the area. © 2013 Elsevier B.V.


News Article | January 20, 2016
Site: phys.org

Photosynthesis is one of the most fundamental biological processes on Earth. Normally photosynthesis is performed by plants, but a few bacterial phyla also have the talent. To date, species capable of performing photosynthesis have been reported in six bacterial phyla, and recently researchers have reported that the talent can be observed in a species belonging to the rare and understudied phylum Gemmatimonadetes. The investigated species was isolated from a freshwater lake in the Gobi Desert. When the researchers studied the bacteria in the lab, they were surprised to discover that the genes responsible for the photosynthesis are nicely ordered in a cluster rather than scattered in the whole genome. This makes it possible to remove the gene cluster. "This is highly interesting because it allows you to transfer the gene cluster to another bacterium that can use the genes for a desired purpose. An example is to transfer the gene cluster to the bacterium E. coli and thus make E. coli capable of using sunlight to produce biofuel", explains postdoc Yonghui Zeng, Nordic Center for Earth Evolution, University of Southern Denmark, who has lead the investigations. The newly found bacterium gives a perfect example as how to turn a bacterium photosynthetic. After Zeng and colleagues described the bacterium in Proceedings of the National Academy of Sciences last year, he set out to learn more about it. A new paper in Environmental Microbiology Reports now describes the abundancy of the bacterium. With the help of computational biologist Jan Baumbach's group in Dept. of Mathematics & Computer Science, University of Southern Denmark, Zeng trawled a large number of databases and learned that the bacterium can be found almost everywhere, especially in soil and in water treatment plants. It cannot, however, be found in marine environments. "Now we know that this bacterium has a talent for photosynthesis, we know it is abundant and we know that the relevant genes can easily be transferred to other organisms. That makes it very relevant for future work with focus on, i.e., figuring out a way to turn the biotechnology workhorse E. coli photosynthetic and thus capable of producing biofuels in a more economic way", says Zeng. Explore further: Researchers engineer bacterium to hunt down and kill pathogens More information: Yonghui Zeng et al. Metagenomic evidence for the presence of phototrophic Gemmatimonadetes bacteria in diverse environments, Environmental Microbiology Reports (2015). DOI: 10.1111/1758-2229.12363


Haugaard R.,Copenhagen University | Haugaard R.,Nordic Center for Earth Evolution | Haugaard R.,University of Alberta | Frei R.,Copenhagen University | And 3 more authors.
Precambrian Research | Year: 2013

Here we present new field, petrographic and geochemical data from the ~2.9 Ga Itilliarsuk banded iron formation (BIF) and associated lithologies within the Itilliarsuk supracrustal belt, south-eastern Nussuuaq, West Greenland. The supracrustals represent a volcanic-sedimentary sequence, which rests unconformably on a basement of tonalite-trondhjemite-granodiorite (TTG) lithologies. Felsic metagreywackes, meta-semipelites and thinly bedded ferruginous shales were identified intercalated with the Itilliarsuk BIF. Other associated rocks include metapelites, acidic metavolcanics and metagabbroic sills. The supracrustals have experienced amphibolite-facies metamorphism, which has resulted in complete resetting of the U-Pb system with an apparent age of 1895±48 Ma. This tectono-metamorphic event corresponds well with the Paleoproterozoic Rinkian orogeny known from this region. The Itilliarsuk-(oxide-facies) BIF has been divided into two segments on the basis of major and trace elements chemistry: a shaley-BIF with a strong clastic component and a more chemically pure BIF. The shaley-BIF contains high terrigenous influx as reflected by elevated Al2O3 (up to 12 wt.%), TiO2, high field strength elements (HFSE) and transition metals. The chemically pure BIF is characterised by alternating high iron (~68 wt.%) and high silica (~64 wt.%) bands with low total rare earths and yttrium (REY), Al2O3, TiO2 and HFSE contents, suggesting a low detrital component. The least altered bands of the BIF record diagnostic Archaean seawater features with Post-Archaean Average Shale (PAAS)-normalised positive Laand Eu-anomalies, enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE) [(Pr/Yb)PAAS < 1], and absence of Ce-anomalies which suggests deposition from an anoxic water column. Sm-Nd isotopes of the chemically pure silica-rich bands record TDM model ages of 3.23-2.85 Ga and initial εNd (εNd(i)) values in the range of +0.43 to +4.05, average of +1.35. In contrast, the chemically pure Fe-rich bands yield TDM model ages of 3.61-3.22 Ga and εNd(i) values from -2.87 to +0.09, average of -1.29. The associated supracrustal rocks in the study area have significantly higher, positive εNd(i) values. The 143Nd/144Nd in the Itilliarsuk BIF, therefore, contrasts world BIFs by exhibiting radiogenic, positive εNd(i) values in shallow seawaters where the REY's were controlled by a local, depleted continental crust, whereas the negative εNd(i) values found in the iron-rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle, possibly an older subcontinental lithospheric segment. The felsic metagreywackes are immature, first-cycle (SiO2/Al2O3 ~4.4, [La/Yb » 1]CHON) metasediments with affinities to TTG-suites, primarily extrusives, whereas the meta-semipelites and metapelites contain a larger mafic contribution with higher content of Fe2O3, MgO, Cr, Ni and HREEs. This suggests that the BIF was deposited in a highly unstable basin, presumably in a palaeo-continental slope or outer shelf environment, with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources. The TDM model ages and the use of Th-Sc-Zr and La-Th-Sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting. © 2012 Elsevier B.V.


Berger A.,Copenhagen University | Frei R.,Copenhagen University | Frei R.,Nordic Center for Earth Evolution
Geoderma | Year: 2014

We performed a mineral, geochemical and Cr-Sr-Pb isotope study on a laterite profile developed on ca. 540Ma old tonalitic bedrock in Madagascar with special emphasis on the behavior of chromium during tropical weathering. The observed strong depletions of Ca, Si, and P, and enrichment of Fe and Al, in the soil, relative to bedrock and underlying saprolite, are the characteristic features pertinent to laterites. The enrichment of Fe in topsoil horizon can be correlated with enrichment of P, and the redox sensitive elements Mn and Cr, and indicates redistribution of these elements related to oxidation-reduction processes. The slight scatter of 87Sr/86Sr, and 206,207,208Pb/204Pb values in the profile is one consequence of such redistribution processes. Our results are compatible with a two stage process during alteration: (1) an incipient alteration stage characterized mainly by a pervasive, effective and complete loss of sulfides accompanied by patchy alteration constrained along fissures and cracks, and (2) a main stage of soil formation (actual laterization) with loss of Na, K, Ca, and Si, and accumulation of Fe and Mn relative to the unaltered bedrock. This two stage evolution is also depicted by Cr concentrations and δ53Cr values along the studied profile, where the first incipient alteration stage caused a pronounced depletion of Cr particularly hosted by sulfides, and where the second alteration (laterization) led to redistribution and a small increase in Cr concentration in the uppermost portions of the soil profile relative to stage one altered saprolite. This gain in Cr is accompanied by decreasing δ53Cr values and can be explained by partial immobilization (possibly by adsorption/coprecipitation on/with Fe-oxy-hydroxides) of mobile Cr(III) during upward transport in the weathering profile. The negatively fractionated δ53Cr values measured in the weathering profile relative to the unaltered tonalitic bedrock characterized by a high temperature magmatic inventory Cr isotope signature are consistent with loss of a positively fractionated Cr(VI) pool formed during weathering. The predicted existence of a former, positively fractionated and mobile chromium pool has been experimentally constrained in circumneutral and basic leachates of powdered tonalite bedrock where δ53Cr of +0.21 to +0.48‰ was measured. Our results show that mobilization of chromium is effective under highly oxidative conditions, which in well drained sulfide-bearing parent bedrocks potentially lead to both, acid dissolution of sulfide-hosted Cr and redox-promoted mobilization of Cr(VI) from silicates during later stages of weathering under basic pH conditions. © 2013 Elsevier B.V.

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