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1.
The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ56Fe values for magnetite record a strong inheritance from Fe3+ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe3+ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ56Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ56Fe values and was produced by DIR. Most siderite has slightly negative δ56Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ56Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ56Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.  相似文献   

2.
Banded iron formations (BIFs) are Precambrian chemical marine sedimentary formations that record major transitions of chemical composition, and oxidation–reduction state of oceans at the time of their deposition. In this paper, we report silicon and oxygen isotope compositions of a variety of BIFs from the North China Craton (NCC) in order to deduce the mechanism of their formation. Quartz in the various types of BIFs from the NCC are generally depleted in 30Si, where δ30SiNBS-28 values range from − 2.0‰ to − 0.3‰ (average, − 0.8‰), similar to δ30SiNBS-28 values measured from modern submarine black chimneys and sinters. The δ18OV-SMOW values of quartz in the BIFs are relatively high (8.1‰–21.5‰; average, 13.1‰), similar to those of siliceous rock formed by hydrothermal activities. The δ30SiNBS-28 values of quartz in magnetite bands are commonly lower than those of quartz in adjacent siliceous bands within the same sample, whereas δ18OV-SMOW values are higher in the magnetite bands. A negative correlation is observed between δ30SiNBS-28 and δ18OV-SMOW values of quartz from siliceous and magnetite bands in BIF from Fuping, Hebei Province. The isotopic compositions of silicon and oxygen of quartz in BIFs provide insights for the formation mechanisms of silicon–iron cyclothems in BIFs. After the silicon- and iron-rich hydrothermal solution was injected onto the seabed, the abrupt temperature drop caused oversaturation of silicic acid, resulting in rapid precipitation of SiO2 and deposition of siliceous layers. Ferric hydroxide was precipitated later than SiO2 because of low free-oxygen concentration in the ocean bottom. Progressive mixing of hydrothermal solution with seawater caused a continuous drop in temperature and an increase in Eh values, resulting in gradual oxidation of hydrothermal Fe2 + and deposition of iron-rich layers. In summary, each silicon–iron cyclothem marks a large-scale submarine hydrothermal exhalation. The periodic nature of these exhalations resulted in the formation of regular silicon–iron cyclothems. The widespread distribution of BIFs indicates that volcanism and submarine hydrothermal exhalation were extensive; the low δ30SiNBS-28 and high δ18O V-SMOW values of the BIFs indicate that the temperature of seawater was relatively high at the time of BIF formation, and that concentrations of Fe2 + and H4SiO4 in seawater were saturated.  相似文献   

3.
Estimates of early atmosphere compositions from metamorphosed banded iron formations(BIFs)including the well-studied ≥3.7 BIFs of the Isua supracrustal belt(Greenland)are dependent on knowledge of primary versus secondary Fe-mineralogical assemblages.Using new observations from locally well preserved domains,we interpret that a previously assumed primary redox indicator mineral,magnetite,is secondary after sedimentary Fe-clays(probably greenalite)±carbonates.Within ~3.7 Ga Isua BIF,pre-tectonic nodules of quartz+Fe-rich amphibole±calcite reside in a finegrained(≤100 μm)quartz+magnetite matrix.We interpret the Isua nodule amphibole as the metamorphosed equivalent of primary Fe-rich clays,armoured from diagenetic oxidative reactions by early silica concretion.Additionally,in another low strain lacunae,~3.76 Ga BIF layering is not solid magnetite but instead fine-grained magnetite+quartz aggregates.These magnetite+quartz aggregates are interpreted as the metamorphosed equivalent of Fe-clay-rich layers that were oxidised during diagenesis,because they were not armoured by early silicification.In almost all Isua BIF exposures,this evidence has been destroyed by strong ductile deformation.The Fe-clays likely formed by abiotic reactions between aqueous Fe~(2+)and silica.These clays along with silica±carbonate were deposited below an oceanic Fe-chemocline as the sedimentary precursors of BIF.Breakdown of the clays on the sea floor may have been by anaerobic oxidation of Fe~(2+),a mechanism compatible with iron isotopic data previously published on these rocks.The new determinations of the primary redoxsensitive Fe-mineralogy of BIF significantly revise estimates of early Earth atmospheric oxygen and CO_2 content,with formation of protolith Fe-rich clays and carbonates compatible with an anoxic Eoarchean atmosphere with much higher CO_2 levels than previously estimated for Isua and in the present-day atmosphere.  相似文献   

4.
Silicon (Si) isotope variability in Precambrian chert deposits is significant, but proposed explanations for the observed heterogeneity are incomplete in terms of silica provenance and fractionation mechanisms involved. To address these issues we investigated Si isotope systematics, in conjunction with geochemical and mineralogical data, in three well-characterised and approximately contemporaneous, ∼3.5 Ga chert units from the Pilbara greenstone terrane (Western Australia).We show that Si isotope variation in these cherts is large (−2.4‰ to +1.3‰) and was induced by near-surface processes that were controlled by ambient conditions. Cherts that formed by chemical precipitation of silica show the largest spread in δ30Si (−2.4‰ to +0.6‰) and are characterised by positive Eu, La and Y anomalies and overall depletions in lithophile trace elements. Silicon isotope systematics in these orthochemical deposits are explained by (1) mixing between hydrothermal fluids and seawater, and/or (2) fractionation of hydrothermal fluids by subsurface losses of silica due to conductive cooling. Rayleigh-type fractionation of hydrothermal fluids was largely controlled by temperature differences between these fluids and seawater. Lamina-scale Si isotope heterogeneity within individual chemical chert samples up to 2.2‰ is considered to reflect the dynamic nature of hydrothermal activity. Silicified volcanogenic sediments lack diagnostic REE+Y anomalies, are enriched in lithophile elements, and exhibit a much more restricted range of positive δ30Si (+0.1‰ to +1.1‰), which points to seawater as the dominant source of silica.The proposed model for Si isotope variability in the Early Archaean implies that chemical cherts with the most negative δ30Si formed from pristine hydrothermal fluids, whereas silicified or chemical sediments with positive δ30Si are closest to pure seawater deposits. Taking the most positive value found in this study (+1.3‰), and assuming that the Si isotope composition of seawater is governed by input of fractionated hydrothermal fluids, we infer that the temperature of ∼3.5 Ga seawater was below ∼55 °C.  相似文献   

5.
We present detailed petrographic surveys of apatite grains in association with carbonaceous material (CM) in two banded iron formations (BIFs) from the Paleoproterozoic of Uruguay and Michigan for comparison with similar mineral associations in the highly debated Akilia Quartz-pyroxene (Qp) rock. Petrographic and Raman spectroscopic surveys of these Paleoproterozoic BIFs show that apatite grains typically occur in bands parallel to bedding and are more often associated with CM when concentrations of organic matter are high. Carbonaceous material in the Vichadero BIF from Uruguay is generally well-crystallized graphite and occurs in concentrations around 0.01 wt% with an average δ13Cgra value of −28.6 ± 4.4‰ (1σ). In this BIF, only about 5% of apatite grains are associated with graphite. In comparison, CM in the Bijiki BIF from Michigan is also graphitic, but occurs in concentrations around 2.4 wt% with δ13Cgra values around −24.0 ± 0.3‰ (1σ). In the Bijiki BIF, more than 78% of apatite grains are associated with CM. Given the geologic context and high levels of CM in the Bijiki BIF, the significantly higher proportion of apatite grains associated with CM in this rock is interpreted to represent diagenetically altered biomass and shows that such diagenetic mineral associations can survive metamorphism up to the amphibolite facies.Isotope compositions of CM in muffled acidified whole-rock powders from the Akilia Qp rock have average δ13Cgra values of −17.5 ± 2.5‰ (1σ), while δ13Ccarb values in whole-rock powders average −4.0 ± 1.0‰ (1σ). Carbon isotope compositions of graphite associated with apatite and other minerals in the Akilia Qp rock were also measured with the NanoSIMS to have similar ranges of δ13Cgra values averaging −13.8 ± 5.6‰ (1σ). The NanoSIMS was also used to semi-quantitatively map the distributions of H, N, O, P, and S in graphite from the Akilia Qp rock, and relative abundances were found to be similar for graphite associated with apatite or with hornblende, calcite, and sulfides. These analyses revealed generally lower abundances of trace elements in the Akilia graphite compared to graphite associated with apatite from Paleoproterozoic BIFs.Graphite associated with hornblende, calcite, and sulfides in the Akilia Qp rock was fluid-deposited at high-temperature from carbon-bearing fluids, and since this graphite has similar ranges of δ13Cgra values and of trace elements compared to graphite associated with apatite, we conclude that the Akilia graphite in different mineral associations formed from the same source(s) of CM. Collectively our results do not exclude a biogenic origin of the carbon in the Akilia graphite, but because some observations can not exclude graphitization of abiogenic carbon from CO2- and CH4-bearing mantle fluids, there remain ambiguities with respect to the exact origin of carbon in this ancient metasedimentary rock. Accordingly, there may have been several generations of graphite formation along with possibly varying mixtures of CO2- and CH4-bearing fluids that may have resulted in large ranges of δ13Cgra values. The possibility of fluid-deposited graphite associated with apatite should be a focus of future investigations as this may prove to be an alternative pathway of graphitization from phosphate-bearing fluids. Correlated micro-analytical approaches tested on terrestrial rocks in this work provide insights into the origin of carbon in ancient graphite and will pave the way for the search for life on other ancient planetary surfaces.  相似文献   

6.
To investigate the genesis of BIFs, we have determined the Fe and Si isotope composition of coexisting mineral phases in samples from the ∼2.5 billion year old Kuruman Iron Formation (Transvaal Supergroup, South Africa) and Dales Gorges Member of the Brockman Iron Formation (Hamersley Group, Australia) by UV femtosecond laser ablation coupled to a MC-ICP-MS. Chert yields a total range of δ30Si between −1.3‰ and −0.8‰, but the Si isotope compositions are uniform in each core section examined. This uniformity suggests that Si precipitated from well-mixed seawater far removed from its sources such as hydrothermal vents or continental drainage. The Fe isotope composition of Fe-bearing mineral phases is much more heterogeneous compared to Si with δ56Fe values of −2.2‰ to 0‰. This heterogeneity is likely due to variable degrees of partial Fe(II) oxidation in surface waters, precipitation of different mineral phases and post-depositional Fe redistribution. Magnetite exhibits negative δ56Fe values, which can be attributed to a variety of diagenetic pathways: the light Fe isotope composition was inherited from the Fe(III) precursor, heavy Fe(II) was lost by abiotic reduction of the Fe(III) precursor or light Fe(II) was gained from external fluids. Micrometer-scale heterogeneities of δ56Fe in Fe oxides are attributed to variable degrees of Fe(II) oxidation or to isotope exchange upon Fe(II) adsorption within the water column and to Fe redistribution during diagenesis. Diagenetic Fe(III) reduction caused by oxidation of organic matter and Fe redistribution is supported by the C isotope composition of a carbonate-rich sample containing primary siderite. These carbonates yield δ13C values of ∼−10‰, which hints at a mixed carbon source in the seawater of both organic and inorganic carbon. The ancient seawater composition is estimated to have a minimum range in δ56Fe of −0.8‰ to 0‰, assuming that hematite and siderite have preserved their primary Fe isotope signature. The long-term near-zero Fe isotope composition of the Hamersley and Transvaal BIFs is in balance with the assumed composition of the Fe sources. The negative Fe isotope composition of the investigated BIF samples, however, indicates either a perturbation of the steady state, or they have to be balanced spatially by deposition of isotopically heavy Fe. In the case of Si, the negative Si isotope signature of these BIFs stands in marked contrast to the assumed source composition. The deviation from potential source composition requires a complementary sink of isotopically heavy Si in order to maintain steady state in the basin. Perturbing the steady state by extraordinary hydrothermal activity or continental weathering in contrast would have led to precipitation of light Si isotopes from seawater. Combining an explanation for both elements, a likely scenario is a steady state ocean basin with two sinks. When all published Fe isotope records including BIFs, microbial carbonates, shales and sedimentary pyrites, are considered, a complementary sink for heavy Fe isotopes must have existed in Precambrian ocean basins. This Fe sink could have been pelagic sediments, which however are not preserved. For Si, such a complementary sink for heavy Si isotopes might have been provided by other chert deposits within the basin.  相似文献   

7.
The iron isotope, trace and major element compositions of Eoarchean supracrustal rocks from southern West Greenland (Isua Supracrustal Belt, the islands of Akilia and Innersuartuut) were analyzed in order to identify protoliths and characterize the imprints of metamorphism and metasomatism. Banded iron formations (BIFs) from the Isua Supracrustal Belt (ISB) have trace element characteristics that are consistent with seawater derivation, including high Y/Ho ratios, positive Eu/Eu∗ anomalies, positive La/La∗ anomalies, and concave upward REE patterns. These rocks also have heavy Fe isotopic compositions relative to surrounding igneous rocks (∼+0.4‰/amu). The most likely interpretation is that this signature was inherited from partial oxidation in a marine setting of Fe emanating from a source similar to modern mid-ocean ridge hydrothermal vents (∼−0.15‰/amu).Banded quartz-rich rocks from the island of Akilia with high Fe/Ti ratios share many similarities with bona fide BIFs from Isua (heavy Fe isotopic compositions up to +0.4‰/amu, elevated Y/Ho ratios compared to igneous rocks, sometimes positive Eu/Eu∗ anomalies) suggesting a chemical sedimentary origin.Iron-poor metacarbonates from the southwestern part of the ISB have light Fe isotopic compositions (∼−0.4‰/amu). This is consistent with derivation of these rocks by fluid flow through surrounding ultramafic rocks and deposition as metasomatic carbonates. Iron-rich metacarbonates from the northwest and northeast parts of the ISB have Fe isotopic compositions (from +0.1 to +0.4‰/amu) and trace element patterns (high Y/Ho ratios, positive Eu/Eu∗ and La/La∗ anomalies, and concave upward REE) similar to associated BIFs. The most likely interpretation is that these iron-rich metacarbonates were derived from mobilization of Fe in BIFs by metasomatic fluids.  相似文献   

8.
In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ13C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from -18 to +2‰ and from -7 to -3‰, respectively. The maximum δ13C value of graphite globules in the metasediment rises from -14 to -5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ13C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ13C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively 13C-depleted graphite indicates that the regional metamorphism increased the δ13C values of the Isua graphite. This is consistent with the regional trend of 13C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused 13C-enrichment of Isua graphite. The δ13C values of graphite reported here (δ13C > -18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ13C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13C-enrichment of Isua graphite, previously reported large 13C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga).  相似文献   

9.
The oxygen and carbon isotopic compositions of minerals from banded iron formations (BIFs) and high-grade ore in the region of the Kursk Magnetic Anomaly (KMA) were determined in order to estimate the temperature of regional metamorphism and the nature of rock-and ore-forming solutions. Magnetite and hematite of primary sedimentary or diagenetic origin have δ18O within the range from +2 to 6‰. During metamorphism, primary iron oxides, silicates, and carbonates were involved in thermal dissociation and other reactions to form magnetite with δ18O = +6 to +11‰. As follows from a low δ18Oav = ?3.5‰ of mushketovite (magnetite pseudomorphs after hematite) in high-grade ore, this mineral was formed as a product of hematite reduction by organic matter. The comparison of δ18O of iron oxides, siderite, and quartz from BIFs formed at different stages of the evolution of the Kursk protogeosyncline revealed specific sedimentation (diagenesis) conditions and metamorphism of the BIFs belonging to the Kursk and Oskol groups. BIF of the Oskol Group is distinguished by a high δ18O of magnetite compared to other Proterozoic BIFs. Martite ore differs from host BIF by a low δ18O = ?0.2 to ?5.9‰. This implies that oxygen from infiltration water was incorporated into the magnetite lattice during the martite formation. Surface water penetrated to a significant depth through tectonic faults and fractures.  相似文献   

10.
Pillow basalt and chert form integral lithologies comprising many Archean greenstone belt packages. To investigate details of these lithologies in the >3.7 Ga Isua Greenstone Belt, SW Greenland, we measured silicon isotope compositions of quartz crystals, by secondary ion mass spectrometry, from a quartz‐cemented, quartz‐amygdaloidal basaltic pillow breccia, recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions. The recrystallized chert, chert clasts and quartz cement have overlapping δ30Si values, while the δ30Si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age, despite amphibolite facies metamorphism. We suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting, consistent with the pillow breccia origin. On the basis of the present data, we conclude that the geological context of each sample must be carefully evaluated when interpreting δ30Si values of quartz.  相似文献   

11.
In soils, silicon released by mineral weathering can be retrieved from soil solution through clay formation, Si adsorption onto secondary oxides and plant uptake, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers. Here we use these proxies to study the contribution of biogenic Si (BSi) in a soil-plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping. δ30Si and Ge/Si ratios were determined in bulk soils (<2 mm), sand (50-2000 μm), silt (2-50 μm), amorphous Si (ASi, 2-50 μm) and clay (<2 μm) fractions: δ30Si by MC-ICP-MS Nu Plasma in medium resolution, operating in dry plasma with Mg doping (δ30Si vs. NBS28 ± 0.12‰ ± 2σSD), Ge/Si computed after determination of Ge and Si concentrations by HR-ICP-MS and ICP-AES, respectively. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes). Compared to fresh ash (δ30Si = −0.38‰; Ge/Si = 2.21 μmol mol−1), soil clay fractions (<2 μm) were enriched in light Si isotopes and Ge: with increasing weathering degree, δ30Si decreased from −1.19 to −2.37‰ and Ge/Si increased from 4.10 to 5.25 μmol mol−1. Sand and silt fractions displayed δ30Si values close to fresh ash (−0.33‰) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic signatures of bulk soils (<2 mm) were strongly governed by the relative proportions of primary and secondary minerals: the bulk soil Si-isotopic budget could be closed indicating that all the phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ30Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution, available for Si sequestration in clay-sized secondary minerals (clay minerals formation and Si adsorption onto Fe-oxide). In the soil-plant system, δ30Si and Ge/Si are thus highly relevant to trace weathering and input of DSi from phytoliths in secondary minerals, although not quantifying the net input of BSi to DSi.  相似文献   

12.
We report here the silicon isotopic composition (δ30Si) of dissolved silicon (DSi) from 42 surface water samples from the Drake Passage, the Weddell Gyre, other areas south of the Southern Boundary of the Antarctic Circumpolar Current (ACC), and the ACC near the Kerguelen Plateau, taken between the beginning of February and the end of March 2007. From the beginning to end of the cruise (ANTXXIII/9), DSi diminished in the Antarctic by 50 μmol L−1 while concentrations of nitrate + nitrite and phosphate showed no net decline, indicating that the high seasonal Si/N removal ratios well known for the Southern Ocean may be more related to the strength of the silicate pump in the Southern Ocean than to the instantaneous Si/N uptake ratio of diatoms. The δ30Si of DSi in samples containing more than 20 μM DSi were strongly negatively correlated to DSi concentrations, supporting the use of δ30Si as a proxy for DSi removal. The “open system” fractionation observed, ε = −1.2 ± 0.11‰, agrees well with results from previous work in other areas, and the estimate of the initial δ30Si of DSi of +1.4‰ is not far off observations of the δ30Si of DSi in Winter Water (WW) in this area. Results were used to model DSi draw down in the past from the δ30Si of sediment cores, although isotopic fractionation during silica dissolution appeared to influence the δ30Si of some surface water samples, inviting further study of this phenomenon.  相似文献   

13.
The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H4SiO40), thereby influencing Si concentration in soil solution. Here we study the impact of H4SiO40 adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0-72 h) and initial concentration (ic) of Si in solution (0.61-1.18 mM) at 20 °C, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ30Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σSEM). The quantitative adsorption of H4SiO40 by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ30Si = +0.02 ± 0.07‰ (±2σSD)), the solutions were systematically enriched in 30Si reaching maximum δ30Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H4SiO40 adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.  相似文献   

14.
Silicon isotopes in meteorites and planetary core formation   总被引：1，自引：0，他引：1  
The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ30Si differences are resolved between chondrites (δ30Si = −0.49 ± 0.15‰, 2σSD) and achondrites (δ30Si = −0.47 ± 0.11‰, 2σSD), although enstatite chondrites are consistently lighter (δ30Si = −0.63 ± 0.07‰, 2σSD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ30SiBSE−meteorite∗ = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ30SiBSE−meteorite∗ value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.  相似文献   

15.
Silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4°N and 3°S latitude at 110°W longitude in the eastern Equatorial Pacific. Silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of ε estimated by applying a steady-state isotope fractionation model to data from all stations between 4°N and 3°S was −0.77 ± 0.12‰ (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of ε was −1.08 ± 0.27‰ (std. err.) similar to the value established in culture studies (−1.1‰). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of ε was significantly more positive, −0.61 ± 0.16‰ (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of −1.1‰ for the fractionation factor for isotopes of Si in the sea. Without the assumption of steady state the value of ε can only be constrained to be between −0.6 and −1.1‰. Silicic acid in Equatorial Pacific Deep Water below 2000 m had a near constant δ30Si of +1.32 ± 0.05‰. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9-1.0‰) and it is slightly less positive than new measures of the δ30Si of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH)4 > 180  μM, δ30Si = +1.46 ± 0.12‰ (SD, n = 4). We show that the data from the equator and Cascadia basin fit a general trend of increasing δ30Si(OH)4 with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water δ30Si(OH)4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea.  相似文献   

16.
Banded iron formations (BIFs) within the Lvliang region of Shanxi Province, China, are hosted by sediments of the Yuanjiacun Formation, part of the Paleoproterozoic Lvliang Group. These BIFs are located in a zone where sedimentation changed from clastic to chemical deposition, indicating that these are Superior-type BIFs. Here, we present new major, trace, and rare earth element (REE) data, along with Fe, Si, and O isotope data for the BIFs in the Yuanjiacun within the Fe deposits at Yuanjiacun, Jianshan, and Hugushan. When compared with Post Archean Australian Shale (PAAS), these BIFs are dominated by iron oxides and quartz, contain low concentrations of Al2O3, TiO2, trace elements, and the REE, and are light rare earth element (LREE) depleted and heavy rare earth element (HREE) enriched. The BIFs also display positive La, Y, and Eu anomalies, high Y/Ho ratios, and contain 30Si depleted quartz, with high δ18O values that are similar to quartz within siliceous units formed during hydrothermal activity. These data indicate that the BIFs within the Yuanjiacun Formation were precipitated from submarine hydrothermal fluids, with only negligible detrital contribution. None of the BIF samples analyzed during this study have negative Ce anomalies, although a few have a positive Ce anomaly that may indicate that the BIFs within the Yuanjiacun Formation formed during the Great Oxidation Event (GOE) within a redox stratified ocean. The positive Ce anomalies associated with some of these BIFs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed Ho, LREE, and Ce4 +; these deposits formed during reductive dissolution at the oxidation–reduction transition zone or in deeper-level reducing seawater. The loss of Ce, LREE, and Ho to seawater and the deposition of these elements with iron hydroxides caused the positive Ce anomalies observed in some of the BIF samples, although the limited oxidizing ability of surface seawater at this time meant that Y/Ho and LREE/HREE ratios were not substantially modified, unlike similar situations within stratified ocean water during the Late Paleoproterozoic. Magnetite and hematite within the BIFs in the study area contain heavy Fe isotopes (56Fe values of 0.24–1.27‰) resulting from the partial oxidation and precipitation of Fe2 + to Fe3 + in seawater. In addition, mass-independent fractionation of sulfur isotopes within pyrite indicates that these BIFs were deposited within an oxygen-deficient ocean associated with a similarly oxygen-deficient atmosphere, even though the BIFs within the Yuanjiacun Formation formed after initiation of the GOE.  相似文献   

17.
The Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon 206Pb/238U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO2, Zr(Nb) and Al2O3 and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al2O3 contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.  相似文献   

18.
Variations in the oxygen isotope composition (δ18O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ18O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ18O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ18O values, (δ18O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ18O value.At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ18O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ18O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature (Tsw at which the amorphous silica precipitated) and diagenesis temperature (Tdiagenesis at which microquartz formed) that reproduce the δ18O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ18O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for Tsw and Tdiagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.  相似文献   

19.
Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.  相似文献   

20.
We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ25Mg = −0.14 ± 0.06; δ26Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ7Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ7Li = 3.5 ± 0.5‰, δ25Mg = −0.10 ± 0.03‰ and δ26Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ7Li and δ26Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ7Li and δ26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ7Li seen in some oceanic basalts.  相似文献   

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