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1.
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   

2.
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   

3.
Silicon isotope fractionation during magmatic differentiation   总被引：3，自引：0，他引：3  
The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ30Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.At an average SiO2 content of ∼60 wt.%, the predicted δ30Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ30Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.  相似文献   

4.
Banded iron formations (BIFs) are chemical marine sediments dominantly composed of alternating iron-rich (oxide, carbonate, sulfide) and silicon-rich (chert, jasper) layers. Isotope ratios of iron, carbon, and sulfur in BIF iron-bearing minerals are biosignatures that reflect microbial cycling for these elements in BIFs. While much attention has focused on iron, banded iron formations are equally banded silica formations. Thus, silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in BIFs. BIFs are banded by definition, and microlaminae, or sub-mm banding, are characteristic of many BIFs. In situ microanalysis including secondary ion mass spectrometry is well-suited for analyzing such small features. In this study we used a CAMECA IMS-1280 ion microprobe to obtain highly accurate (±0.3‰) and spatially resolved (∼10 μm spot size) analyses of silicon and oxygen isotope ratios for quartz from several well known BIFs: Isua, southwest Greenland (∼3.8 Ga); Hamersley Group, Western Australia (∼2.5 Ga); Transvaal Group, South Africa (∼2.5 Ga); and Biwabik Iron Formation, Minnesota, USA (∼1.9 Ga). Values of δ18O range from +7.9‰ to +27.5‰ and include the highest reported δ18O values for BIF quartz. Values of δ30Si have a range of ∼5‰ from −3.7‰ to +1.2‰ and extend to the lowest δ30Si values for Precambrian cherts. Isua BIF samples are homogeneous in δ18O to ±0.3‰ at mm- to cm-scale, but are heterogeneous in δ30Si up to 3‰, similar to the range in δ30Si found in BIFs that have not experienced high temperature metamorphism (up to 300 °C). Values of δ30Si for quartz are homogeneous to ±0.3‰ in individual sub-mm laminae, but vary by up to 3‰ between multiple laminae over mm-to-cm of vertical banding. The scale of exchange for Si in quartz in BIFs is thus limited to the size of microlaminae, or less than ∼1 mm. We interpret differences in δ30Si between microlaminae as preserved from primary deposition. Silicon in BIF quartz is mostly of marine hydrothermal origin (δ30Si < −0.5‰) but silicon from continental weathering (δ30Si ∼ 1‰) was an important source as early as 3.8 Ga.  相似文献   

5.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.  相似文献   

6.
The terrestrial biogenic Si (BSi) pool in the soil-plant system is ubiquitous and substantial, likely impacting the land-ocean transfer of dissolved Si (DSi). Here, we consider the mechanisms controlling DSi in forest soil in a temperate granitic ecosystem that would differ from previous works mostly focused on tropical environments. This study aims at tracing the source of DSi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions, using stable Si isotopes and Ge/Si ratios. Relative to granitic bedrock, clays minerals were enriched in 28Si and had high Ge/Si ratios, while BSi from phytoliths was also enriched in 28Si, but had a low Ge/Si ratio. Such a contrast is useful to infer the relative contribution of silicate weathering and BSi dissolution in the shallow soil on the release of DSi in forest floor leachate solutions. The δ30Si values in forest floor leachates (−1.38‰ to −2.05‰) are the lightest ever found in natural waters, and Ge/Si ratios are higher in forest floor leachates relative to soil solution. These results suggest dissolution of 28Si and Ge-enriched secondary clay minerals incorporated by bioturbation in organic-rich horizons in combination with an isotopic fractionation releasing preferentially light Si isotopes during this dissolution process. Ge/Si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals. Tree species influence Si-isotopic compositions and Ge/Si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and, to a lesser extent, through differing Si recycling.  相似文献   

7.
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   

8.
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   

9.
The silicon isotopic composition of dissolved silicon and suspended particulate matter (SPM) were systematically investigated in water samples from the mainstem of the Yellow River and 4 major tributaries. The SPM content of the Yellow River varied from 1.4 to 38,560 mg/L, averaging 3568 mg/L, and the δ30Si of suspended particulate matter (δ30SiSPM) varied from 0.3‰ to −0.4‰, averaging −0.02‰. The major factors affecting the SPM content and the δ30SiSPM values in the Yellow River were inferred to be the mineralogical, chemical and isotopic characteristics of the sediments from the Loess Plateau and a combination of the climate and the flow discharge of the river.The major ions in the Yellow River water were Na+, Ca2+, Mg2+, HCO3−, SO42− and Cl−. High salt concentration was observed in samples from the middle and lower reaches, likely reflecting the effects of evaporation and irrigation because the Na+, Mg2+, SO42− and K+ concentrations were correlated with the Cl− concentration. The dissolved Si concentration (DSi) increased downstream, varying from 0.016 to 0.323 mM. The δ30Si of dissolved Si (δ30SiDiss) varied from 0.4‰ to 2.5‰, averaging 1.28‰. The major processes controlling the DSi and δ30SiDiss of the Yellow River are (a) the weathering of silicate rocks, (b) the formation of phytoliths in plants, (c) the evaporation of water from and the addition of meteoric water to the river system, which only affects concentrations, (d) the adsorption and desorption of aqueous monosilicic acid on iron oxide, and (e) the dissolution of phytoliths in soils.The DSi and δ30SiDiss values of global rivers vary spatially and temporally in response to changes in climate, chemical weathering intensity and biological activity. The moderately positive δ30SiDiss values observed in the Yellow River may be attributed to the higher rates of chemical weathering and biological activities that have been observed in this catchment in comparison with those of other previously studied catchments, excluding the Yangtze River. Human activities may also potentially influence chemical weathering and biological activities and affect the DSi and δ30SiDiss values of the major rivers of the world. Further river studies should be performed to gain a better understanding of the global Si isotope budget.  相似文献   

10.
Boron isotope compositions (δ11B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The 11B/10B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in 10B compared to the ocean value (δ11B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)3 present in precipitations. A boron meteoric water line, δD = 2.6δ11B − 133, is proposed which describes the relationship between δD and δ11B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ11B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ11B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.  相似文献   

11.
The isotopic compositions of commercially available herbicides were analyzed to determine their respective 15N, 13C and 37Cl signatures for the purposes of developing a discrete tool for tracing and identifying non-point source contaminants in agricultural watersheds. Findings demonstrate that of the agrochemicals evaluated, chlorine stable isotopes signatures range between δ37Cl = −4.55‰ and +3.40‰, whereas most naturally occurring chlorine stable isotopes signatures, including those of road salt, sewage sludge and fertilizers, vary in a narrow range about the Standard Mean Ocean Chloride (SMOC) between −2.00‰ and +1.00‰. Nitrogen stable isotope values varied widely from δ15N = −10.86‰ to +1.44‰ and carbon stable isotope analysis gave an observed range between δ13C = −37.13‰ and −21.35‰ for the entire suite of agro-chemicals analyzed. When nitrogen, carbon and chlorine stable isotope analyses were compared in a cross-correlation analysis, statistically independent isotopic signatures exist suggesting a new potential tracer tool for identifying herbicides in the environment.  相似文献   

12.
Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1-1.4 ppm, average values) compared to pyroxenes (0.2-0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5-5.0 ppm), opx (1.1-2.4 ppm) and cpx (2.4-5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8-1.3 ppm). Average δ7Li values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx (−0.6 and −2‰ average δ7Li values, respectively). In the hydrous lherzolites average δ7Li values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx (−2.7 to +9.7‰). Low δ7Li values are observed in some opx (−10.4‰) and cpx (−13‰) from sample Sdi, and in cpx from three hydrous samples (from −9.7 to −5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of δ7Li values on the intra-grain scale (2-27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature.  相似文献   

13.
Bulk chemical compositions and oxygen isotopic compositions were analyzed for 48 stony cosmic spherules (melted micrometeorites) collected from the Antarctic ice sheet using electron- and ion-microprobes. No clear correlation was found between their isotopic compositions and textures. The oxygen isotopic compositions showed an extremely wide range from −28‰ to +93‰ in δ18O and from −21‰ to +13‰ in Δ17O. In δ18O-δ17O space, most samples (38 out of 48) plot close to the terrestrial fractionation line, but 7 samples plot along the carbonaceous chondrite anhydrous mineral (CCAM) line. Three samples plot well above the terrestrial fractionation line. One of these has a Δ17O of +13‰, the largest value ever found in solar system materials. One possible precursor for this spherule could be 16O-poor planetary material that is still unknown as a meteorite. The majority of the remaining spherules are thought to be related to carbonaceous chondrites.  相似文献   

14.
Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ7Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ7Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ7Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ7Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ7Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton.  相似文献   

15.
Silicon isotope compositions of main channel samples of the Yangtze River were systematically investigated along with their chemical compositions. The concentration of suspended matter in the Yangtze River tends to decrease from the upper reaches to the lower reaches, corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow. The silica contents of suspended matter vary from 52.1% to 56.9% and their δ30Si values vary from 0 to −0.7‰, both similar to those of shales. From the upper to lower reaches, the silica contents of suspended matter tend to increase, whilst their δ30Si values tend to decrease. Both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter.The concentrations of dissolved silicon vary from 97 to 121 μmol/L and their δ30Si values vary over a wide range from 0.7 to 3.4‰. From the upper to lower reaches, dissolved silica concentrations tend to decrease and their δ30Si values tend to increase. These trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches. The major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass (in wetlands) and rice (in paddy fields) in drainage areas of the middle and lower reaches.There is no correlation between δ30Si of dissolved silicon and that of suspended matter. The Δ30SiDiss-SPM values vary over a wide range of 1.0-3.7‰, indicating that (1) they are out of isotopic equilibrium, (2) dissolved silicon and the associated suspended matter do not belong to one physico-chemical system, and (3) isotopic exchange rate between them is very slow.The δ30Si value of dissolved silicon output from the Yangtze River to the East Sea is estimated to be 3.0‰, much higher than the values reported for the Amazon and Congo rivers. This increases the δ30Si range of dissolved silicon in the world’s rivers from 0.4-1.2%; to 0.4-3.4%.  相似文献   

16.
The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U-233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41 ± 0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old.  相似文献   

17.
We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m−2) is present as sulfates (2.5 kmol m−2), and to a lesser extent carbonates (0.4 kmol m−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear correlation among δ44/40Ca, δ34S and δ18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ34S values are positively correlated with sulfate-δ18O values (R2 = 0.78) and negatively correlated with sulfate δ44/40CaE values (R2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ44/40Ca fractionation factor of −0.4‰ in CaSO4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.  相似文献   

18.
Holocene sediments from the Gotland Deep basin in the Baltic Sea were investigated for their Fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron. The sediments display variations in δ56Fe (differences in the 56Fe/54Fe ratio relative to the IRMM-14 standard) from −0.27 ± 0.09‰ to +0.21 ± 0.08‰. Samples deposited in a mainly limnic environment with oxygenated bottom water have a mean δ56Fe of +0.08 ± 0.13‰, which is identical to the mean Fe isotopic composition of igneous rocks and oxic marine sediments. In contrast, sediments that formed in brackish water under periodically euxinic conditions display significantly lighter Fe isotope signatures with a mean δ56Fe of −0.14 ± 0.19‰. Negative correlations of the δ56Fe values with the Fe/Al ratio and S content of the samples suggest that the isotopically light Fe in the periodically euxinic samples is associated with reactive Fe enrichments and sulfides. This is supported by analyses of pyrite separates from this unit that have a mean Fe isotopic composition of −1.06 ± 0.20‰ for δ56Fe. The supply of additional Fe with a light Fe isotopic signature can be explained with the shelf to basin Fe shuttle model. According to the Fe shuttle model, oxides and benthic ferrous Fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins. The data furthermore suggest that the euxinic water has a negative dissolved δ56Fe value of about −1.4‰ to −0.9‰. If negative Fe isotopic signatures are characteristic for euxinic sediment formation, widespread euxinia in the past might have shifted the Fe isotopic composition of dissolved Fe in the ocean towards more positive δ56Fe values.  相似文献   

19.
Komatiites from Alexo, Canada, are well preserved and represent high-degree partial mantle melts (∼50%). They are thus well suited for investigating the Mg and Fe isotopic compositions of the Archean mantle and the conditions of magmatic differentiation in komatiitic lavas. High precision Mg and Fe isotopic analyses of 22 samples taken along a 15-m depth profile in a komatiite flow are reported. The δ25Mg and δ26Mg values of the bulk flow are −0.138 ± 0.021‰ and −0.275 ± 0.042‰, respectively. These values are indistinguishable from those measured in mantle peridotites and chondrites, and represent the best estimate of the composition of the silicate Earth from analysis of volcanic rocks. Excluding the samples affected by secondary Fe mobilization, the δ56Fe and δ57Fe values of the bulk flow are +0.044 ± 0.030‰, and +0.059 ± 0.044‰, respectively. These values are consistent with a near-chondritic Fe isotopic composition of the silicate Earth and minor fractionation during komatiite magma genesis. In order to explain the early crystallization of pigeonite relative to augite in slowly cooled spinifex lavas, it was suggested that magmas trapped in the crystal mush during spinifex growth differentiated by Soret effect, which should be associated with large and coupled variations in the isotopic compositions of Mg and Fe. The lack of variations in Mg and Fe isotopic ratios either rules out the Soret effect in the komatiite flow or the effect is effaced as the solidification front migrates downward through the flow crust. Olivine separated from a cumulate sample has light δ56Fe and slightly heavy δ26Mg values relative to the bulk flow, which modeling shows can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion in olivine. Such variations can be used to identify diffusive processes involved in the formation of zoned minerals.  相似文献   

20.
Sixteen groundwater samples collected from production wells tapping Lower Cretaceous Nubian Sandstone and fractured basement aquifers in Sinai were analyzed for their stable isotopic compositions, dissolved noble gas concentrations (recharge temperatures), tritium activities, and 14C abundances. Results define two groups of samples: Group I has older ages, lower recharge temperatures, and depleted isotopic compositions (adjusted 14C model age: 24,000–31,000 yr BP; δ18O: − 9.59‰ to − 6.53‰; δ2H: − 72.9‰ to − 42.9‰; < 1 TU; and recharge T: 17.5–22.0°C) compared to Group II (adjusted 14C model age: 700–4700 yr BP; δ18O: − 5.89‰ to − 4.84‰; δ2H: − 34.5‰ to − 24.1‰; < 1 to 2.78 TU; and recharge T: 20.6–26.2°C). Group II samples have isotopic compositions similar to those of average modern rainfall, with larger d-excess values than Group I waters, and locally measurable tritium activity (up to 2.8 TU). These observations are consistent with (1) the Nubian Aquifer being largely recharged prior to and/or during the Last Glacial Maximum (represented by Group I), possibly through the intensification of paleowesterlies; and (2) continued sporadic recharge during the relatively dry and warmer interglacial period (represented by Group II) under conditions similar to those of the present.  相似文献   

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