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1.
Experimental studies in the Fe3C–SiO2–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron–wustite buffer.  相似文献   

2.
Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe0- or Fe3C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 1300–1500°С) in the carbide–oxide–carbonate systems (Fe3C–SiO2–(Mg,Ca)CO3 and Fe3C–SiO2–Al2O3–(Mg,Ca)CO3) are reported. Conditions of formation of mantle silicates with metallic or metal–carbon melt inclusions are determined and their stability in the presence of CO2-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO2-fluid are formed in the carbide–oxide–carbonate system through decarbonation, with subsequent redox interaction between CO2 and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a Fe–C melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO2-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of Fe–C melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.  相似文献   

3.
Experimental research in the Fe3C–(Fe,Ni)S system was carried out. The objective of the investigation was to model the reactions of carbide–sulfide interaction related to graphite (diamond) formation in reduced lithosphere mantle domains. T ≤ 1200°C is the formation temperature of the Ni-cohenite + graphite assemblage coexisting with two immiscible melts such as sulfide (Fe60–Ni3–S37)L and metal–sulfide (Fe71–Ni7–S21–C1)L containing dissolved carbon. Т ≥ 1300°C is the generation temperature of a unified melt such as (Fe80–Ni6–S10–C4)L characterized by graphite crystallization and diamond growth. The extraction of carbide carbon during the interaction with the sulfide melt can be considered as one of the potential mechanisms of graphite and diamond formation in the reduced mantle.  相似文献   

4.
Metallic Fe content and S abundance are inversely correlated in mare basalts. Either S volatilization from the melt results in reduction of Fe2+ to Fe0 or else high S content decreases Fe0 activity in the melt, thus explaining the correlation. All considerations favor the model that metallic iron in mare basalts is due to sulfur loss. The Apollo 11 and 17 mare basalt melts were probably saturated with S at the time of eruption; the Apollo 12 and 15 basalts were probably not saturated.Non-mare rocks show a positive correlation of S abundance with metallic Fe content; it is proposed that this is due to the addition of meteoritic material having a fairly constant Fe0/S ratio. If true, metallic Fe content or S abundance in non-mare rocks provides a measure of degree of meteoritic contamination.  相似文献   

5.
Experimental studies in the system Fe,Ni–olivine–carbonate–S (P = 6.3 GPa, T = 1050–1550°C, t = 40–60 h) aimed at modeling of the interaction of subducted carbonates and sulfur with rocks of the silicate mantle and at investigation of the likely mechanism of the formation of mantle sulfides were performed. It is shown that an association of olivine + orthopyroxene + magnesite + pyrite coexisting with a sulfur melt/fluid with dissolved Fe, Ni, and O is formed at T ≤ 1250°C. An association of low-Fe olivine, orthopyroxene, and magnesite and two immiscible melts of the carbonate and S–Fe–Ni–O compositions are formed at T ≥ 1350°C. It is shown that the reduced S-bearing fluids may transform silicates and carbonates, extract metals from the solid-phase matrix, and provide conditions for generation of sulfide melts.  相似文献   

6.
The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) facies banded iron formation (BIF). It consists of three stratabound orebodies; Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz‐chlorite‐ferroan dolomite‐albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite‐dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral‐chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz‐carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel‐bedded and generally chert‐hosted pyrites are in sharp contact with re‐crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite–hematite re‐crystallization. The Fe/(Fe+Mg) ratio of the dolomite–ankerite series and chlorite decreased from veins through mineralized BIF and non‐mineralized BIF, in contrast to most Archean BIF‐hosted gold deposits. This is interpreted to be due to the effect of a high sulfur activity and increase in fO2 in a H2S‐dominant fluid during progressive fluid‐rock interaction. High sulfur activity of the hydrothermal fluid fixed pyrite in the BIF by consuming Fe2+ released into the chert layers and leaving the co‐precipitating carbonates and chlorites with less available ferrous iron content. Alternatively, the occurrence of hematite in the alteration assemblage of the host BIF caused a structural limitation in the assignment of Fe3+ in chlorite which favored the incorporation of magnesium (rather than ferric iron) in chlorite under increasing fO2 conditions, and is consistent with deposits hosted in hematite‐bearing rocks. The combined effects of reduction in sulfur contents due to sulfide precipitation and increasing fO2 during progressive fluid‐rock interactions are likely to be the principal factors to have caused gold deposition. Arsenopyrite–pyrite geothermometry indicated a temperature range of 300–350°C for the associated gold mineralization. The estimated δ34SΣS (= +1.8 to +2.5‰) and low base metal contents of the sulfide ore mineralogy are consistent with sulfides that have been sourced from magma or derived by the dissolution of magmatic sulfides from volcanic rocks during fluid migration.  相似文献   

7.
The Fe3+/Fetot of all Fe-bearing minerals has been analysed by Mössbauer spectroscopy in a suite of biotite-rich to biotite-free graphitic metapelite xenoliths, proxies of an amphibolite-granulite transition through progressive biotite melting. Biotite contains 9 to 16% Fe3+/Fetot, whereas garnet, cordierite and ilmenite are virtually Fe3+ -free (0–1% Fe3+/Fetot) in all samples, regardless of biotite presence. Under relatively reducing conditions (graphite-bearing assemblages), biotite is the only carrier of Fe3+ during high-temperature metamorphism; therefore, its disappearance by melting represents an important event of iron reduction during granulite formation, because haplogranitic melts usually incorporate small amounts of ferric iron. Iron reduction is accompanied by the oxidation of carbon and the production of CO2, according to the redox reaction:
Depending on the nature of the peritectic Fe-Mg mineral produced (garnet, cordierite, orthopyroxene), the CO2 can either be present as a free fluid component, or be completely stored within melt and cordierite. The oxidation of graphite by iron reduction can account for the in situ generation of CO2, implying a consequential rather than causal role of CO2 in some granulites and migmatites. This genetic model is relevant to graphitic rocks more generally and may explain why CO2 is present in some granulites although it is not required for their formation.  相似文献   

8.
We present new high-pressure temperature experiments on melting phase relations of Fe-C-S systems with applications to metallic core formation in planetary interiors. Experiments were performed on Fe-5 wt% C-5 wt% S and Fe-5 wt% C-15 wt% S at 2-6 GPa and 1050-2000 °C in MgO capsules and on Fe-13 wt% S, Fe-5 wt% S, and Fe-1.4 wt% S at 2 GPa and 1600 °C in graphite capsules. Our experiments show that: (a) At a given P-T, the solubility of carbon in iron-rich metallic melt decreases modestly with increasing sulfur content and at sufficiently high concentration, the interaction between carbon and sulfur can cause formation of two immiscible melts, one rich in Fe-carbide and the other rich in Fe-sulfide. (b) The mutual solubility of carbon and sulfur increases with increasing pressure and no super-liquidus immiscibility in Fe-rich compositions is likely expected at pressures greater than 5-6 GPa even for bulk compositions that are volatile-rich. (c) The liquidus temperature in the Fe-C-S ternary is significantly different compared to the binary liquidus in the Fe-C and Fe-S systems. At 6 GPa, the liquidus of Fe-5 wt% C-5 wt% S is 150-200 °C lower than the Fe-5 wt% S. (d) For Fe-C-S bulk compositions with modest concentration of carbon, the sole liquidus phase is iron carbide, Fe3C at 2 GPa and Fe7C3 at 6 GPa and metallic iron crystallizes only with further cooling as sulfur is concentrated in the late crystallizing liquid. Our results suggest that for carbon and sulfur-rich core compositions, immiscibility induced core stratification can be expected for planets with core pressure less than ∼6 GPa. Thus planetary bodies in the outer solar system such as Ganymede, Europa, and Io with present day core-mantle boundary (CMB) pressures of ∼8, ∼5, and 7 GPa, respectively, if sufficiently volatile-rich, may either have a stratified core or may have experienced core stratification owing to liquid immiscibility at some stage of their accretion. A similar argument can be made for terrestrial planetary bodies such as Mercury and Earth’s Moon, but no such stratification is predicted for cores of terrestrial planets such as Earth, Venus, and Mars with the present day core pressure in the order ?136 GPa, ?100 GPa, and ?23 GPa. (e) Owing to different expected densities of Fe-rich (and carbon-bearing) and sulfur-rich metallic melts, their settling velocities are likely different; thus core formation in terrestrial planets may involve rain of more than one metallic melt through silicate magma ocean. (f) For small planetary bodies that have core pressures <6 GPa and have a molten core or outer core, settling of denser carbide-rich liquid or flotation of lighter, sulfide-rich melt may contribute to an early, short-lived geodynamo.  相似文献   

9.
It is generally accepted that carbonates can be subducted to the mantle depths, where they are reduced with iron metal to produce a diamond. In this work, we found that this is not always the case. The mantle carbonates from inclusions in diamonds show a wide range of cation compositions (Mg, Fe, Ca, Na, and K). Here we studied the reaction kinetics of these carbonates with iron metal at 6–6.5 GPa and 1000–1500 °C. We found that the reduction of carbonate with Fe produces C-bearing species (Fe, Fe-C melt, Fe3C, Fe7C3, C) and wüstite containing Na2O, CaO, and MgO. The reaction rate constants (k = Δx2/2t) are log-linear relative to 1/T and their temperature dependences are determined to bekMgCO3 (m2/s) = 4.37 × 10?3 exp [?251 (kJ/mol)/RT]kCaMg(CO3)2 (m2/s) = 1.48 × 10?3 exp [?264 (kJ/mol)/RT]kCaCO3 (m2/s) = 3.06 × 10?5 exp [?245 (kJ/mol)/RT] andkNa2CO3 (m2/s) = 1.88 × 10?10 exp [?155 (kJ/mol)/RT].According to obtained results at least, 45–70 vol% of carbonates preserve during subduction down to the 660-km discontinuity if no melting occurs. The slab stagnation and warming, subsequent carbonate melting, and infiltration into the mantle saturated with iron metal are accompanied by a reduction of carbonate melt with Fe. The established sequence of reactivity of carbonates: FeCO3 ≥ MgCO3 > CaMg(CO3)2 > CaCO3 ? Na2CO3, where K2CO3 does not react at all with iron metal, implies that during reduction carbonate melt with Fe evolves toward alkali-rich. The above conclusions are consistent with the findings of carbonates in inclusions in diamonds from the lower mantle and high concentrations of alkalis, particularly K, in mantle carbonatite melts entrapped by diamonds from kimberlites and placers worldwide.  相似文献   

10.
Interactions in a Fe–C–O–H–N system that controls the mobility of siderophile nitrogen and carbon in the Fe0-saturated upper mantle are investigated in experiments at 6.3–7.8 GPa and 1200–1400 °C. The results show that the γ-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 °C. The interactions of Fe3C with an N-rich fluid in a graphite-saturated system produce the ε-Fe3N phase (space group P63/mmc or P6322) at subsolidus conditions of 1200–1300 °C, while N-rich melts form at 1400 °C. At IW- and MMO-buffered hydrogen fugacity (fH2), fluids vary from NH3- to H2O-rich compositions (NH3/N2?>?1 in all cases) with relatively high contents of alkanes. The fluid derived from N-poor samples contains less H2O and more carbon which mainly reside in oxygenated hydrocarbons, i.e., alcohols and esters at MMO-buffered fH2 and carboxylic acids at unbuffered fH2 conditions. In unbuffered conditions, N2 is the principal nitrogen host (NH3/N2?≤?0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (γ-Fe, melt, or Fe3N) are stable at the upper mantle pressures and temperatures. According to our estimates, the metal/fluid partition coefficient of nitrogen is higher than that of carbon. Thus, nitrogen has a greater affinity for iron than carbon. The general inference is that reduced fluids can successfully transport volatiles from the metal-saturated mantle to metal-free shallow mantle domains. However, nitrogen has a higher affinity for iron and selectively accumulates in the metal phase, while highly mobile carbon resides in the fluid phase. This may be a controlling mechanism of the deep carbon and nitrogen cycles.  相似文献   

11.
Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe2O3) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450 °C and pressures from saturated vapor pressure (Psat) to 600 bar. Precipitation experiments at 200 °C and Psat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (FeIII) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ57Fefluid-hematite = δ57Fefluid − δ57Fehematite = 0.01 ± 0.08‰ (2 × standard error, 2SE). In contrast, experiments at 300 °C and Psat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ57Fefluid-hematite = −0.54 ± 0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 °C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ57Fefluid-hematite values close to zero within errors, 0.15 ± 0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous FeII and FeIII fractions, and Fe isotope ratios in solution, and identical Δ57Fe values from dissolution and precipitation runs. Our measured equilibrium Δ57Fefluid-hematite values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid-hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ57Fe Fe(aq)-hematite value at 200 °C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite-saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions.  相似文献   

12.
A method of in situ X-ray diffraction at Spring-8 (Japan) was used to analyze simultaneously the hydrogen incorporation into Fe and Fe3C, as well as to measure the relative stability of carbides, nitrides, sulfides, and hydrides of iron at pressures of 6–20 GPa and temperatures up to 1600 K. The following stability sequence of individual iron compounds was established in the studied pressure and temperature interval: FeS > FeN > FeC > FeH > Fe. A change in the unit-cell volume as compared to the known equations of state was used to estimate the hydrogen contents in carbide Fe3C and hydride FeHx. Data on hydride correspond to stoichiometry with x ≈ 1. Unlike iron sulfides and silicides, the solubility of hydrogen in Fe3C seemed to be negligibly low—within measurement error. Extrapolating obtained data to pressures of the Earth’s core indicates that carbon and hydrogen are mutually incpompatible in the iron–nickel core, while nitrogen easily substitutes carbon and may be an important component of the inner core in the light of the recent models assuming the predominance of iron carbide in its composition.  相似文献   

13.
A first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions. Experiments were performed in an internally heated gas pressure vessel at 500 MPa and temperatures between 840 and 1000 °C for 120-168 h. Three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by MC-ICP-MS. (i) Kinetic experiments using 57Fe-enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h. (ii) Isotope exchange experiments—using mixtures of hydrous peralkaline rhyolitic glass powder (∼4 wt% H2O) and natural pyrrhotites (Fe1 − xS) as starting materials— and (iii) crystallisation experiments, in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt, consistently showed that pyrrhotite preferentially incorporates light iron. No temperature dependence of the fractionation factor was found between 840 and 1000 °C, within experimental and analytical precision. An average fractionation factor of Δ 56Fe/54Fepyrrhotite-melt = −0. 35 ± 0.04‰ (2SE, n = 13) was determined for this temperature range. Predictions of Fe isotope fractionation between FeS and ferric iron-dominated silicate minerals are consistent with our experimental results, indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt. Consequently, the fractionation factor determined in this study is representative for the specific Fe2+/ΣFe ratio of our peralkaline rhyolitic melt of 0.38 ± 0.02. At higher Fe2+/ΣFe ratios a smaller fractionation factor is expected. Further investigation on Fe isotope fractionation between other mineral phases and silicate melts is needed, but the presented experimental results already suggest that even at high temperatures resolvable variations in the Fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems.  相似文献   

14.
Assessing the ferric-ferrous ratio in magmas prior to eruption remains a challenging task. X-ray absorption near-edge structure (μXANES) spectra were collected at the iron K-edge in water-rich peralkaline silicic melt/glass inclusions trapped in quartz. These experiments were carried out between 800 and 20 °C. The chemical environment of iron was also determined in the naturally quenched samples (glass inclusions and matrix glass) and in the peralkaline rhyolitic reference glasses, with variable [Fe3+ / ∑Fe] ratios.In the reference glasses, both the intensity of the pre-peaks (Fe2+, Fe3+) and site geometry of iron change as the oxidation state increases. Fourfold-coordinated Fe3+ prevails in highly oxidised peralkaline silicic glasses, using alkalis for charge balance. The position of the pre-edge centroid of the 1s-3d transition correlates with the Fe3+ / ΣFe ratios that allowed calibration of the redox state of iron of our natural samples.At high temperatures, Fe2+ dominates in the pre-edge structure of melt inclusions. Upon cooling down to 20 °C, the intensity of the Fe3+ peak increases, the centroid position of the pre-edge features shifts by nearly 0.5 eV and the main edge moves slightly towards higher energies. The slower the cooling rate, the higher the ferric iron contribution. Iterative μXANES experiments performed on the same samples show that the process is reversible. However, this apparent oxidation of iron upon cooling is an artefact of changes in Fe coordination. It implies that the [Fe3+ / ΣFe] ratio of glassy samples, measured at 20 °C, may be overestimated by a factor > 1.7, and that this ratio cannot be reliably retrieved by probing naturally cooled glass inclusions, and most silicate glasses. High temperature μXANES experiments led first to an assessment of the ferric-ferrous ratio in the water-rich peralkaline melt in pre-eruptive magmatic conditions and second to the determination of the corresponding oxygen fugacity at 740 °C.  相似文献   

15.
The fluid inclusions in minerals and isotope composition of sulfur in sulfides and carbon and oxygen in carbonates are studied for the Novoshirokinskii gold-polymetallic deposit. The ore-forming fluids are characterized by the following physico-chemical and isotope-geochemical parameters: temperature of 290–100°C, salinity of 13–2.5 wt % NaCl-equiv., δ18O from +8 to 0‰, δ13C of 2.5 ± 0.5‰, and δ34S of 10.5 ± 1.0‰. It is concluded that the Late Proterozoic-Early Cambrian carbonaceous-terrigenous and carbonate rocks were involved in the Late Jurassic ore-magmatic system.  相似文献   

16.
Sediment and pore water samples have been collected from the coastal tidal flat in the Shuangtaizi estuary, China, in order to investigate the geochemical behavior of iron, cadmium, and lead during diagenesis and to assess the degree of contamination. The calculated enrichment factors and geoaccumulation indices for separate elements show that anthropogenic activities have had no significant influence on the distribution of Fe and Pb in the study area, whereas the distribution of Cd has been closely influenced in this way. The high percentage of exchangeable Cd (average of 56.34%) suggests that Cd represents a potential hazard to benthic organisms in the estuary. The calculated diffusive fluxes of metals show that the most mobilized metal is Fe (9.22 mg m?2 a?1), followed by Cd (0.54 mg m?2 a?1) and Pb (0.42 mg m?2 a?1). Low Fe2+ contents in surface pore water, alongside high chromium-reducible sulfur contents, and low acid-volatile sulfur, and elemental sulfur contents at 0–25 cm depth in sediments show that Fe2+ is formed by the reduction of Fe oxides and is transformed first to a solid phase of iron monosulfides (FeS) and eventually to pyrite (FeS2). The release of adsorbed Pb due to reductive dissolution of Fe/Mn oxides during early diagenesis could be a source of Pb2+ in pore water. From the relatively low total organic carbon contents measured in sediments (0.46–1.28%, with an average of 0.94%) and the vertical variation of Cd2+ in pore water, sulfide or Fe/Mn oxides (instead of organic matter) are presumed to exert a significant influence on carrying or releasing Cd by the sediments.  相似文献   

17.
The paper presents the results of study of ferromanganese carbonate rocks in the Sob area (Polar Urals), which is located between the Rai-Iz massif and the Seida–Labytnangi Railway branch. These rocks represent low-metamorphosed sedimentary rocks confined to the Devonian carbonaceous siliceous and clayey–siliceous shales. In terms of ratio of the major minerals, ferromanganese rocks can be divided into three varieties composed of the following minerals: (1) siderite, rhodochrosite, chamosite, quartz, ± kutnahorite, ± calcite, ± magnetite, ± pyrite, ± clinochlore, ± stilpnomelane; (2) spessartite, rhodochrosite, and quartz, ± hematite, ± chamosite; (3) rhodochrosite, spessartite, pyroxmanite, quartz ± tephroite, ± fridelite, ± clinochlore, ± pyrophanite, ± pyrite. In all varieties, the major concentrators of Mn and Fe are carbonates (rhodochrosite, siderite, kutnahorite, Mn-calcite) and chlorite group minerals (clinochlore, chamosite). The chemical composition of rocks is dominated by Si, Fe, Mn, carbon dioxide, and water (L.O.I.): total SiO2 + Fe2O 3 tot + MnO + L.O.I. = 85.6?98.4 wt %. The content of Fe and Mn varies from 9.3 to 55.6 wt % (Fe2O 3 tot + MnO). The Mn/Fe ratio varies from 0.2 to 55.3. In terms of the aluminum module AlM = Al/(Al + Mn + Fe), the major portion of studied samples corresponds to metalliferous sediments. The δ13Ccarb range (–30.4 to–11.9‰ PDB) corresponds to authigenic carbonates formed with carbon dioxide released during the microbial oxidation of organic matter in sediments at the dia- and/or catagenetic stage. Ferromanganese sediments were likely deposited in relatively closed seafloor zones (basin-traps) characterized by periodic stagnation. Fe and Mn could be delivered from various sources: input by diverse hydrothermal solutions, silt waters in the course of diagenesis, river discharges, and others. The diagenetic delivery of metals seems to be most plausible. Mn was concentrated during the stagnation of bottom water in basin-traps. Interruption of stagnation promoted the precipitation of Mn. The presence of organic matter fostered a reductive pattern of postsedimentary transformations of metalliferous sediments. Fe and Mn were accumulated initially in the oxide form. During the diagenesis, manganese and iron oxides reacted with organic matter to make up carbonates. Relative to manganese carbonates, iron carbonates were formed under more reductive settings and higher concentrations of carbon dioxide in the interstitial solution. Crystallization of manganese and iron silicates began already at early stages of lithogenesis and ended during the regional metamorphism of metalliferous sediments.  相似文献   

18.
Parts of the Fe–C–N system were studied in experiments at 7.8 GPa and 1350°C. It was shown that the admixture of nitrogen extends considerably the domain of melt stability in the system at temperatures close to the Fe–Fe3C eutectic temperatures. Nitrogen solubility in cementite in equilibrium with the nitrogen- rich melt is below the detection limit of the EMPA technique applied. The metal melt is the only nitrogen concentrator (up to 4 wt % of N) in the range of compositions considered. The data obtained permit the conclusion that, in the case of complete dissolution of carbon and nitrogen, which might occur in the enriched mantle, native iron at ~250 km depth should either be completely molten or consist of a melt and carbide of iron.  相似文献   

19.
In iron-manganese nodules from the floor of Pacific ocean, Baltic, White Sea and Kara Sea, iron bydroxide '-FeOOH was analysed in the laboratory. In buried ooze, reduction processes generate Fe(HCO3)2 which migrates into the upper part of the bottom ooze and into near bottom sea water where Fe(OH)2 is formed. The oxidation process of Fe2+ to Fe3+, without participation of iron bacteria, leads to the topotactic transformation of Fe(OH)2 to '-FeOOH. Marine water does not contain Fe2+ and cannot be a direct source of iron deposited in the nodules. Discovery of '-FeOOH in marine nodules permits the consideration that both iron and manganese were derived from the buried bottom mud, which during diagenetic processes led to the transfer of these metals in solutions and their upward migration.  相似文献   

20.
Sulfur partitioning between melt and fluid phase largely controls the environmental impact of volcanic eruptions. Fluid/melt partitioning data also provide the physical basis for interpreting changes in volcanic gas compositions that are used in eruption forecasts. To better constrain some variables that control the behavior of sulfur in felsic systems, in particular the interaction between different volatiles, we studied the partitioning of sulfur between aqueous fluids and haplogranitic melts at 200 MPa and 750–850 °C as a function of oxygen fugacity (Ni–NiO or Re–ReO2 buffer), melt composition (Al/(Na?+?K) ratio), and fluid composition (NaCl and CO2 content). The data confirm a first-order influence of oxygen fugacity on the partitioning of sulfur. Under “reducing conditions” (Ni–NiO buffer), Dfluid/melt is nearly one order of magnitude larger (323?±?14 for a metaluminous melt) than under “oxidizing conditions” (Re–ReO2 buffer; 74?±?5 for a metaluminous melt). This effect is likely related to a major change in sulfur speciation in both melt and fluid. Raman spectra of the quenched fluids show the presence of H2S and HS? under reducing conditions and of SO42? and HSO4? under oxidizing conditions, while SO2 is undetectable. The latter observation suggests that already at the Re–ReO2 buffer, sulfur in the fluid is almost completely in the S6+ state and, therefore, more oxidized than expected according to current models. CO2 in the fluid (up to xCO2?=?0.3) has no effect on the fluid/melt partitioning of sulfur, neither under oxidizing nor under reducing conditions. However, the effect of NaCl depends on redox state. While at oxidizing conditions, Dfluid/melt is independent of xNaCl, the fluid/melt partition coefficient strongly decreases with NaCl content under reducing conditions, probably due to a change from H2S to NaSH as dominant sulfur species in the fluid. A decrease of Dfluid/melt with alkali content in the melt is observed over the entire compositional range under reducing conditions, while it is prominent only between the peraluminous and metaluminous composition in oxidizing experiments. Overall, the experimental results suggest that for typical oxidized, silicic to intermediate subduction zone magmas, the degassing of sulfur is not influenced by the presence of other volatiles, while under reducing conditions, strong interactions with chlorine are observed. If the sulfur oxidation state is preserved during an explosive eruption, a large fraction of the sulfur released from oxidized magmas may be in the S6+ state and may remain undetected by conventional methods that only measure SO2. Accordingly, the sulfur yield and the possible climatic impact of some eruptions may be severely underestimated.  相似文献   

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