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1.
Despite the common belief that AuI complexes with hydrogen sulfide ligands (H2S/HS−) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous AuI–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local atomic structure around the dissolved metal in S–NaOH–Na2SO4–H2SO4 aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)2− is the dominant Au species in neutral-to-basic solutions (5.5  pH  8.5; H2O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur atoms are in a linear geometry around AuI at an average distance of 2.29 ± 0.01 Å. At acidic conditions (1.5  pH  5.0; H2O–S–Na2SO4–H2SO4), the Au atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)0 complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H2S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H2S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of AuI complexes with these ligands (e.g., AuHS(SO2)0, Au(HS2O3)2−, Au(HSn)2−) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.  相似文献   

2.
OH structure of metamorphic fluids has been studied by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in quartz veins, over the temperature range 25–370 °C. Blueschist-facies veins from Tinos island core complex (Cyclades, Greece) display H2O–NaCl–CaCl2–CO2 inclusions whereas greenschist-facies veins contain H2O–NaCl ± CO2 inclusions. From 25 to 370 °C, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 cm–1 °C–1 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to 400 °C yield peak positions of 3,475 cm–1 for blueschist inclusions and 3,585 cm–1 for greenschist inclusions. Because the smaller wave number indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a loose structure compared with blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of aqueous fluids suggested by structural observation and stable isotope analysis.Editorial responsibility: J. Hoefs  相似文献   

3.
Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2+H2O inclusions + Al2SiO5 ± brines in garnet (type 1); early CO2-rich inclusions (± brines) in quartz (type 2); early CO2+CH4 (up to 4 mol%)±H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4+CO2+N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease off O 2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andf O 2 can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.Contribution within the network Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis. CEC program: Human Capital and Mobility  相似文献   

4.
The Daenam mine, which produced over 9250 tons of iron oxide ore from 1958 to 1962, is situated in the Early Cretaceous Yeongyang subbasin of the Gyeongsang basin. It consists of two lens-shaped, hematite-bearing quartz veins that occur along faults in Cretaceous leucocratic granite. The hematite-bearing quartz veins are mainly composed of massive and euhedral quartz and hematite with minor amounts of pyrite, pyrrhotite, mica, feldspar and chlorite.Fluid inclusions in quartz can be divided into three main types: CO2-rich, CO2–H2O, and H2O-rich. Hydrothermal fluids related to the formation of hematite are composed of either H2O–CO2–NaCl ± CH4 (homogenization temperature: 262–455 °C, salinity <7 eq. wt.% NaCl) or H2O–NaCl (homogenization temperature: 182–266 °C, and salinity <5.1 eq. wt.% NaCl), both of which evolved by mixing with deeply circulating meteoric water. Hematite from the quartz veins in the Daenam mine was mainly deposited by unmixing of H2O–CO2–NaCl ± CH4 fluids with loss of the CO2 + CH4 vapor phase and mixing with downward percolating meteoric water providing oxidizing conditions.  相似文献   

5.
Widespread mud volcanism across the thick (≤ 14 km) seismically active sedimentary prism of the Gulf of Cadiz is driven by tectonic activity along extensive strike–slip faults and thrusts associated with the accommodation of the Africa–Eurasia convergence and building of the Arc of Gibraltar, respectively. An investigation of eleven active sites located on the Moroccan Margin and in deeper waters across the wedge showed that light volatile hydrocarbon gases vented at the mud volcanoes (MVs) have distinct, mainly thermogenic, origins. Gases of higher and lower thermal maturities are mixed at Ginsburg and Mercator MVs on the Moroccan Margin, probably because high maturity gases that are trapped beneath evaporite deposits are transported upwards at the MVs and mixed with shallower, less mature, thermogenic gases during migration. At all other sites except for the westernmost Porto MV, δ13C–CH4 and δ2H–CH4 values of ~ − 50‰ and − 200‰, respectively, suggest a common origin for methane; however, the ratio of CH4/(C2H6 + C3H8) varies from ~ 10 to > 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C2+ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C2+-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH4 having δ13C–CH4 ~ − 50‰, as at most sites, but markedly lower δ2H–CH4 values < − 250‰, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH4 is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments ~ 2–4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH4.  相似文献   

6.
Investigations of fluid inclusions in granulitefacies metapelites of southern Calabria enable characterization of the fluid composition of these lower crustal rocks, and constrain the petrologically deduced retrograde P-T path characterized by isothermal uplift prior to isobaric cooling in middle crustal levels. Fluid inclusions in cordierite, garnet and sillimanite have a CO2-rich composition. Inclusions in cordierite rarely contain minor amounts of N2 and H2O, and in garnets some CO2–CH4–N2 inclusions have been analyzed by Raman microprobe. Quartz reveals the most complex fluid melusion compositions (1) CO2-rich, (2) CO2–CH4–N2, (3) CH4–N2, (4) H2O–MgCl2–CaCl2–NaCl, (5) H2O–NaCl and (6) H2O–CO2. The earliest fluid inclusions after peak metamorphism are rich in CO2 with minor amounts of N2 and H2O. An early CO2–(H2O–N2) fluid composition has been confirmed by detection of CO2, H2O and N2 in the channels of the cordierite structure. Most of the early CO2-rich fluid inclusions were modified during the uplift from the lower to the middle crustal level, resulting in a density decrease with CO2 still dominant. The subsequent isobaric cooling led to further modifications of the fluid inclusions. High-density inclusions around implosion textures or scattered amongst lower-density ones must have formed during this cooling episode. Aqueous inclusions in quartz are mostly formed late and are consistent with trapping during retrograde rehydration.This project has been supported by the DFG as contribution to the special program Continental Lower Crust  相似文献   

7.
Syngenetic garnet of eclogitic/pyroxenitic composition included in a polycrystalline diamond aggregate from the Venetia kimberlite, Limpopo Belt, South Africa shows multiple inclusions of spherules consisting of 61±5 vol% Fe3C (cohenite), 30±2 vol% Fe-Ni and 9±3 vol% FeS (troilite). Troilite forms shells around the native iron-cohenite assemblage, implying that both compositions were immiscible melts and were trapped rapidly by the silicate. It is proposed that this polycrystalline diamond-silicate-metallic spherule assemblage formed in very local pressure and fO2 conditions in cracks at the base of the subcratonic lithosphere from a C-H-O fluid that reacted with surrounding silicate at about 1,300–1,400 °C. In a mantle fluid consisting of CH4>H2O>H2 near fO2=IW, the H2 activity increases rapidly when carbon from the fluid is consumed by diamond precipitation, driving the oxygen fugacity of the system to lower values along the diamond saturation curve. Water from the fluid induces melting of surrounding silicate material, and hydrogen reduces metals in the silicate melt, reflected by an unusually low Ni content of the garnet. The carbon isotopic composition of 13C=–13.69 (PDB) and the lack of nitrogen as an impurity is consistent with formation of the diamond from non-biogenic methane, whereas 18O=7.4 (SMOW) of the garnet implies derivation of the silicate from subduction-related material. Hence, very localized and transient reducing conditions within the subcratonic lithosphere can be created by this process and do not necessarily call for involvement of fluids derived from subducted material of biogenic origin.Editorial responsibility: J. Hoefs  相似文献   

8.
Equilibrium 2H/1H fractionation factors (αeq) for various H positions in alkanes, alkenes, ketones, carboxylic acids, esters, alcohols, and ethers were calculated between 0 and 100 °C using vibrational frequencies from ab initio QM calculations (B3LYP/6-311G**). Results were then corrected using a temperature-dependent linear calibration curve based on experimental data for Hα in ketones (Wang et al., 2009). The total uncertainty in reported αeq values is estimated at 10–20‰. The effects of functional groups were found to increase the value of αeq for H next to electron-donating groups, e.g. OR, OH or O(CO)R, and to decrease the value of αeq for H next to electron-withdrawing groups, e.g. (CO)R or (CO)OR. Smaller but significant functional group effects are also observed for Hβ and sometimes Hγ. By summing over individual H positions, we estimate the equilibrium fractionation relative to water to be −90‰ to −70‰ for n-alkanes and around −100‰ for pristane and phytane. The temperature dependence of these fractionations is very weak between 0 and 100 °C. Our estimates of αeq agree well with field data for thermally mature hydrocarbons (δ2H values between −80‰ and −110‰ relative to water). Therefore the observed δ2H increase of individual hydrocarbons and the disappearance of the biosynthetic δ2H offset between n-alkyl and linear isoprenoid lipids during maturation of organic matter can be confidently attributed to H exchange towards an equilibrium state. Our results also indicate that many n-alkyl lipids are biosynthesized with δ2H values that are close to equilibrium with water. In these cases, constant down-core δ2H values for n-alkyl lipids cannot be reliably used to infer a lack of isotopic exchange.  相似文献   

9.
The Rushan gold deposit in the Jiaodong Peninsula is currently the largest lode gold in China. Gold occurs mainly in pyrite- and polymetallic sulfide–quartz vein/veinlet stockworks. Fluid inclusions in the deposit are divided into three main types, namely CO2–H2O, H2O–CO2 ± CH4 and aqueous ones. Microthermometric data show that the pre-gold fluids were CO2-dominant (XCO2 up to 0.53), and the total homogenization temperatures fall in the range of 298377 °C. These fluids, modified by fluid/wallrock reactions, gradually evolved into fluids with less CO2 (XCO2 = 0.010.19) in the main ore-forming stage, and the total homogenization temperatures range from 170 to 324 °C. Hydrogen and oxygen stable isotope data suggest that ore-forming fluids were mixture of magmatic and meteoritic origin. Co-occurrence of gold and sulfides implies that gold was most likely transported in the form of gold–sulfide complexes. The wide distribution of CO2 inclusions means that the pH variation during gold transportation was controlled by CO2 buffering.  相似文献   

10.
Fluid inclusion data are presented for the successive stages of limestone, dolomite, magnesite and sulphide-bearing quartz veins in Proterozoic carbonate rocks of the Lesser Himalaya. Subsurface fluids were H2O–NaCl–KCl ± MgCl2 ± CaCl2 and showed successive increase in salinity and temperature. The salinity of the pore fluid during limestone diagenesis was in the range of 7.5–15 eq wt.% NaCl and the magnesite-forming fluids had a salinity of about 9 to 19 eq wt.% NaCl. This progressive rise in salinity is attributed to a more saline fluid in the deeper zones. The inverse relation between homogenization temperatures and final melting temperatures suggests mixing of the fluids during diagenesis, and highly depleted δ18O values rule out participation of magmatic fluid in the mixing. A late stage carbonic fluid is linked with talc formation. The low temperature of sulphide-forming epigenetic solutions, as obtained from fluid inclusions, is also substantiated by the chemical data from these sulphides. δ34S values in galena infer that magmatic sulphur was probably not involved, and the sulphur of the galena is derived from an isotopically heavy source.  相似文献   

11.
Application of hornblende thermobarometry and fluid inclusion studies to the Palaeoproterozoic (1.7 Ga) basement rocks from Maddhapara, NW Bangladesh, provide information on the pressure and temperature (P–T) conditions of crystallization, the emplacement depth and composition of magmatic fluid. The basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase, hornblende, biotite, quartz, K-feldspar, titanite, and secondary epidote and chlorite. The calculated P–T conditions of the dioritic rocks are 680–725 °C and 4.9–6.4 kbar, which probably correspond to crystallization conditions. Fluid inclusion studies suggest that low- to medium-salinity (0–6.4 wt.% NaCleq) H2O-rich fluids are trapped during the crystallization of quartz and plagioclase. The isochore range calculated for primary aqueous inclusions is consistent with the P–T condition obtained by geothermobarometry. The basement rocks likely crystallized at a depth of 17–22 km, with a minimum average exhumation rate of 12–15 m/Ma during Palaeoproterozoic to Lopingian time. Such slow exhumation indicates low relief continental shield surface during this period.  相似文献   

12.
Solutions of H2O–NaCl–CH4 occur in fluid inclusions enclosed by quartz, apatite and feldspar from gabbroic pegmatitites, anorthositic structures and intercumulus minerals within the Skaergaard intrusion. The majority of the fluid inclusions resemble 10 m diameter sub-to euhedral negative crystals. A vapour phase and a liquid phase are visible at room temperature, solids are normally absent. The salinity of the fluids ranges from 17.5 to 22.8 wt.% NaCl. CH4, which comprises less than six mole percent of the solution, was detected in the vapour phase of the fluid inclusions with Raman microprobe analysis. Homogenization of the fluid inclusions occurred in the liquid phase in the majority of the fluid inclusions, though 10% of the inclusions homogenized in the gas phase. Thermodynamic consideration of the stability of feldspars + quartz, and the C–O–H system, indicates that the solutions were trapped at temperatures between 655 and 770°C, at oxygen fugacities between 1.5 and 2.0 log units below the QFM oxygen buffer. Textural evidence and the composition of the solutions suggest that the fluids coexisted with late-magmatic intercumulus melts and the melts which formed gabbroic pegmatites. These solutions are thought to have contributed to late-magmatic metasomatism of the primocryst assemblages of the Skaergaard intrusion.  相似文献   

13.
Carbonic fluid inclusions were observed in quartz-bearing veins at the Proterozoic Bidjovagge AuCu deposit within the Kautokeino greenstone belt in Norway, where mineralization occurred in oxidation zones of graphitic schists. A primary fluid inclusion zonation was observed with C02-rich fluid inclusions in the structural footwall of the deposit, and CH4-rich inclusions within the ore zone in the oxidation zone. Microthermometry of the primary hydrocarbon inclusions revealed 2 groups; (1) a group which homogenized between −125°C and the critical temperature of CH4 (−82.1°C), which indicated the presence of pure CH4, and (2) a group which homogenized between the critical temperature of CH4 and −42°C, which indicated the presence CH4 and higher hydrocarbons (HHC). Raman microprobe analyses of the first group confirmed the presence of CH4. The second inclusion group were fluorescent, and Raman spectra clearly displayed CH4,C2H6, and rarer C3H8 peaks. A typical feature of the Raman spectra were elevated baselines at the hydrocarbon peaks. Carbon peaks were also usually detected in each inclusion by Raman analysis. Bulk gas chromatography analyses of samples containing the first group (CH4) indicated the presence of CH4 and low concentrations of C2H6 and C3H8. Gas chromatography analyses of samples containing the second group (CH4 and higher hydrocarbons) confirmed the presence of CH4, and higher hydrocarbons such as C2H6 and C3H8 and also butanes. Based on the spacial zonation of hydrocarbons and the estimated PT conditions of 300 to 375°C and 2 to 4 kbars, the authors suggest an abiotic origin for the hydrocarbons. It is suggested that the hydrothermal fluids oxidized the graphitic schist, precipitated Cu and Au and formed light gas hydrocarbons.  相似文献   

14.
Fluid inclusions and mineral associations were studied in late-stage charnockitic granites from the Bjerkreim-Sokndal lopolith (Rogaland anorthosite province). Because the magmatic and tectonic evolutions of this complex appear to be relatively simple, these rocks are a suitable case for investigation of the origin and evolution of granulitic fluids. Fluid inclusions, primarily contained in quartz, can be divided into four types: carbonic (type I), N2-bearing (type II), CO2+H2O (type III) and aqueous inclusions (type IV). For each type, the role of leakage and fluid mixing are discussed from microthermometric and Raman spectrometric data. The most striking features of CO2-rich inclusions (the predominant fluid) is the presence of graphite in numerous, trail-bound inclusions (Ib) and its absence in a few isolated, very dense (d=1.16), pure CO2 inclusions (Ia) and in the late carbonic inclusions (Ic). Fluid chronology and mineral assemblages suggest that carbonic Ia inclusions represent the first fluid (pure CO2) trapped at or close to magmatic conditions (T=780–830° C, fO2=10-15 atm and P=7.4±1 kb), outside the graphite stability field. In contrast, type Ib inclusions enclosed graphite particles from a channelized fluid during retrograde rock evolution (P=3–4 kb and T=600° C). Decreases in T-fO2 could explain a progressive evolution from a CO2-rich fluid to an H2O-rich fluid in a closed C–O–H system. However, graphite destabilization observed in type Ic inclusions implies some late introduction of external water during the last stage of retrogression. The main results of this study are the following: (1) a carbonic fluid was present in an early stage of rock evolution (probably in the charnockitic magma) and (2) this granulite occurrence offers good evidence of crossing the graphite stability field during post-magmatic evolution.  相似文献   

15.
The structure of H2O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na2O·4SiO2–H2O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H2O at nominal densities, ρfluid, (from EOS of pure H2O) of 0.90 and 0.78 g/cm3. With ρfluid = 0.78 g/cm3, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρfluid = 0.90 g/cm3, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na2O·4SiO2 starting material.Molecular H2O (H2O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H2O)-ratio is positively correlated with temperature. The OH/(H2O)° in melts is greater than in coexisting fluids. Structural units of Q3, Q2, Q1, and Q0 type are observed in all phases under all conditions. An expression of the form, 12Q3 + 13H2O2Q2 + 6Q1 + 4Q0, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.  相似文献   

16.
 Equilibrium C–O–H fluid speciation calculations predict that graphite will precipitate from initially graphite saturated fluid inclusions during cooling and exhumation of metamorphic rocks. In the case that no mass is gained or lost by the inclusions, the original X O ratio [O/(O+H)] of the fluid phase must be maintained. Given this closed system constraint, the down-temperature progress of graphite precipitation can easily be monitored as a function of the varible X O, and produces some effects that are of significance to fluid inclusion studies: 1. Variation of the H2O : CO2 : CH4 relationship in the graphite-saturated COH fluid, namely increase of X H2 O and decrease of the carbonic fraction; 2. Decrease of fluid density due to precipitation of graphite, which is denser than the residual fluid; 3. Alteration of the CO2 : CH4 ratio of the fluid, depending on the initial O : H ratio of the fluid: for X O>1/3, fluids increase their CO2 : CH4 ratio with decreasing temperature, and vice-versa. This implies that the CO2 : CH4 ratio measured at room T will not represent the trapping value, which is in any case closer to unity. As a consequence of density reduction, isochores extrapolated from densities observed at room temperature do not pass through the pressure-temperature conditions at which the inclusion was trapped, with pressure underestimates of up to 2 kbar. Actual P-T trapping conditions are located along the equilibrium “bulk isochore” (curve of constant-X O, constant-volume) of the fluid. Alteration of the CO2 : CH4 ratio is a mechanism by which a CO2-rich or CH4-rich carbonic phase can be formed from aqueous fluids that are slightly off the neutral X O=1/3 value. Subsequent segregation of this phase from the aqueous counterpart may account for the formation of pure CO2 and CH4 fluids in the upper crust. Received: 15 March 1995 / Accepted: 1 June 1995  相似文献   

17.
We collected sediment samples and pore water samples from the surface sediment on the Daini Atsumi Knoll, and analyzed the sediments for CH4, C2H6, and δ13CCH4, and the pore fluids for CH4, C2H6, δ13CCH4, Cl−, SO42−, δ18OH2O, and δDH2O, respectively. A comparison of the measured concentration and isotopic composition of methane in pore water samples with those in sediment samples revealed that methane was present in the sediment samples at a higher concentration and was isotopically heavier than those in the pore water samples. It suggests that the effect of the release of a sorbed gas bound to organic particles when heated prior to analysis of hydrocarbons was larger than that of the degassing process. A large amount of a sorbed gas would be a significant source of natural gas. Two striking features are the chemical and isotopic composition of the pore water samples taken from the different sites around the Daini Atsumi Knoll. In the KL09, KL10, and KP07 samples, Cl− concentrations in the pore water samples showed depletion to a minimum of 460 mmol/kg, correspond to  17% dilution of seawater, however the latter was not enriched in CH4. The isotopic compositions of pore water samples suggested the low-Cl− fluids in the pore water were not derived from dissociation of methane hydrate, but were derived from input of meteoric water. In contrast, in the KP05 samples from the north flank of the Daini Atsumi Knoll, pore water were characterized by CH4 enrichment more than 370 μmol/kg, but not depleted in Cl− concentrations. The observed methane concentration in the KP05 samples is not sufficient for methane hydrate to form in situ, indicating that the existence of methane hydrate in the surface sediment is negligible, as supported by Cl− concentration. Based on the stable carbon isotope ratio of methane in the pore fluid from the KP05 site (δ13CCH4 < − 50‰PDB), methane is thought to be of microbial origin. The pore waters in the surface sediments in the north flank of the Daini Atsumi Knoll were not directly influenced by upward fluid bearing methane of thermogenic origin from a deeper part of the sedimentary layer. However, extremely high methane concentration in the north flank site as compared with the concentration of pore water taken from the normal seafloor suggests that the north flank site is not the normal seafloor. We hypothesize that upward migration of chemically-reduced fluids from a deeper zone of the sedimentary layer reduces chemically-oxidized solutes in the surface sediment. As a consequence methane production replaced sulfate reduction as the microbial metabolism in the reduced environment of the surface sediment.  相似文献   

18.
Phase equilibria in the system CaO–MgO–SiO2–CO2–H2O–NaCl are calculated to illustrate phase relations in metacarbonates over a wide-range of P–T–X[H2O–CO2–NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (Geochim Cosmochim Acta 59:2869–2882, 1995) for H2O–CO2–NaCl fluids and the internally consistent data set of Gottschalk (Eur J Mineral 9:175–223, 1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P–T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H2O–CO2–NaCl system. Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in the presence of a single phase H2O–CO2 fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO2 is immiscible and may separate from the brine as a low salinity, low density H2O–CO2 fluid. Most parameters of fluid–rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.  相似文献   

19.
The origin of Neoproterozoic intrusions (ca. 860–750 Ma) along the western part of the Yangtze Craton has been the subject of debate in recent years, with two competing models proposed. The plume model argues for an extensional setting and emphasizes the role of a superplume in the Rodinia breakup, whereas the arc model argues for the presence of a subduction zone in the Yangtze Craton. As a contribution to this animated dispute, geochronologic and geochemical analyses have been carried out on the Mianning granite, which is the largest pluton (700 km2) in the northern Kangdian rift of the western Yangtze Craton. It is shown that the Mianning granites were emplaced at ca. 780 Ma and display highly fractionated feature (i.e., SiO2 > 75 wt%; Eu/Eu* = 0.03–0.50; enrichment of K, Rb, Th, U, Zr, Hf, Y and REEs; depletion of Nb, Ta, Ba, Sr, P, Eu and Ti). They are metaluminous to strongly peraluminous (A/CNK = 0.93–1.55) and contain abundant perthite and minor alkali riebeckite and sphene, sharing the petrological and geochemical characters of A2-type granites. Positive Nd (t) (2.97–5.24) and zircon Hf (t) (9.2–12.1) values are consistent with a derivation by partial melting of a relatively young crust formed about 1000–900 Ma. Given the general absence of A-type granites in arc settings, the Mianning A-type granites are suggestive of an anorogenic, crustal extensional environment for the western Yangtze Craton during the Neoproterozoic. The data presented in this study are therefore consistent with an intracontinental rift model, but are not sufficient to identify plume involvement in the Neoproterozoic magmatism.  相似文献   

20.
The composition of a reduced C–O–H fluid was studied by the method of chromatography–mass spectrometry under the conditions of 6.3 GPa, 1300–1400°C, and fO2 typical of the base of the subcratonic lithosphere. Fluids containing water (4.4–96.3 rel. %), methane (37.6–0.06 rel. %), and variable concentrations of ethane, propane, and butane were obtained in experiments. With increasing fO2, the proportion of the CH4/C2H6 peak areas on chromatograms first increases and then decreases, whereas the CH4/C3H8 and CH4/C4H10 ratios continually decrease. The new data show that ethane and heavier HCs may be more stable to oxidation, than previously thought. Therefore, when reduced fluids pass the “redox-front,” carbon is not completely released from the fluid and may be involved in diamond formation.  相似文献   

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