Experimental studies of equilibrium iron isotope fractionation in ferric aquo-chloro complexes     

Pamela S. Hill  Edwin A. Schauble  Eric Tonui 《Geochimica et cosmochimica acta》2009,73(8):2366-6654

Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H₂O)₆³⁺, FeCl(H₂O)₅²⁺, FeCl₂(H₂O)₄⁺, FeCl₃ (H₂O)₃, and FeCl₄^-), using the Fe-Cl-H₂O system as a simple, easily-modeled example of the larger variety of iron-ligand compounds, such as chlorides, sulfides, simple organic acids, and siderophores. Isotopic fractionation (⁵⁶Fe/⁵⁴Fe) among naturally occuring iron-bearing species at Earth surface temperatures (up to ∼3‰) is usually attributed to redox effects in the environment. However, theoretical modeling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar magnitude due to non-redox changes in speciation (i.e., ligand bond strength and coordination number). In the present study, fractionations are measured in a series of low pH ([H⁺] = 5 M) solutions of ferric chloride (total Fe = 0.0749 mol/L) at chlorinities ranging from 0.5 to 5.0 mol/L. Advantage is taken of the unique solubility of FeCl₄^- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Δ⁵⁶Fe_aq-eth = δ⁵⁶Fe (total Fe remaining in aqueous phase)−δ⁵⁶Fe (FeCl₄^- in ether phase) is determined for each solution via MC-ICPMS analysis.Both experiments and theoretical calculations of Δ⁵⁶Fe_aq-eth show a downward trend with increasing chlorinity: Δ⁵⁶Fe_aq-eth is greatest at low chlorinity, where FeCl₂(H₂O)₄⁺ is the dominant species, and smallest at high chlorinity where FeCl₃(H₂O)₃ is dominant. The experimental Δ⁵⁶Fe_aq-eth ranges from 0.8‰ at [Cl^-] = 0.5 M to 0.0‰ at [Cl^-] = 5.0 M, a decrease in aqueous-ether fractionation of 0.8‰. This is very close to the theoretically predicted decreases in Δ⁵⁶Fe_aq-eth, which range from 1.0 to 0.7‰, depending on the ab initio model.The rate of isotopic exchange and attainment of equilibrium are shown using spiked reversal experiments in conjunction with the two-phase aqueous-ether system. Equilibrium under the experimental conditions is established within 30 min.The general agreement between theoretical predictions and experimental results points to substantial equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in ⁵⁶Fe/⁵⁴Fe as the Cl^-/Fe³⁺ ion ratio increases. The effects on isotopic fractionation shown by the modeling of this simple iron-ligand system imply that ligands present in an aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due to other speciation effects (such as iron-sulfide systems or iron bonding with organic ligands), and must be considered when interpreting iron isotope fractionation in the geological record.  相似文献   

Experimental study of Pd hydrolysis in aqueous solutions at 25–70°C     

L. A. Koroleva  N. D. Shikina  P. G. Kolodina  A. V. Zotov  B. R. Tagirov  Yu. V. Shvarov  V. A. Volchenkova  Yu. K. Shazzo 《Geochemistry International》2012,50(10):853-859

The hydrolysis of the Pd²⁺ ion in HClO₄ solutions was examined at 25–70°C, and the thermodynamic constants of equilibrium K ₍₁₎⁰ and K ₍₂₎⁰were determined for the reactions Pd²⁺ + H₂O = PdOH⁺ + H⁺ and Pd²⁺ + 2H₂O = Pd(OH)₂⁰ + 2H⁺, respectively. The values of log K ₍₁₎⁰ = −1.66 ± 0.5 (25°C) and −0.65 ± 0.25 (50°C) and log K ₍₂₎⁰ = −4.34 ± 0.3 (25°C) and −3.80 ± 0.3 (50°C) were derived using the solubility technique at 0.95 confidence level. The values of log K ₍₁₎⁰ = −1.9 ± 0.6 (25°C), −1.0 ± 0.4 (50°C), and −0.5 ± 0.3 (70°C) were obtained by spectrophotometric techniques. The palladium ion is significantly hydrolyzed at elevated temperatures (50–70°C) even in strongly acidic solutions (pH 1–1.5), and its hydrolysis is enhanced with increasing temperature.  相似文献   

Spectroscopic investigation of the formation of PuO2Cl+ and PuO2Cl2 in NaCl solutions and application for natural brine solutions     

《Geochimica et cosmochimica acta》1999,63(19-20):3443-3449

The chloride complexation of the PuO₂²⁺ ion has been studied in acidic NaCl solutions with electrolyte concentrations as high as 5 mol kg⁻¹ at 23°C by using conventional absorption spectrophotometry. Plutonyl and its complexes have ionic strength-dependent molar absorptivities that were determined in NaClO₄, the first essential step in the quantitative analysis of chloride complexation. The distributions of species for the Pu complexes, PuO₂²⁺, PuO₂Cl⁺, and PuO₂Cl₂^o, formed under the conditions investigated, were determined by peak-fitting of optical absorption spectra. The apparent stability constants of the Pu(VI) chloro complexes were calculated at each NaCl concentration. Specific ion-interaction theory parameters were determined for the plutonyl chloro complexes and the electrolyte constituents, then compared with the literature data. The calculated values for log β° were determined to be 0.23 ± 0.03 and −1.7 ± 0.2 for the mono and bis chloro complexes, respectively. Spectra of Pu(VI) in brines representative of waters at the Waste Isolation Pilot Plant, the licensed nuclear waste repository in a salt formation at Carlsbad, NM, USA, were measured and modeled by using the thermodynamic data and ion interaction parameters were determined. In these brines, less than 10% of the total Pu(VI) concentration exists as the Pu(VI) aquo ion, whereas about 90% is present as Pu(VI) chloro complexes.  相似文献   

Ab initio calculations of 17O and nT NMR parameters (nT=31P, 29Si) in H3 TOTH3 dimers and T 3O9 trimeric rings     

C. G. Lindsay  J. A. Tossell 《Physics and Chemistry of Minerals》1991,18(3):191-198

NMR shieldings (σ) and electric field gradients (eq) are calculated using ab initio methods at the O and T nuclei (where T=P, Si) in two different types of molecules-TH₃ dimers, i.e. H₃SiOSiH₃ and H₃POPH ₃ ²⁺ , and TO₄ trimeric rings, i.e., Si₃O ₉ ^6- and P₃O ₉ ^3- , which serve as models for assessing the effects of polymerization, bond length and bond angle variation on the NMR properties of polymerized silicates and phosphates. In agreement with earlier ab initio studies on H₃SiOSiH₃ we confirm that σ(²⁹Si), σ(³¹P), σ(¹⁷O) and eq(¹⁷O) all decrease as θ(SiOSi) decreases in the range from 180° to 100°. However, correction for artifacts due to distant core electrons leads to a considerably reduced value for the anisotropy in σ ^O, bringing it into better agreement with estimated experimental values. The qualitative change in σ(²⁹Si) with θ(SiOSi) can be understood on the basis of changes in the energies of the highest energy occupied MO's and consequent variations in their contributions to the paramagnetic part of the shielding. For H₃POPH ₃ ²⁺ we calculate a larger value of eq^O than for the analog Si compound but the same type of variation of σ(¹⁷O) with θ(TOT). The change in σ(³¹P) with θ(POP) is, however, calculated to be much smaller than in the Si case and a maximum is predicted for intermediate angles. For the trimeric rings we obtain energy optimized geometries in good agreement with x-ray structural data, with T-O terminal distances systematically shorter than the T-O bridging distances. Calculated σ(T) anisotropies are also in good agreement with experiment and can be simply related to the calculated structure. After correction for distant core effects we obtain a change in σ(³¹P) between PO ₄ ^3- and P₃O ₉ ^3- in reasonable agreement with experiment.  相似文献   

Crystal chemistry of selenates with mineral-like structures: V. Crystal structures of (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 and (H3O)2[(UO2)(SeO4)2(H2O)](H2O), new compounds with rhomboclase and goldichite topology     

S. V. Krivovichev 《Geology of Ore Deposits》2008,50(8):789-794

The crystal structures of two new compounds (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O)₂ (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) ų) and (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O) (2, monoclinic, P2₁/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) ų) have been solved by direct methods and refined to R ₁ = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO₂)(SeO₄)₂]^2? formed by cornershared [(UO₂)O₄(H₂O)] bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO₂)(SeO₄)₂(H₂O)]^2? layers are linked by hydrogen bonds via interlayer groups H₂O and H₃O⁺. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H₂O₂)⁺[Fe(SO₄)₂(H₂O)₂] and goldichite K[Fe(SO₄)₂(H₂O)₂](H₂O)₂, respectively.  相似文献   

Experimental determination of zirconium speciation in hydrothermal solutions     

B. N. Ryzhenko  N. I. Kovalenko  N. I. Prisyagina  N. P. Starshinova  V. V. Krupskaya 《Geochemistry International》2008,46(4):328-339

The solubility of ZrO₂(baddeleyite) in HCl, HF, H₂SO₄, NaOH, and Na₂CO₃ solutions was determined by the capsule method at 500°C and 1000 bar. Baddeleyite is the only solid phase detected in the experimental products. Based on the ZrO₂(baddeleyite) solubility measurements, the values of equilibrium constants at 500°C and 1000 bar (consistent with the Gibbs free energies of all the reactants) were obtained for the following reactions: ZrO₂(cr) + H₂SO ₄ ⁰ = Zr(OH)₂OH ₄ ⁰ (pK^o = 4.95), ZrO₂(cr) + 2H₂SO ₄ ⁰ = Zr(SO₄) ₂ ⁰ ) + 2H₂O (pK^o = 3.74), ZrO₂(cr) + H₂O + HF⁰ = Zr(OH)₃F⁰ (pK^o = 3.35), ZrO₂(cr) + 2HF⁰ = Zr(OH)₂F ₂ ⁰ (pK^o = 2.37), and ZrO₂(cr) + 2H₂O + OH^? = Zr(OH) ₅ ^? (pK^o = 4.39). Ionization constants were estimated for the chloride, fluoride, sulfate, and hydroxo complexes of zirconium. Using the experimental data and thermodynamic information derived from experiments and the electrostatic model of the ionization of electrolytes, it was shown that no more than n mg zirconium per one kilogram H₂O can be accumulated in high-temperature fluids at 500°C and 1000 bar.  相似文献   

Theoretical and experimental investigation of nickel isotopic fractionation in species relevant to modern and ancient oceans     

Toshiyuki Fujii  Frédéric Moynier  Minori Abe 《Geochimica et cosmochimica acta》2011,75(2):469-7643

Nickel plays a central role as an enzyme co-factor in the metabolism of methanogenic Archaea. Methanogens can fractionate Ni isotopes during assimilation, opening the possibility of using the stable isotopic composition of Ni as a biomarker. However, other sources of Ni isotopic variations need to be evaluated before one can establish Ni isotopes as an unambiguous tracer of methanogenesis in the rock record. Equilibrium exchange of Ni between the different species present in the ocean is a potential source of isotopic fractionation. Through controlled laboratory experiments and theoretical calculations, we quantify equilibrium Ni isotope fractionation between different species relevant to the modern and ancient ocean: Ni(H₂O)₆²⁺, Ni(H₂O)₁₈²⁺, NiOH(H₂O)₅⁺, Ni(OH)₂(H₂O)₄, NiCl(H₂O)₅⁺, cis-NiCl₂(H₂O)₄, trans-NiCl₂(H₂O)₄, NiHS(H₂O)₅⁺, Ni(HS)₂(H₂O)₄, NiSO₄(H₂O)₄, NiHCO₃(H₂O)₄⁺, NiCO₃(H₂O)₄, and organic ligands (crown ether and oxalic acid). The magnitude of ligand-controlled Ni isotopic fractionation, approximately 1.25‰/amu (2.5‰ for the ⁶⁰Ni/⁵⁸Ni ratio), is similar to that previously measured in cultures of methanogenic Archaea.  相似文献   

The stability of palladium(II) hydroxide and hydroxy–chloride complexes: an experimental solubility study at 25–85°C and 1 bar     

《Geochimica et cosmochimica acta》1999,63(11-12):1751-1765

Solubility methods were employed to determine conditional equilibrium constants for the formation of hydroxide and mixed hydroxy–chloride complexes of Pd(II). Measurements were made over a temperature range of 25–85°C, a pH range from 0 to 12, and ionic strengths of 0.1, 0.2, 0.5 and 1.0 molal in both KCl and NaClO₄ media. Several speciation models were fit to the data using nonlinear regression, and the model yielding the best fit with the fewest number of species was accepted for each temperature and ionic strength. The conditional equilibrium constants were then fit to a function of ionic strength and temperature (including a Debye–Hückel term) to facilitate interpolation and extrapolation to infinite dilution. The following species were found to be important in KCl solutions: PdCl₄²⁻, PdCl₃(OH)²⁻, and Pd(OH)₂⁰. The relative proportions of the species are dependent on pH and ionic strength (chloride concentration). In perchlorate media the predominant species were Pd(OH)₃⁻, Pd(OH)₂⁰, PdOH⁺ and Pd²⁺, depending on pH. Conditional stability constants determined in this study agree well with those reported in previous work for the simple chloride and hydroxide complexes, but our results suggest that mixed complexes may be more important than previously thought, and that PdCl₃(OH)²⁻ may be the dominant species in seawater, followed by Pd(OH)₂⁰.  相似文献   

Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases     

Gleb S PokrovskiDenis Testemale  Jean-Louis HazemannAndrew Y.u Bychkov  Galina V Golikova 《Geochimica et cosmochimica acta》2002,66(19):3453-3480

The stoichiometry and stability of arsenic gaseous complexes were determined in the system As-H₂O ± NaCl ± HCl ± H₂S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As^(III) and As^(V) vapor-liquid and vapor-solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As^(III)-bearing aqueous fluids. Vapor-aqueous solution partitioning for As^(III) was measured from 250 to 450°C at the saturated vapor pressure of the system (P_sat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H₃AsO₃) between an aqueous solution (pure H₂O) and its saturated vapor (K = mAs_vapor /mAs_liquid) were found to be independent of As^(III) solution concentrations (up to ∼1 to 2 mol As/kg) and equal to 0.012 ± 0.003, 0.063 ± 0.023, and 0.145 ± 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH)₃(gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H₂S (up to ∼0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As^(III)-H₂O (±NaCl) solutions up to 500°C demonstrate that the As(OH)₃ complex dominates As speciation both in dense H₂O-NaCl fluids and low-density supercritical vapor. Vapor-liquid partition coefficients for As^(III) measured in the H₂O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic^(III) partitioning between vapor and As-concentrated solutions (>2 mol As/kg) or As₂O₃ solid is consistent with the formation, in the vapor phase, of both As₄O₆ and As(OH)₃. Arsenic^(V) (arsenic acid, H₃AsO₄) vapor-liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH)₃ in the vapor phase.The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH)₃(gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH)₃(gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn, -W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H₂O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.  相似文献   

Speciation of gold in hydrosulphide-rich ore-forming fluids: Insights from first-principles molecular dynamics simulations   总被引：1，自引：0，他引：1  

Xiandong Liu  Xiancai Lu  Rucheng Wang  Huiqun Zhou  Shijin Xu 《Geochimica et cosmochimica acta》2011,75(1):185-194

To shed light on gold speciation in sulfur-containing ore-forming fluids, we perform first principles molecular dynamics (FPMD) simulations to investigate gold-hydrosulphide complexing under representative geological conditions. With this advanced technique, the electronic structures of solutes and solvents are calculated with density functional theory and the thermal motions are sampled with molecular dynamics. The molecular structures, solvated structures and stabilities of possible complexes are characterized in detail and the following insights have been gained. (1) The previously hypothesized species Au(HS)(H₂S)₃ and Au(HS) are found unstable under ore-forming conditions. Au(HS)(H₂S)₃ would dissociate to LAu(HS) (L = H₂S or H₂O) and free H₂S molecules spontaneously. Au(HS) is highly reactive and tends to capture a second ligand to form a double-coordinated complex. (2) In the thin vapor-like phases of low pressures, the stable complexes include Au(HS)(H₂O), Au(HS)(H₂S) and Au(HS)₂⁻ and their relative stability is Au(HS)₂⁻ > Au(HS)(H₂S) > Au(HS)(H₂O). In dense aqueous phases of high pressures, Au(HS)(H₂S) would spontaneously deprotonate to Au(HS)₂⁻ and thus Au(HS)(H₂O) and Au(HS)₂⁻ are the stable forms. All of these complexes can retain to the upper-limit of ore-forming temperatures. (3) The gold ions in the complexes do not favor coordinating more molecules and therefore the solvations happen mainly through H-bonding interactions between the ligands and environmental waters. H-bonds are found in vapor, liquid, and dense supercritical phases, whereas in the thin supercritical phase the hydration is very weak. These results provide quantitative and microscopic basis for understanding the speciation of gold in hydrothermal fluids.  相似文献   

Ultraviolet spectroscopic evidence for polynuclear lead (II) complexes in synthetic Red Sea brines     

P. Tsai  R.P. Cooney 《Chemical Geology》1976,18(3):187-202

The Pb complexes in a synthetic brine solution with a composition comparable to that of the Atlantis II Deep 56°C Brine have been identified by UV absorption spectroscopy. The important species at ambient temperatures are PbCl^2?₄ and Pb(OH)BrCl₂^2?. At 56°C the former complex partially dissociates to form lower chloro complexes while the latter undergoes halide exchange reactions forming Pb(OH)Br₂Cl^2?. Evidence has also been found for the following polymeric lead hydroxo complexes: Pb₄(OH)₄Cl₄, Pb₆(OH)₃Cl₁₂ and Pb₃(OH)₁₂Cl₄. The predominant polynuclear complex in the brine, Pb₄(OH)₄Cl₄, tends to dissociate at 56°C to form lower polymeric species. The formulation of the limiting binary chloro complex as PbCl₄^2? rather than PbCl₆^4? is supported by the reflectance spectrum of Cs₄PbCl₆.  相似文献   

Physicochemical modeling as applied to study of sulfoarsenide complexes in hydrothermal solutions     

《Russian Geology and Geophysics》2007,48(6):457-467

System As–Na–S–Cl–H–O was studied. The research was carried out in three stages: (1) selection of the most likely complexes resulting from arsenic sulfide dissolution, (2) calculation of their thermodynamic constants, and (3) comparison of calculated data with thermodynamic database obtained in tests with the solution of inverse thermodynamic problems using the Selektor program complex. The system As–Na–S–Cl–H–O included more than 230 dependent components, which were divided into two groups, base and functional. The former group includes components of the solution (NaCl, NaOH, Na₂S, NaHS, HCl, H₂S, H₂SO₄, sulfates, H₂SO₃, sulfites, thiosulfates, Na⁺, Cl⁻,HS⁻, S²⁻), gas phase (43 components), and solid phase (orpiment, red arsenic, arsenolite, claudetite, arsenic, sulfur, sodium salts). Thermodynamic constants of the base components are contained in the Selektor database (they were borrowed from reference-books). The latter group includes 77 complexes labile in the solution but determining the solubility of arsenic and stability of its solid phases. Physicochemical modeling was performed in H₂S (≥0.01 m, pH = 1–10), Na₂S, and NaHS solutions at 25–250 °C and saturated-vapor pressure. It has been established that the dissolution of arsenic sulfide mineral phases in subneutral and alkaline solutions at low oxidation potential is favored by the formation of sulfoarsenides, which are more stable than arsenides and arsenates. Thermodynamic constants of functional complexes determining the orpiment solubility were calculated within the experimental error. It is shown that in hydrothermal iron-free systems with a low oxidation potential, the concentration of As in the solution decreases on cooling and with acidity increase.  相似文献   

Experimental study of chloritoid stability at high pressure and various fO2 conditions     

Olivier Vidal  Thomas Theye  Christian Chopin 《Contributions to Mineralogy and Petrology》1994,118(3):256-270

The reaction chloritoid (ctd)=almandine (alm)+diaspore+H₂O (CAD) has been reversed using Fe³⁺-free synthetic chloritoid and almandine, under fO₂ conditions of the solid oxygen buffer Fe/FeO (CADWI), and using partially oxidized synthetic minerals under fO₂ conditions of the solid oxygen buffer Ni/NiO (CADNNO). Experiments have been conducted between 550 and 700°C, 25 and 45 kbar. The equilibrium pressure and temperature conditions are strongly dependent on the fO₂ conditions (CADNNO lies some-what 50°C higher than CADWI). This can be explained by a decrease in aH₂O for experiments conducted on the Fe/FeO buffer, and a decrease in actd and aalm (through incorporation of ferric iron preferentially in chloritoid) for experiments conducted on the Ni/NiO buffer. The H₂O activity has been calculated using the MRK equation of state, and the values obtained checked against the shift of the equilibrium diaspore=corundum+H₂O bracketed on the Fe/FeO buffer and under unbuffered fO₂ conditions. For fO₂ buffered by the assemblage Fe/FeO, aH₂O increases with pressure from about 0.85 at 600°C, 12 kbar to about 0.9 at 605°C, 25 kbar and 1 above 28 kbar. For fO₂ buffered by the assemblage Ni/NiO, aH₂O=1. The aH₂O decrease from Ni/NiO to Fe/FeO is, however, too small to be entirely responsible for the temperature shift between CADNNO and CADWI. In consequence, the amount of ferric iron in almandine and chloritoid growing in the CADNNO experiments must be significant and change along the CADNNO, precluding calculation of the thermodynamic properties of chloritoid from this reaction. Our experimental data obtained on the Fe/FeO buffer are combined, using a thermodynamic analysis, with Ganguly's (1969) reversal of the reaction chloritoid=almandine+corundum +H₂O (CAC) on the same oxygen buffer. Experimental brackets are mutually consistent and allow extraction of the thermodynamic parameters H ^o f,ctd and S ^octd. Our thermodynamic data are compared with others, generally calculated using Ganguly's bracketing of CACNNO. The agreement between the different data sets is relatively good at low pressure, but becomes rapidly very poor toward high pressure conditions. Using our thermodynamic data for chloritoid and K_D=(Fe³⁺/Al)ctd/(Fe³⁺/Al)alm estimated from natural assemblages, we have calculated the composition of chloritoid and almandine growing from CADNNO and CACNNO. The Fe³⁺ content in chloritoid and almandine increases with pressure, from less than 0.038 per FeAl₂SiO₅(OH)₂ formula unit at 10 kbar to at least 0.2 per formula unit above 30 kbar. This implies that chloritoid and almandine do contain Fe³⁺ in most natural assemblages. The reliability of our results compared to natural systems and thermodynamic data for Mg-chloritoid is tested by comparing the equilibrium conditions for the reaction chloritoid+quartz=garnet (gt)+kyanite+H₂O (CQGK), calculated for intermediate Fe–Mg chloritoid and garnet compositions, from the system FASH and from the system MASH. For 0.65<(XFe)gt<0.8, CQKG calculated from FASH and MASH overlap for K_D=(Mg/Fe)ctd/(Mg/Fe)gt=2. This is in good agreement with the K_D values reported from chloritoid+garnet+quartz+kyanite natural assemblages.  相似文献   

显生宙全球海水化学成分演化及其对蒸发岩沉积的约束     

沈立建  刘成林 《岩石学报》2018,34(6):1819-1834

通过搜集显生宙以来不同地质时期内海相碳酸盐岩鲕粒及胶结物矿物成分、钾盐矿床矿物种类及组合特征、蒸发岩盆地中石盐流体包裹体成分,并利用这些资料与人工海水模拟实验得到的石盐中Br分配特征的对比,得出海水成分在5.5亿年以来的显生宙期间,经历了五个阶段:其中晚元古代至寒武纪早期、二叠纪早期至中生代早期、新生代早期至现今,这些时期的原始海水组成特征系数m(SO_4~(2-))+m(HCO_3~-)/2m(Ca~(2+)),为Na-Mg-K-SO_4-Cl型海水,此期间沉积的钾盐矿床的钾镁盐矿物主要为钾盐镁矾、无水钾镁矾、杂卤石、硫酸镁石等含MgSO_4矿物,海相鲕粒和碳酸盐胶结物矿物成分为文石;而寒武纪早期至石炭纪、中生代早期至新生代早期,原始海水组成特征系数m(Ca~(2+))m(SO_4~(2-))+m(HCO_3~-)/2,为Na-Mg-KCa-Cl型海水,此期间沉积的钾镁盐矿物主要为光卤石和钾石盐,甚至含有溢晶石,海相鲕粒和碳酸盐胶结物矿物成分为方解石。根据石盐流体包裹体成分计算得出:显生宙期间,海水K+含量大部分时间变化幅度较小,为9.3～11.5mmol/kg H_2O(除了石炭纪和晚元古代),平均为10.55mmol/kg H_2O。Mg~(2+)含量在早寒武世≥67mmol/kg H_2O、晚志留世至中泥盆世31～41mmol/kg H_2O、晚古生代≥48mmol/kg H22O、晚白垩世34mmol/kg H_2O和现代55.1mmol/kg H_2O。Ca~+含量在晚元古代至古生代早期≤11mmol/kg H_2O、古生代早期至石炭纪22～35mmol/kg H_2O、石炭纪至中生代早期≤17mmol/kg H_2O、中生代早期至新生代早期19～39mmol/kg H_2O及新生代早期至今7～21mmol/kg H_2O。SO_4~(2-)含量在晚元古代至古生代早期≥23mmol/kg H_2O、古生代早期至石炭纪5～17mmol/kg H_2O、石炭纪至中生代早期13～22mmol/kg H_2O、中生代早期至新生代早期5～19mmol/kg H_2O及新生代早期至今12～29.2mmol/kg H_2O。海水Ca~(2+)与SO_4~(2-)含量的相对变化是控制海相钾盐矿床钾镁盐矿物类型的基本因素。同时,利用以上数据计算得到的显生宙各时期海水[m(Mg~(2+))+m(SO_4~(2-))]/[m(K~+)+m(Ca~(2+))]的变化与各时期海相蒸发岩系石盐层底部的Br含量变化具有同步性,进一步验证了显生宙期间海水成分是不断变化的,是约束海相蒸发岩钾盐矿物类型的主要因素。海水成分变化的控制因素为洋中脊热液和陆地水,其中洋中脊热液起主要作用,而控制这些因素变化的根本原因为板块构造运动。  相似文献