Laboratory simulation of meteoritic noble gases. II. Sorption of xenon on carbon: Etching and heating experiments     

M.G. Zadnik  John F. Wacker  Roy S. Lewis 《Geochimica et cosmochimica acta》1985,49(4):1049-1059

Sixteen amorphous carbon (lampblack) samples that had been exposed to Xe¹²⁷ and pumped for >9 hrs to remove the most labile gas were examined by etching with HNO₃, for comparison with the release pattern of meteoritic xenon. Samples originally exposed at 100–200°C lost 90% of their Xe very readily, when the surface had been etched to a mean depth of only ~0.2 Å. This suggests that the Xe is adsorbed mainly at rare sites that are unusually reactive to HNO₃. The adsorbed Xe survived several months' storage in vacuum, but on exposure to air, part of it was lost within a few hours, while the remainder persisted without measurable exchange. Samples exposed at 800–1000°C had a similar adsorbed component, as well as a second, tightly bound component extending to a mean depth of up to 30 Å; this component had apparently diffused into the carbon during exposure. The (microscopic) diffusion coefficient for graphitic crystallites is 5 × 10^?20 cm²/sec at 1000°C.PVDC carbon lost its adsorbed Xe at about the same rate as lampblack on exposure to air or HNO₃, though it differs from lampblack in being non-graphitizable and more porous. It had only a small diffused component, however.The most tightly bound part of the Xe adsorbed on lampblack resembles planetary Xe in most characteristics: surface siting, etchability, persistence in vacuum, and lack of exchange with atmospheric Xe. The Xe concentrations—if interpreted as equilibrium distribution coefficients—are some 10⁶× too small to account for meteoritic Xe, but it appears that equilibrium had not been reached by any of the samples, even after 1 day's exposure to Xe. If the uptake of Xe is controlled by rate rather than equilibrium, then the high noble gas concentrations in meteorites may simply reflect the much longer uptake times in the solar nebula. It seems likely that the trapping mechanisms discussed here can also explain two other features: elemental fractionations of noble gases, and the close correlation between planetary Xe and CCFXe.  相似文献   

Sorption of noble gases by solids,with reference to meteorites. I. Magnetite and carbon     

Jongmann Yang  Roy S. Lewis  Edward Anders 《Geochimica et cosmochimica acta》1982,46(6):841-860

To simulate trapping of meteoritic noble gases by solids, 18 samples of Fe₃O₄ were synthesized in a noble gas atmosphere at 350–720 K by the reactions: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (Ne, Ar, Kr, Xe) 3Fe + 4CO → Fe₃O₃ + 4C + carbides (Xe only) Phases were separated by selective solvents (HgCl₂, HCl). Noble gas contents were analyzed by mass spectrometry, or, in runs where 36 d Xe¹²⁷ tracer was used, by γ-counting. Surface areas, as measured by the BET method, ranged from 1 to 400 m²/g. Isotopic fractionations were below the detection limit of 0.5%/m.u.Sorption of Xe on Fe₃O₄ and C obeys Henry's Law between $\text{1 × 10}{}^{\mathrm{?8}}$ and $\text{4 × 10}{}^{\mathrm{?5}}$ atm, but shows only a slight temperature dependence between 650 and 720 K ($\text{ΔH}{}_{\mathrm{sol}}\text{= ?4 ± 2}\text{kcal/mole}$). The mean distribution coefficient K_Xe is $\text{0.28 ± 0.09}$ cc STP/g atm for Fe₃O₄ and only a factor of $\text{1.2 ± 0.4}$ greater for C; such similarity for two cogenetic phases was predicted by Lewis et al. (1977). Stepped heating and etching experiments show that 20–50% of the total Xe is physically adsorbed and about 20% is trapped in the solid. The rest is chemisorbed with $\text{ΔH}{}_{\mathrm{s}}\text{? ?13}\text{kcal/mole}$. The desorption or exchange half-time for the last two components is >10² yr at room temperature.Etching experiments showed a possible analogy to “Phase $\text{Q}$” in meteorites. A typical carbon + carbide sample, when etched with HNO₃, lost 47% of its Xe but only 0.9% of its mass, corresponding to a ~0.6 Å layer. Though this etchable, surficial gas component was more thermolabile than $\text{Q}$ (release $\text{T}$ below 1000°C, compared to 1200–1600°C), another experiment shows that the proportion of chemisorbed Xe increases upon moderate heating (1 hr at 450°C). Apparently adsorbed gases can become “fixed” to the crystal, by processes not involving volume diffusion (recrystallization, chemical reaction, migration to traps, etc.). Such mechanisms may have acted in the solar nebula, to strengthen the binding of adsorbed gases.Adsorbed atmospheric noble gases are present in all samples, and dominate whenever the noble gas partial pressure in the atmosphere is greater than that in the synthesis. Many of the results of Lancet and Anders (1973) seem to have been dominated by such an atmospheric component; others are suspect for other reasons, whereas still others seem reliable. When the doubtful samples of Lancet and Anders are eliminated or corrected, the fractionation pattern—as in our samples—no longer peaks at Ar, but rises monotonically from Ne to Xe. No clear evidence remains for the strong temperature dependence claimed by these authors.  相似文献   

Noble gases in the Allende and Abee meteorites and a gas-rich mineral fraction: investigation by stepwise heating     

B. Srinivasan  Roy S. Lewis  Edward Anders 《Geochimica et cosmochimica acta》1978,42(2):183-198

Noble gases in three meteoritic samples were examined by stepwise heating, in an attempt to relate peaks in the outgassing curves to specific minerals: NeKrXe in Allende (C3V) and an Allende residue insoluble in HF-HCl, and Xe in Abee (E4). In Allende, chromite and carbon contain most of the trapped Ne (²⁰Ne/²²Ne ≈ 8.7) and anomalous Xe enriched in light and heavy isotopes, and release it at ~850°C (bulk meteorite) or 1000°C (residue). Mineral Q, containing most of the trapped Ar, Kr, Xe as well as some Ne (²⁰Ne/²²Ne ≈ 10.4), releases its gases mainly between 1200 and 1600°C, well above the release temperatures of organic polymers (300–500°) or amorphous carbon (800–1000°). The high noble-gas release temperature, ready solubility in oxidizing acids, and correlation with acid-soluble Fe and Cr all point to an inorganic rather than carbonaceous nature of Q.All the radiogenic ¹²⁹Xe is contained in HCl, HF-soluble minerals, and is distributed as follows over the peaks in the release curve: Attend 1000° (75%), 1300° (25%); Abee (data of Hohenberg and Reynolds, 1969) ~850° (15%), 1100° (60%), 1300° (25%). No conclusive identifications of host phases can yet be given; possible candidates are troilite and silicates for Allende, and djerfisherite, troilite and silicates for Abee.Mineral Q strongly absorbs air xenon, and releases some of it only at 800–1000°C. Dilution by air Xe from Q and other minerals may explain why temperature fractions from bulk meteorites often contain less ^124–130Xe for a given enrichment in heavy isotopes than does xenon from etched chromitecarbon samples, although chromite-carbon is the source of the anomalous xenon in either case. Air xenon contamination thus is an important source of error in the derivation of fission xenon spectra.  相似文献   

The thermodynamic properties and phase relations of some minerals in the system CaO-AI2O3-SiO2-H2O     

Dexter Perkins  Edgar F. Westrum  Eric J. Essene 《Geochimica et cosmochimica acta》1980,44(1):61-84

The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K^?1 mol^?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K^?1 mol^?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10^?3T ? 15.27 × 10⁵T^?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10^?3T ? 30.24 × 10⁵T^?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10^?3T ? 25.02 × 10⁵T^?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10^?3T ? 24.08 × 10⁵T^?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al₂O₃-SiO₂-H₂O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H₂O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.  相似文献   

Enthalpy effects associated with Al/Si ordering in anhydrous Mg-cordierite     

Michael A Carpenter  Andrew Putnis  Alexandra Navrotsky  J.Desmond C McConnell 《Geochimica et cosmochimica acta》1983,47(5):899-906

Measurements of the heats of solution (ΔH_soln) in molten Pb₂B₂O₅ at 708°C of anhydrous magnesian cordierites, prepared with a range of structural states, show that the enthalpy effect associated with Al/Si ordering is substantial (? 9.76 ± 1.56 kcal mole^?1). Differences in the state of order between synthetic cordierites used in phase equilibrium studies and cordierites in the natural environment could lead to significant errors in the estimation of palaeo-pressures and temperatures. A continuous change of ΔH_soln with annealing time supports the suggestion of putnis (1980) that the hexagonal → orthorhombic transformation in cordierite, which can occur via a modulated structure, is truly continuous under metastable conditions. In addition, a linear relation between ΔH_soln and the logarithm of annealing time has been found, which provides some insight into the nature of the ordering mechanisms at an atomic level. Al and Si exchanges occur continuously between neighbouring tetrahedral sites with a net drift towards increasing order. No kinetic or thermochemical distinction can be made between the development of long range and short range order.The enthalpy of vitrification (~ 12 kcal mole^?1) for a metastable stuffed β-quartz polymorph of cordierite composition is similar to that for pure quartz (on a per two oxygen basis), while the heat of vitrification for even the most disordered cordierite seen in this study is more than a factor of three greater (~40 kcal mole^?1). This is consistent with the view that cordierite glass resembles the quartz structure more closely than the crystalline cordierite structure, and that crystallisation of the glass below ~900°C is controlled by a tetrahedral framework.  相似文献   

Carbon isotopic thermometry calibrated by dolomite-calcite solvus temperatures   总被引：1，自引：0，他引：1  

Hideki Wada  Kazuhiro Suzuki 《Geochimica et cosmochimica acta》1983,47(4):697-706

The temperature dependence of carbon isotopic fractionations between calcite and graphite, and between dolomite and graphite are calibrated by the calcite-dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the Kasuga area, central Japan. The carbon isotopic fractionations (Δ¹³C_Cc-Gr and Δ¹³C_DoGr) systematically decrease with increasing metamorphic temperature. The concordant relationships between the fractionations and solvus temperatures are approximately linear with T^?2 over the temperature range. 400° to 680°C: Δ¹³C_CcGr (%.) = 5.6 × 10⁶ × T^?2 (K) ? 2.4 Δ¹³C_DoGr (%.) = 5.9 × 10⁶ × T^?2 (K) ? 1.9 These systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases. The equation for the calcite-graphite system has a slope steeper than Bottinga's (1969) results. It is, however, in good agreement with that of Valley and O'Neil (1981) in the temperature range from 600° to 800°C.Because of the relatively high sensitivity to temperature, these isotopic geothermometers are useful for determining the temperatures in moderate- to high-grade metamorphosed carbonate rocks.  相似文献   

Isotopic anomalies in meteorites and their origins—V. Search for fission fragment recoils in Allende sulfides     

Roy S. Lewis  Jan Hertogen  Leo Alaerts  Edward Anders 《Geochimica et cosmochimica acta》1979,43(11):1743-1752

Three troilite- and pentlandite-rich samples from the Allende C3 chondrite were analyzed for Xe (and in one case Ne and Ar) by mass spectrometry, in 13–22 temperature steps. All samples released a small ‘CCFXe’ component (enriched in the heavy isotopes Xe^{134, 136}) at the relatively low temperature of 700–800°C, ahead of adsorbed atmospheric Xe (~900°C), radiogenic Xe¹²⁹ (1000°C), and primordial Xe (1250°C). Though such a labile component suggests implanted fission recoils, the simultaneous release of Ne, Ar, and Xe^{124, 126} shows that it instead comes from carbon and perhaps chromite, two major host phases of CCFXe. Apparently small amounts of these phases are occluded in sulfides, and decompose by chemical reaction upon heating. Thus the experiment fails to resolve the nature of CCFXe.A marked enrichment of Xe¹²⁴, without corresponding enrichments in Xe¹²⁶ or Xe^131–136, was observed in the 550–650° and 1400–1500° fractions. Though requiring confirmation, it supports earlier evidence for the complexity and variability of the light xenon component, contrary to claims that it is an integral part of CCFXe.  相似文献   

Retention of ion-implanted-xenon in olivine: Dependence on implantation dose     

C.L. Melcher  T.A. Tombrello  D.S. Burnett 《Geochimica et cosmochimica acta》1983,47(10):1707-1712

The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500°C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 × 10¹⁵ Xe ions/cm². Retention was less at lower doses, with ≥ 50% loss at 1 × 10¹⁴ Xe ions/cm². Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 μm layer for 10⁷ years was calculated as a function of metamorphic temperature. For example, an activation energy of 50 kcal/mole implies Xe retention may be possible for metamorphic temperatures below 500°C.  相似文献   

Distribution of gold and rhenium between nickel-iron and silicate melts: implications for the abundance of siderophile elements on the Earth and Moon     

Kan Kimura  Roy S Lewis  Edward Anders 《Geochimica et cosmochimica acta》1974,38(5):683-701

The distribution equilibrium of Au and Re between nickel-iron and basaltic melts was studied at 1400–1600°C, using radioactive tracers. Metal/silicate distribution coefficients were 1–3 orders of magnitude higher than earlier estimates, as follows. Mauna Loa basalt—Fe₁₀Ni₉₀: D_Au = 3.3 × 10⁴, D_Re = (2.4?89) × 10⁴. Gorda Ridge basalt—Fe₁₀Ni₉₀: D_Au = (18?75) × 10⁴. Synthetic lunar basalt—Fe₇₀ Ni₃₀: D_Au≥ 2 × 10⁴, D_Re ≥ 2 × 10³. The experimental ΔG₁₈₀₀^° for the distribution of Au between nickel-iron and Mauna Loa basalt is ?40 kcal/mole, compared to a calculated value of about ?110 kcal/mole for a reaction involving simple Au³⁺ ions. Presumably the difference represents stabilization of Au(III) by complex formation with ligands such as Cl^?, H₂O, etc.Gold abundances in lunar basalts are roughly consistent with the measured D_Au, but those in terrestrial basalts are two orders of magnitude too high. This discrepancy may reflect complexing by volatiles in the Earth's upper lithosphere, as well as oxidative destruction of metal in the final stages of accretion. In the absence of a metal phase, siderophile trace elements would remain trapped in the upper mantle and crust.  相似文献   

Oxygen diffusion in amphiboles     

John R. Farver  Bruno J. Giletti 《Geochimica et cosmochimica acta》1985,49(6):1403-1411

The kinetics of oxygen isotope self-diffusion in natural samples of hornblende, tremolite, and richterite have been measured. Samples were run under hydrothermal conditions using ¹⁸O enriched water. Profiles of ${}^{18}\text{O}\text{(}{}^{16}\text{O +}{}^{18}\text{O)}$vs depth into the crystal were obtained using an ion microprobe; the depths of sputtered holes were measured using an optical interferometer. At 1000 bars (100 MPa) water pressure, the activation energies (Q) and pre-exponential factors (D₀) for diffusion parallel to c are: D₀(cm²/sec) Q (kcal/gm-atom) T (°C) Hornblende 1⁺²⁰_?1 × 10^?741 ± 6 650–800 Tremolite 2⁺³⁰_?2× 10^?8 39 ± 5 650–800 Richterite 3⁺⁵_?2 × 10^?4 57 ± 2 650–800The diffusion coefficient (D) for hornblende at 800°C and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions. An increase in water pressure from 200 to 2000 bars increases D by a factor of 2.7 for diffusion parallel to c at 800°C. The D value for hornblende at 800°C is about 0.01 that for quartz and 0.001 that for anorthite. As a result, closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars. It is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions.  相似文献   

Thermal metamorphism of primitive meteorites—I. Variation of six trace elements in Allende carbonaceous chondrite heated at 400–1000°C     

Mohammed Ikramuddin  Michael E. Lipschutz 《Geochimica et cosmochimica acta》1975,39(3):363-375

Determination by neutron activation of 6 trace elements retained in Allende (C3) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~10^?5 atm H₂) atmosphere reveals loss of small proportions of Ga and Se and large proportions of Bi, In and Tl-Co being unaffected. The retentivity patterns for the 5 volatile elements differ and in no way duplicate a step-function. In contrast to these trace elements, sulfur is initially present in discrete mineral(s) and visually it appears to be released over a narrow temperature range. Elements are lost more easily from powder than from chips but the difference is ≤35 per cent. Above 600°C, the process of loss appears due to process(es) with apparent activation energies of 2 kcal/mole (Bi, Tl), 4 kcal/mole (Se) and 22 kcal/mole (In). Loss of Bi, Se and Tl below 600°C involves higher apparent activation energies. Two-element correlation diagrams involving Bi, In and Tl are consistent with the idea that trends among highly-volatile elements in enstatite chondrites arise from metamorphism.  相似文献   

Argon retention properties of silicate glasses and implications for <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age and noble gas diffusion studies     

Garrett?B.?Hazelton  Gary?AxenEmail author  Oscar?Lovera 《Contributions to Mineralogy and Petrology》2003,145(1):1-14

Sized aggregates of glasses (47–84 wt% SiO₂) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from ~400 to >1,200 °C to obtain their ³⁹Ar diffusion properties (average E=33,400 cal mol^?1; D _o=4.63×10^?3 cm² s^?1). At T<~1,000 °C, glasses containing <~69 wt% SiO₂ and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log₁₀ unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield ⁴⁰Ar/³⁹Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., <65–175 °C), and coarse particles (e.g., radius (r) >~1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages.  相似文献   

Thio complexes of gold and the transport of gold in hydrothermal ore solutions   总被引：1，自引：0，他引：1  

T.M Seward 《Geochimica et cosmochimica acta》1973,37(3):379-399

The solubility of gold in aqueous sulphide solutions has been determined from pH_20°C ≈ 4 to pH_20°C ≈ 9.5 in the presence of a pyrite-pyrrhotite redox buffer at temperatures from 160 to 300°C and 1000 bar pressure. Maximum solubilities were obtained in the neutral region of pH as, for example, with m_NaHS = 0.15 m, pH_20°C = 5.96, T = 309°C, P = 1000 bar where a gold solubility of 225 mg/kg was obtained. It was concluded that three thio gold complexes contributed to the solubility. The complex Au₂(HS)₂S^2? predominated in alkaline solution, the Au(HS)₂^? complex occurred in the neutral pH region, and in the acid pH region, it was concluded with less certainty that the Au(HS)° complex was present. Formation constants calculated forAu₂(HS)₂S^2? and Au (HS)₂^? emphasize their high stability. In the temperature range from 175 to 250°C, values of pβ for Au₂(HS)₂S^2? vary from ?53.0 to 47.9 (±1.6) and from ?23.1 to ?19.5 ( ± 1.5) for Au(HS)₂^?. Equilibrium constante for the dissolution reactions, $\text{Au° + H}{}_2\text{S + HS}{}^{\mathrm{?}}\text{? Au(HS)}{}_2{}^{\mathrm{?}}\text{+}\text{1}\text{2}\text{H}{}_2$ and 2Au° + H₂S + 2H8^? ? Au(HS)₂^? + H₂ vary from pK_m = +2.4 to +2.55 (±0.10) for Au₂(HS)₂S^2? and from pK_n = + 1.29 to + 1.19 (±0.10) for Au(HS)₂^? over the temperature range 175 to 250°C. Enthalpies of these dissolution reactions were calculated to be ΔH_m° = ?5.2 ±2.0 kcal/mol and ΔH_n° = +1.7 ±2.0 kcal/mol respectively. It was concluded that gold is probably transported in hydrothermal ore solutions as both thio and chloro complexes and may be deposited in response to changes in temperature, pressure, pH, oxidation potential of the system and total sulphur concentration.  相似文献   

An Experimental Study of Oxygen Isotope Fractionation in the Quartz—Wolframite—Water System     

Zhang Ligang  Liu Jingxiu  Zhou Huanbo  Chen Zhensheng 《中国地球化学学报》1993,12(3):220-227

Oxygen isotope fractionation was experimentally studied in the quartz-wolframite-water system from 200 to 420 °C. The starting wolframite was synthesized in aqueous solutions of Na₂WO₄ · 2H₂O + FeCl₂ · 4H₂O or MnCl₂ · 4H₂O. The starting solutions range in salinity from 0 to 10 equivalent wt.% NaCl. Experiments were conducted in a gold-lined stainless steel autoclave, with filling degrees of about 50%. The results showed no significant difference in equilibrium isotope fractionation between water and wolframite, ferberite and huebnerite at the same temperature (310 °C ). The equilibrium oxygen isotope fractionation factors of wolframite and water tend to be equal with increasing temperature above 370 °C, but to increase significantly with decreasing temperature below 370 °C: 1000 ln α_wf-H₂o= 1.03×10⁶T−2-4.91 (370 °C ±200 °C ) 1000 ln α_wf-H₂o = 0.21×10⁶T −2-2.91 (420 °C -370 °C ±) This projects was financially supported by the National Natural Science Foundation of China.  相似文献   

The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations   总被引：5，自引：0，他引：5  

T.Mark Harrison  E.Bruce Watson 《Geochimica et cosmochimica acta》1984,48(7):1467-1477

The solubility and dissolution kinetics of apatite in felsic melts at 850°–1500°C have been examined experimentally by allowing apatite crystals to partially dissolve into apatite-undersaturated melts containing 0–10 wt% water. Analysis of P and Ca gradients in the crystal/melt interfacial region enables determination of both the diffusivities and the saturation levels of these components in the melt. Phosphorus diffusion was identified as the rate-limiting factor in apatite dissolution. Results of four experiments at 8 kbar run in the virtual absence of water yield an activation energy (E) for P diffusion of 143.6 ± 2.8 kcal-mol^?1 and frequency factor (D₀) of 2.23^+2.88_?1.26 × 10⁹cm²-sec^?1. The addition of water causes dramatic and systematic reduction of both E and D₀ such that at 6 wt% H₂O the values are ~25 kcal-mol^?1 and 10^?5 cm²-sec^?1, respectively. At 1300°C, the diffusivity of P increases by a factor of 50 over the first 2% of water added to the melt, but rises by a factor of only two between 2 and 6%, perhaps reflecting the effect of a concentration-dependent mechanism of H₂O solution. Calcium diffusion gradients do not conform well to simple diffusion theory because the release of calcium at the dissolving crystal surface is linked to the transport rate of phosphorus in the melt, which is typically two orders of magnitude slower than Ca. Calcium chemical diffusion rates calculated from the observed gradients are about 50 times slower than calcium tracer diffusion.Apatite solubilities obtained from these experiments, together with previous results, can be described as a function of absolute temperature (T) and melt composition by the expression: In D^apatite/melt_P = [(8400 + ((SiO₂ ? 0.5)2.64 × 10⁴))/T] ? [3.1 + (12.4(SiO₂ ? 0.5))] where SiO₂ is the weight fraction of silica in the melt. This model appears to be valid between 45% and 75% SiO₂, 0 and 10% water, and for the range of pressures expected in the crust.The diffusivity information extracted from the experiments can be directly applied to several problems of geochemical interest, including I) dissolution times for apatite during crustal anatexis, and 2) pileup of P, and consequent local saturation in apatite, at the surfaces of growing major-mineral phases.  相似文献   

http://www.sciencedirect.com/science/article/pii/S1674987112000643   总被引：2，自引：1，他引：1  

Takahiro Endo  Toshiaki Tsunogae  M.Santosh  E.Shaji 《地学前缘(英文版)》2012,3(6):801-811

Incipient charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust.Here we report new petrological data from an incipient charnockite locality at Rajapalaiyam in the Madurai Block,southern India,and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature（UHT） metamorphism have been reported.Proximal to the UHT rocks are patches and lenses of charnockite（Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm） occurring within Opx-free Grt-Bt gneiss（Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt） which we report in this study.The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p-T range of～820℃and～9 kbar.Modeling of the charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T（～760℃and～7.5 kbar）. which is consistent with the results obtained from geothermobarometry（710—760℃,6.7—7.5 kbar）. but significantly lower than the peak temperatures（>1000℃） recorded from the UHT rocks in this locality,suggesting that charnockitization is a post-peak event.The modeling of T versus molar H₂O content in the rock（M（H₂O）） demonstrates that the Opx-bearing assemblage in charnockite and Opxfree assemblage in Grt-Bt gneiss are both stable at M（H₂O） = 0.3 mol%-0.6 mol%.and there is no significant difference in water activity between the two domains.Our finding is in contrast to the previous petrogenetic model of incipient charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO₂-rich fluid.T-X_Fe³⁺（= Fe₂O₃/（FeO + Fe₂O₃） in mole） pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene:Opx is stable at X_Fe³⁺ <0.03 in charnockite.while Opx-free assemblage in Grt-Bt gneiss is stabilized at X_Fe³⁺ >0.12.Such low oxygen fugacity conditions of X_Fe³⁺ <0.03 in the charnockite compared to Grt-Bt gneiss might be related to the infiltration of a reduced fluid（e.g.,H₂O + CH₄） during the retrograde stage.  相似文献   

Methane: An equation of state with application to the ternary system H₂O-CO₂-CH₄     

Gary K Jacobs  Derrill M Kerrick 《Geochimica et cosmochimica acta》1981,45(5):607-614

The following hardsphere modified Redlich-Kwong (HSMRK) equation of state was obtained by least squares fitting to available P-V-T data for methane (P in bars; T in Kelvins; v in cm³ mol^?1; b = 60.00 cm³ mol^?1; R = 83.14 cm³barmol^?1K^?1): $\text{P}\text{RT(1 + y + y}{}^2\text{?y}{}^3\text{v(1?y)}{}^3\text{)}\text{-}\text{c(T) + d(T)}\text{v}\text{+}\text{e(T)}\text{v}{}^2\text{/v(v + b)T}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$$\text{y =}\text{b}\text{4v}$c(T) = 13.403 × 10⁶ + (9.28 × 10⁴)T + 2.7 T²d(T) = 5.216 × 10⁹ ? (6.8 × 10⁶)T + (3.28 × 10³)T²e(T) = (?2.3322 × 10¹¹) + (6.738 × 10⁸)T + (3.179 × 10⁵)T² For the P-T range of experimental data used in the fit (50 to 8600 bars and from 320 to 670 K), calculated volumes and fugacity coefficients for CH₄ relative to experimentally determined volumes and fugacity coefficients have average percent deviations of 0.279 and 1.373, respectively. The HSMRK equation, which predicts linear isochores over a wide P-T range, should yield reasonable estimates of fugacity coefficients for CH₄ to pressures and temperatures well outside the P-T range of available P-V-T data. Calculations for the system H₂O-CO₂-CH₄, using the HSMRK equations for H₂O and CO₂ of Kerrick and Jacobs (1981) and the HSMRK equation for CH₄ of this study, indicate that compared to the binary H₂O-CO₂ system, small amounts of CH₄ in the ternary system H₂O-CO₂-CH₄ slightly increases the activity of H₂O, and significantly decreases the activity of CO₂.  相似文献   

Fluids at crustal pressures and temperatures   总被引：1，自引：0，他引：1  

S. K. Saxena  Y. Fei 《Contributions to Mineralogy and Petrology》1987,95(3):370-375

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The heat capacity of two natural chlorite group minerals derived from differential scanning calorimetry     

C. Bertoldi  A. Benisek  L. Cemic✓  E. Dachs 《Physics and Chemistry of Minerals》2001,28(5):332-336

The heat capacity of natural chamosite (X_Fe=0.889) and clinochlore (X_Fe=0.116) were measured by differential scanning calorimetry (DSC). The samples were characterised by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy. DSC measurements between 143 and 623?K were made following the procedure of Bosenick et?al. (1996). The fitted data for natural chamosite (CA) in J?mol^?1?K^?1 give: C _p,CA = 1224.3–10.685?×?10³?×?T ^??0.5???6.4389?× 10⁶?×T ^??2?+?8.0279?×?10⁸?×?T ^??3 and for the natural clinochlore (CE): C _p,CE = 1200.5–10.908?×?10³?×T ^??0.5?? 5.6941?×?10⁶?×?T ^??2?+?7.1166?×?10⁸?×?T ^??3. The corrected C _p-polynomial for pure end-member chamosite (Fe₅Al)[Si₃AlO₁₀](OH)₈ is C _p,CAcor = 1248.3–11.116?× 10³?×?T ^??0.5???5.1623?×?10⁶?×?T ^??2?+?7.1867?×?10⁸×T ^??3 and the corrected C _p-polynomial for pure end-member clinochlore (Mg₅Al)[Si₃AlO₁₀](OH)₈ is C _p,CEcor = 1191.3–10.665?×?10³?×?T ^??0.5???6.5136?×?10⁶?×?T ^??2?+ 7.7206?×?10⁸?×?T ^??3. The corrected C _p-polynomial for clinochlore is in excellent agreement with that in the internally consistent data sets of Berman (1988) and Holland and Powell (1998). The derived C _p-polynomial for chamosite (C _p,CAcor) leads to a 4.4% higher heat capacity, at 300?K, compared to that estimated by Holland and Powell (1998) based on a summation method. The corrected C _p-polynomial (C _p,CAcor) is, however, in excellent agreement with the computed C _p-polynomial given by Saccocia and Seyfried (1993), thus supporting the reliability of Berman and Brown's (1985) estimation method of heat capacities.  相似文献   

Oxygen isotope fractionation during the dolomitization of calcium carbonate     

Alan Matthews  Amitai Katz 《Geochimica et cosmochimica acta》1977,41(10):1431-1438

The oxygen isotope fractionation accompanying the hydrothermal dolomitization of CaCO₃ between 252 and 295°C has been investigated. Dolomitization (which occurs via the crystallization of one or more intermediate magnesian calcite phases) is characterised by a progressive lowering in δ⁸O, which smoothly correlates with the change in the Mg/(Mg + Ca) and the $\text{Sr}\text{(Mg + Ca)}$ ratios and with the sequential phase formation. The data support the proposals of Katz and Matthews (1977) that (a) all reaction occurs by solution and reprecipitation, (b) intermediate phases and dolomite form sequentially and (c) the intermediate phases form within limited solution zones surrounding the dissolving precursor. Calculated volumes of the solution zone for the aragonite → low magnesian calcite transformation are within the range 3.7–6.7 × 10^?5 liters (out of 5 × 10^?3 liters, the volume of the bulk solution used in the present study), and agree well with those calculated from strontium and magnesium partitioning data. Dolomite precipitates in apparent isotopic equilibrium with the bulk solution. The temperature dependence of the fractionation is defined by the equation 1000 Inα_D-H2O = 3.06 × 10⁶T^?2 ? 3.24 Dolomite-water fractionations from this equation are significantly lower than those obtained by extrapolation of the Northrop And Clayton (1966) calibration. The reaction zone model can be applied to explain near zero dolomite-calcite oxygen isotope fractionations reported by Epsteinet al. (1964).  相似文献