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1.
Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different
geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree.
In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals
that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the
temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in
the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed
for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content
(ppm by weight), pressure (GPa) and reciprocal absolute temperature:
The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic
disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the
presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks
of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization
temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth
conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over
the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the
Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
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2.
Titanium is one of many trace elements to substitute for silicon in the mineral quartz. Here, we describe the temperature dependence of that substitution, in the form of a new geothermometer. To calibrate the “TitaniQ” thermometer, we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt, at temperatures ranging from 600 to 1,000°C, at 1.0 GPa. The Ti contents of quartz (in ppm by weight) from 13 experiments increase exponentially with reciprocal T as described by: Application of this thermometer is straightforward, typically requiring analysis of only one phase (quartz). This can be accomplished either by EPMA for crystallization temperatures above 600°C, or by SIMS for temperatures down to at least 400°. Resulting temperature estimates are very precise (usually better than ±5°C), potentially allowing detailed characterization of thermal histories within individual quartz grains. Although calibrated for quartz crystallized in the presence of rutile, the thermometer can also be applied to rutile-absent systems if TiO 2 activity is constrained. 相似文献
3.
Rutile is an important carrier of high field strength elements (HFSE; Zr, Nb, Mo, Sn, Sb, Hf, Ta, W). Its Zr content is buffered in systems with quartz and zircon as coexisting phases. The effects of temperature ( T) and pressure ( P) on the Zr content in rutile have been empirically calibrated in this study by analysing rutile–quartz–zircon assemblages of 31 metamorphic rocks spanning a T range from 430 to 1,100°C. Electron microprobe measurements show that Zr concentrations in rutile vary from 30 to 8,400 ppm across this temperature interval, correlating closely with metamorphic grade. The following thermometer has been formulated based on the maximum Zr contents of rutile included in garnet and pyroxene: No pressure dependence was observed. An uncertainty in absolute T of ±50°C is inherited from T estimates of the natural samples used. A close approach to equilibrium of Zr distribution between zircon and rutile is suggested based on the high degree of reproducability of Zr contents in rutiles from different rock types from the same locality. At a given locality, the calculated range in T is mostly ±10°C, indicating the geological and analytical precision of the rutile thermometer. Possible applications of this new geothermometer are discussed covering the fields of ultrahigh temperature (UHT) granulites, sedimentary provenance studies and metamorphic field gradients. 相似文献
4.
Experimental and thermodynamic data and the apparent immobility of Ti under metamorphic conditions suggest that rutile is very insoluble in aqueous fluids at upper crustal conditions. New solubility measurements at 1.0–2.93 GPa and 800–1200°C show, however, that under certain pressure and temperature conditions rutile is quite soluble in H 2O. Solubilities were estimated from the measured weight loss of a single crystal equilibrated with a known mass of fluid in a piston cylinder apparatus. Measured solubilities in H 2O range from 0.15 wt% (wt loss crystal/wt fluid) at 2.93 GPa and 1000°C to 1.9% at 1.0 GPa and 1100°C. Solubility increases with increasing temperature and with decreasing pressure in a manner given by the following fit to the experimental data: |
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5.
The electrical conductivity of upper-mantle rocks—dunite, pyroxenite, and lherzolite—was measured at ∼2–3 GPa and ∼1,273–1,573 K
using impedance spectra within a frequency range of 0.1–10 6 Hz. The oxygen fugacity was controlled by a Mo–MoO 2 solid buffer. The results indicate that the electrical conductivity of lherzolite and pyroxenite are approximately half and
one order of magnitude higher than that of dunite, respectively. A preliminary model involving water and iron content effects
on the electrical conductivity was derived and is summarized by the relation: The results also indicate that pyroxenes dominate the bulk conductivity of upper mantle in hydrous conditions and suggest
the maximum water content in oceanic upper mantle is as high as ∼0.09 wt%. 相似文献
6.
The models recognize that ZrSiO 4, ZrTiO 4, and TiSiO 4, but not ZrO 2 or TiO 2, are independently variable phase components in zircon. Accordingly, the equilibrium controlling the Zr content of rutile
coexisting with zircon is ZrSiO 4 = ZrO 2 (in rutile) + SiO 2. The equilibrium controlling the Ti content of zircon is either ZrSiO 4 + TiO 2 = ZrTiO 4 + SiO 2 or TiO 2 + SiO 2 = TiSiO 4, depending whether Ti substitutes for Si or Zr. The Zr content of rutile thus depends on the activity of SiO 2
as well as T, and the Ti content of zircon depends on and as well as T. New and published experimental data confirm the predicted increase in the Zr content of rutile with decreasing and unequivocally demonstrate that the Ti content of zircon increases with decreasing . The substitution of Ti in zircon therefore is primarily for Si. Assuming a constant effect of P, unit and that and are proportional to ppm Zr in rutile and ppm Ti in zircon, [log(ppm Zr-in-rutile) + log] = A 1 + B 1/ T(K) and [log(ppm Ti-in-zircon) + log − log] = A 2 + B 2/ T, where the A and B are constants. The constants were derived from published and new data from experiments with buffered by either quartz or zircon + zirconia, from experiments with defined by the Zr content of rutile, and from well-characterized natural samples. Results are A 1 = 7.420 ± 0.105; B 1 = −4,530 ± 111; A 2 = 5.711 ± 0.072; B 2 = −4,800 ± 86 with activity referenced to α-quartz and rutile at P and T of interest. The zircon thermometer may now be applied to rocks without quartz and/or rutile, and the rutile thermometer
applied to rocks without quartz, provided that and are estimated. Maximum uncertainties introduced to zircon and rutile thermometry by unconstrained and can be quantitatively assessed and are ≈60 to 70°C at 750°C. A preliminary assessment of the dependence of the two thermometers
on P predicts that an uncertainty of ±1 GPa introduces an additional uncertainty at 750°C of ≈50°C for the Ti-in-zircon thermometer
and of ≈70 to 80°C for the Zr-in-rutile thermometer. 相似文献
7.
In order to evaluate the effect of trace and minor elements (e.g., P, Y, and the REEs) on the high-temperature solubility of Ti in zircon (zrc), we conducted 31 experiments on a series of synthetic and natural granitic compositions [enriched in TiO 2 and ZrO 2; Al/(Na + K) molar ~1.2] at a pressure of 10 kbar and temperatures of ~1,400 to 1,200 °C. Thirty of the experiments produced zircon-saturated glasses, of which 22 are also saturated in rutile (rt). In seven experiments, quenched glasses coexist with quartz (qtz). SiO 2 contents of the quenched liquids range from 68.5 to 82.3 wt% (volatile free), and water concentrations are 0.4–7.0 wt%. TiO 2 contents of the rutile-saturated quenched melts are positively correlated with run temperature. Glass ZrO 2 concentrations (0.2–1.2 wt%; volatile free) also show a broad positive correlation with run temperature and, at a given T, are strongly correlated with the parameter (Na + K + 2Ca)/(Si·Al) (all in cation fractions). Mole fraction of ZrO 2 in rutile $ \left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) $ in the quartz-saturated runs coupled with other 10-kbar qtz-saturated experimental data from the literature (total temperature range of ~1,400 to 675 °C) yields the following temperature-dependent expression: $ {\text{ln}}\left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) + {\text{ln}}\left( {a_{{{\text{SiO}}_{2} }} } \right) = 2.638(149) - 9969(190)/T({\text{K}}) $ , where silica activity $ a_{{{\text{SiO}}_{2} }} $ in either the coexisting silica polymorph or a silica-undersaturated melt is referenced to α-quartz at the P and T of each experiment and the best-fit coefficients and their uncertainties (values in parentheses) reflect uncertainties in T and $ \mathop X\nolimits_{{{\text{ZrO}}_{2} }}^{\text{rt}} $ . NanoSIMS measurements of Ti in zircon overgrowths in the experiments yield values of ~100 to 800 ppm; Ti concentrations in zircon are positively correlated with temperature. Coupled with values for $ a_{{{\text{SiO}}_{2} }} $ and $ a_{{{\text{TiO}}_{2} }} $ for each experiment, zircon Ti concentrations (ppm) can be related to temperature over the range of ~1,400 to 1,200 °C by the expression: $ \ln \left( {\text{Ti ppm}} \right)^{\text{zrc}} + \ln \left( {a_{{{\text{SiO}}_{2} }} } \right) - \ln \left( {a_{{{\text{TiO}}_{2} }} } \right) = 13.84\left( {71} \right) - 12590\left( {1124} \right)/T\left( {\text{K}} \right) $ . After accounting for differences in $ a_{{{\text{SiO}}_{2} }} $ and $ a_{{{\text{TiO}}_{2} }} $ , Ti contents of zircon from experiments run with bulk compositions based on the natural granite overlap with the concentrations measured on zircon from experiments using the synthetic bulk compositions. Coupled with data from the literature, this suggests that at T ≥ 1,100 °C, natural levels of minor and trace elements in “granitic” melts do not appear to influence the solubility of Ti in zircon. Whether this is true at magmatic temperatures of crustal hydrous silica-rich liquids (e.g., 800–700 °C) remains to be demonstrated. Finally, measured $ D_{\text{Ti}}^{{{\text{zrc}}/{\text{melt}}}} $ values (calculated on a weight basis) from the experiments presented here are 0.007–0.01, relatively independent of temperature, and broadly consistent with values determined from natural zircon and silica-rich glass pairs. 相似文献
8.
Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0–1.2 GPa) conditions has
been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed
under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted
to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE +3 that might provide charge balance. Diffusion experiments performed in the presence of H 2O–CO 2 fluid were run in a piston–cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon
with oxalic acid added to produce H 2O–CO 2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 7Li(p,γ) 8Be was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion
normal to the c-axis over the temperature range 703–1.151°C at 1 atm for experiments run with the spodumene source:
|
D\textLi = 7.17 ×10 - 7 exp( - 275 ±11 \textkJmol - 1 /\textRT)\textm2 \texts - 1. D_{\text{Li}} = 7.17 \times 10^{ - 7} { \exp }( - 275 \pm 11\,{\text{kJmol}}^{ - 1} /{\text{RT}}){\text{m}}^{2} {\text{s}}^{ - 1}. 相似文献
9.
Interdiffusion of Fe and Mg in (Mg,Fe)O has been investigated experimentally under hydrous conditions. Single crystals of
MgO in contact with (Mg 0.73Fe 0.27)O were annealed hydrothermally at 300 MPa between 1,000 and 1,250°C and using a Ni–NiO buffer. After electron microprobe
analyses, the dependence of the interdiffusivity on Fe concentration was determined using a Boltzmann–Matano analysis. For
a water fugacity of ∼300 MPa, the Fe–Mg interdiffusion coefficient in Fe
x
Mg 1−x
O with 0.01 ≤ x ≤ 0.25 can be described by with and C = −80 ± 10 kJ mol −1. For x = 0.1 and at 1,000°C, Fe–Mg interdiffusion is a factor of ∼4 faster under hydrous than under anhydrous conditions. This enhanced
rate of interdiffusion is attributed to an increased concentration of metal vacancies resulting from the incorporation of
hydrogen. Such water-induced enhancement of kinetics may have important implications for the rheological properties of the
lower mantle.
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10.
The titanium concentrations of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga were measured by ion microprobe. Samples come
from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons
(32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most
of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon
thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO 2 or SiO 2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon
saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (∼15% melt residue
for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous
rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually,
the effects of reduced or , variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low
temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous
melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While
some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included
within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in
Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once.
However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio,
or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and , are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide
overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock
SiO 2 and HfO 2 for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO 2 and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks
average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures
of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons
have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons
from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent
Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration
in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
11.
The incorporation and diffusion of hydrogen in San Carlos olivine (Fo 90) single crystals were studied by performing experiments under hydrothermal conditions. The experiments were carried out either at 1.5 GPa, 1,000°C for 1.5 h in a piston cylinder apparatus or at 0.2 GPa, 900°C for 1 or 20 h in a cold-seal vessel. The oxygen fugacity was buffered using Ni–NiO, and the silica activity was buffered by adding San Carlos orthopyroxene powders. Polarized Fourier transform infrared (FTIR) spectroscopy was utilized to quantify the hydroxyl distributions in the samples after the experiments. The resulting infrared spectra reproduce the features of FTIR spectra that are observed in olivine from common mantle peridotite xenoliths. The hydrogen concentration at the edges of the hydrogenated olivine crystals corresponds to concentration levels calculated from published water solubility laws. Hydrogen diffusivities were determined for the three crystallographic axes from profiles of water content as a function of position. The chemical diffusion coefficients are comparable to those previously reported for natural iron-bearing olivine. At high temperature, hydrogenation is dominated by coupled diffusion of protons and octahedrally coordinated metal vacancies
where the vacancy diffusion rate limits the process. From the experimental data, we determined the following diffusion laws (diffusivity in m 2 s −1, activation energies in kJ mol −1):
for diffusion along [100] and [010];
for diffusion along [001]. These diffusion rates are fast enough to modify significantly water contents within olivine grains in xenoliths ascending from the mantle. 相似文献
12.
We present experiments showing that the lower oceanic crust should melt efficiently and quickly when heated by hot ascending
magmas. Average plagioclase–olivine and plagioclase–augite pairs from the lower crust at the Southwest Indian Ridge have melt–mineral
saturation boundaries at 1,190 and 1,154°C, respectively, and melt rapidly (>0.01 mm/h) at 50°C or more above these temperatures.
Melting experiments performed on olivine–plagioclase and augite–plagioclase mineral pairs from actual oceanic lower crustal
rock samples and under conditions applicable to a MOR setting (1,220–1,330°C, 1 atm, quartz–fayalite–magnetite oxygen buffer,
0.25–24 h) indicate that the resulting disequilibrium melts are linear mixes of the mineral compositions. The rates of melting
are slower than the rate of heat-diffusion into a sample and are approximated as: Our results indicate that great care must be taken in backward models using basalt chemistry alone to explore mantle-melting
processes, assuming only crystallization and fractionation during ascent, as partial melts may mix with intruded hot magma. 相似文献
13.
We evaluate balanced metasomatic reactions and model coupled reactive and isotopic transport at a carbonatite-gneiss contact at Alnö, Sweden. We interpret structurally channelled fluid flow along the carbonatite-gneiss contact at ～640°C. This caused (1) metasomatism of the gneiss, by the reaction: ${\hbox{biotite} + \hbox{quartz} + \hbox{oligoclase} + \hbox{K}_{2} \hbox{O} +\,\hbox{Na}_{2}\hbox{O} \pm \hbox{CaO} \pm \hbox{MgO} \pm \hbox{FeO} = \hbox{albite} + \hbox{K-feldspar} + \hbox{arfvedsonite} + \hbox{aegirene-}\hbox{augite} + \hbox{H}_{2} \hbox{O} + \hbox{SiO}_{2}}We evaluate balanced metasomatic reactions and model coupled reactive and isotopic transport at a carbonatite-gneiss contact
at Aln?, Sweden. We interpret structurally channelled fluid flow along the carbonatite-gneiss contact at ∼640°C. This caused
(1) metasomatism of the gneiss, by the reaction: , (2) metasomatism of carbonatite by the reaction: calcite + SiO2 = wollastonite + CO2, and (3) isotopic homogenization of the metasomatised region. We suggest that reactive weakening caused the metasomatised
region to widen and that the metasomatic reactions are chemically (and possibly mechanically) coupled. Spatial separation
of reaction and isotope fronts in the carbonatite conforms to a chromatographic model which assumes local calcite–fluid equilibrium,
yields a timescale of 102–104 years for fluid–rock interaction and confirms that chemical transport towards the carbonatite interior was mainly by diffusion.
We conclude that most silicate phases present in the studied carbonatite were acquired by corrosion and assimilation of ijolite,
as a reactive by-product of this process and by metasomatism. The carbonatite was thus a relatively pure calcite–H2O−CO2–salt melt or fluid. 相似文献
14.
The density ρ of Caspian Sea waters was measured as a function of temperature (273.15–343.15) K at conductivity salinities
of 7.8 and 11.3 using the Anton-Paar Densitometer. Measurements were also made on one of the samples ( S = 11.38) diluted with water as a function of temperature ( T = 273.15–338.15 K) and salinity (2.5–11.3). These latter results have been used to develop an equation of state for the Caspian
Sea ( σ = ±0.007 kg m −3)
where ρ 0 is the density of water and the parameters A, B and C are given by
Measurements of the density of artificial Caspian Sea water at 298.15 K agree to ± 0.012 kg m −3 with the real samples. These results indicate that the composition of Caspian Sea waters must be close to earlier measurements
of the major components. Model calculations based on this composition yield densities that agree with the measured values
to ± 0.012 kg m −3. The new density measurements are higher than earlier measurements. This may be related to a higher concentration of dissolved
organic carbon found in the present samples (500 μM) which is much higher than the values in ocean waters (~65 μM). 相似文献
15.
A detailed morphological, chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late Precambrian Ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on U-Pb isotopic systematics. Late Paleozoic, amphibolite-grade (approx. 600°C) mylonitization of the Ponaganset gneiss at this locality is associated with movement along the Hope Valley Shear Zone. The response of zircon to metamorphism in each gneiss unit is distinct: zircons in gray augen gneiss are uncorroded and not overgrown, whereas zircons from fluorite-bearing pink granitic gneiss are variably corroded and over 50% bear opaque overgrowths. The zircon overgrowths are chemically distinct from the primary cores, and contain high conentrations of Hf, U, HREE, and Th. Mylonite derived from the gray gneiss contains only a small population of Hf-U-rich metamorphic zircon, but zircons in the pink gneiss-derived mylonite are dominated by the Hf-U-rich metamorphic component. In terms of their U-Pb isotopic systematics, overgrowth-free zircons from both units are markedly discordant (gray, 10–20%, pink, 35%), but overgrown zircons from the pink gneiss are up to 70% discordant. Zircons from the mylonites yield younger Pb–Pb and U–Pb ages than those of the protolith gneisses, and isotopic data from each gneiss + mylonite pair define a linear array on concordia plots. Upper intercept ages of the gray gneiss (621+/–27 Ma) and the pink gneiss (635+/–50 Ma) indicate that the crystallization of both units was coeval, and the lower intercept ages (gray, 270+/–92 Ma; pink, 285+/–26 Ma) fall within the range of other published age estimates for Alleghanian metamorphism in southeastern New England (e.g., Zartman et al. 1988). New growth of zircon suggests that Zr was mobile during metamorphism. The presence of fluorite in the pink gneiss, and a discontinuity in log
values obtained from biotite across the pink gneiss-gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in HF fugacity. Zircon dissolution/reprecipitation in the pink gneiss, and the lack of similar features in the contiguous gray gneiss, suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry, fluid chemistry and/or the degree of fluid-rock interaction. 相似文献
16.
The solubility of water in coexisting enstatite and forsterite was investigated by simultaneously synthesizing the two phases in a series of high pressure and temperature piston cylinder experiments. Experiments were performed at 1.0 and 2.0 GPa at temperatures between 1,100 and 1,420°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on orientated single crystals of each phase. Phase water contents were estimated using the calibration of Libowitzky and Rossman (Am Mineral 82:1111–1115, 1997). Enstatite crystals, synthesized in equilibrium with forsterite and an aqueous phase at 1,350°C and 2.0 GPa, contain 114 ppm H 2O. This is reduced to 59 ppm at 1,100°C, under otherwise identical conditions, suggesting a strong temperature dependence. At 1,350°C and 1.0 GPa water solubility in enstatite is 89 ppm, significantly lower than that at 2.0 GPa. In contrast water solubility in forsterite is essentially constant, being in the range 36–41 ppm for all conditions studied. These data give partition coefficients
in the range 2.28–3.31 for all experiments at 1,350°C and 1.34 for one experiment at 1,100°C. The incorporation of Al 2O 3 in enstatite modifies the OH stretching spectrum in a systematic way, and slightly increases the water solubility. 相似文献
17.
The thermodynamic stability constants for the hydrolysis and formation of mercury (Hg 2+) chloride complexes
have been used to calculate the activity coefficients for Hg(OH)
n
(2–n)+ and HgCl
n
(2–n)+ complexes using the Pitzer specific interaction model. These values have been used to determine the Pitzer parameters for
the hydroxide and chloro complexes and C
ML). The values of and have been determined for the neutral complexes (Hg(OH) 2 and HgCl 2). The resultant parameters yield calculated values for the measured values of log to ±0.01 from I = 0.1 to 3 m at 25°C. Since the activity coefficients of and are in reasonable agreement with the values for Pb(II), we have estimated the effect of temperature on the chloride constants
for Hg(II) from 0 to 300°C and I = 0–6 m using the Pitzer parameters for complexes. The resulting parameters can be used to examine the speciation of Hg(II) with Cl − in natural waters over a wide range of conditions. 相似文献
18.
Kinetic experiments of dolomite dissolution in water over a temperature range from 25 to 250°C were performed using a flow
through packed bed reactor. Authors chose three different size fractions of dolomite samples: 18–35 mesh, 35–60 mesh, and
60–80 mesh. The dissolution rates of the three particle size samples of dolomite were measured. The dissolution rate values
are changed with the variation of grain size of the sample. For the sample through 20–40 mesh, both the release rate of Ca
and the release rate of Mg increase with increasing temperature until 200°C, then decrease with continued increasing temperature.
Its maximum dissolution rate occurs at 200°C. The maximum dissolution rates for the sample through 40–60 mesh and 60–80 mesh
happen at 100°C. Experimental results indicate that the dissolution of dolomite is incongruent in most cases. Dissolution
of fresh dolomite was non-stoichiometric, the Ca/Mg ratio released to solution was greater than in the bulk solid, and the
ratio increases with rising temperatures from 25 to 250°C. Observations on dolomite dissolution in water are presented as
three parallel reactions, and each reaction occurs in consecutive steps as
where the second part is a slow reaction, and also the reaction could occur as follows:
The following rate equation was used to describe dolomite dissolution kinetics
where refers to one of each reaction among the above reactions; k
ij
is the rate constant for ith species in the jth reaction, a
i
stands for activity of ith aqueous species, n is the stoichimetric coefficience of ith species in the jth reaction, and define . The experiments prove that dissolved Ca is a strong inhibitor for dolomite dissolution (release of Ca) in most cases. Dissolved
Mg was found to be an inhibitor for dolomite dissolution at low temperatures. But dissolution rates of dolomite increase with
increasing the concentration of dissolved Mg in the temperature range of 200–250°C for 20–40 mesh sample, and in the temperature
range of 100–250°C for 40–80 mesh sample, whereas the Mg 2+ ion adsorption on dolomite surface becomes progressively the step controlling reaction. The following rate equation is suitable
to dolomite dissolutions at high temperatures from 200 to 250°C. where refers to dissolution rate (release of Ca), and are molar concentrations of dissolved Ca and Mg, k
ad stands for adsorption reaction rate constant, K
Mg refers to adsorption equilibrium constant.
At 200°C for 40–60 mesh sample, the release rate of Ca can be described as: 相似文献
19.
Uranium mineralization in the El Erediya area, Egyptian Eastern Desert, has been affected by both high temperature and low
temperature fluids. Mineralization is structurally controlled and is associated with jasperoid veins that are hosted by a
granitic pluton. This granite exhibits extensive alteration, including silicification, argillization, sericitization, chloritization,
carbonatization, and hematization. The primary uranium mineral is pitchblende, whereas uranpyrochlore, uranophane, kasolite,
and an unidentified hydrated uranium niobate mineral are the most abundant secondary uranium minerals. Uranpyrochlore and
the unidentified hydrated uranium niobate mineral are interpreted as alteration products of petscheckite. The chemical formula
of the uranpyrochlore based upon the Electron Probe Micro Analyzer (EPMA) is . It is characterized by a relatively high Zr content (average ZrO 2 = 6.6 wt%). The average composition of the unidentified hydrated uranium niobate mineral is , where U and Nb represent the dominant cations in the U and Nb site, respectively. Uranophane is the dominant U 6+ silicate phase in oxidized zones of the jasperoid veins. Kasolite is less abundant than uranophane and contains major U,
Pb, and Si but only minor Ca, Fe, P, and Zr. A two-stage metallogenetic model is proposed for the alteration processes and
uranium mineralization at El Erediya. The primary uranium minerals were formed during the first stage of the hydrothermal
activity that formed jasperoid veins in El Eradiya granite (130–160 Ma). This stage is related to the Late Jurassic–Early
Cretaceous phase of the final Pan-African tectono-thermal event in Egypt. After initial formation of El Erediya jasperoid
veins, a late stage of hydrothermal alteration includes argillization, dissolution of iron-bearing sulfide minerals, formation
of iron-oxy hydroxides, and corrosion of primary uranium minerals, resulting in enrichment of U, Ca, Pb, Zr, and Si. During
this stage, petscheckite was altered to uranpyrochlore and oxy-petscheckite. Uranium was likely transported as uranyl carbonate
and uranyl fluoride complexes. With change of temperature and pH, these complexes became unstable and combined with silica,
calcium, and lead to form uranophane and kasolite. Finally, at a later stage of low-temperature supergene alteration, oxy-petscheckite
was altered to an unidentified hydrated uranium niobate mineral by removal of Fe. 相似文献
20.
Dialysis and chemical speciation modelling have been used to calculate activities of Fe 3+ for a range of UK surface waters of varying chemistry (pH 4.3–8.0; dissolved organic carbon 1.7–40.3 mg l −1) at 283 K. The resulting activities were regressed against pH to give the empirical model: . Predicted Fe 3+ activities are consistent with a solid–solution equilibrium with hydrous ferric oxide, consistent with some previous studies
on Fe(III) solubility in the laboratory. However, as has also sometimes been observed in the laboratory, the slope of the
solubility equation is lower than the theoretical value of 3. The empirical model was used to predict concentrations of Fe
in dialysates and ultrafiltrates of globally distributed surface and soil/groundwaters. The predictions were improved greatly
by the incorporation of a temperature correction for , consistent with the temperature dependence of previously reported hydrous ferric oxide solubility. The empirical model,
incorporating temperature effects, may be used to make generic predictions of the ratio of free and complexed Fe(III) to dissolved
organic matter in freshwaters. Comparison of such ratios with observed Fe:dissolved organic matter ratios allows an assessment
to be made of the amounts of Fe present as Fe(II) or colloidal Fe(III), where no separate measurements have been made.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
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