where D0 is in µm2/s, X is mole fraction of H2Ot on a single oxygen basis, T is temperature in K, and P is pressure in GPa.H2Ot diffusivities (DH2Ot, in µm2/s) can be calculated from H2Om diffusivity, or directly from the following expression:
At low H2Ot content (up to 2 wt.% if an error of a factor of 2 is allowed), H2Ot diffusivity is approximately proportional to H2Ot content:
where C is H2Ot content in wt.% and C0 is 1 wt.%. The new expressions for H2O diffusion not only reproduce our own data, but also match data in literature from different laboratories and using different methods, indicating good inter-laboratory and multi-method consistency. The new expressions cover a wide range of geological conditions, and can be applied to H2O diffusion in rhyolitic melts in various volcanic and magmatic processes.  相似文献   

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1.
The self-diffusion of oxygen has been measured for three silicate melts along the join diopsideanorthite. The experiments were done by isotope exchange between an “infinite” reservoir of oxygen gas and spheres of melt. The oxygen self-diffusion coefficients for the three melts are given as: C-1(diopside): D = 1.64 × 101 exp(?(63.2 ± 20)(kcal/mole)/RT) cm2/sec C-2(Di58An42): D = 1.35 × 10?1 exp(?(46.8 ± 9)(kcal/mole)/RT) cm2/sec C-3(Di40An60): D = 1.29 × 10?2 exp(?(44.2 ± 6)(kcal/mole)/RT) cm2/secThe self-diffusion coefficients do not agree with the Eyring equation unless mean ionic jump distances (λ) considerably larger than the diameter of oxygen anion are assumed. However, the sense of variation of the actual diffusivities is as the Eyring equation predicts.Consideration of the results of this study and the bulk of previous work shows that oxygen appears to conform to the compensation law for cationic diffusion in silicate melts and glasses. The range of oxygen diffusivities was also found to encompass the field of divalent cation diffusivities in silicate melts.Those results imply that the diffusion of oxygen in silicate melts may involve a contribution from a cation-like diffusion mechanism (discrete O2? anions) as well as contributions from the diffusion of larger structural units.  相似文献   

2.
Basalt contamination by continental crust: Some experiments and models   总被引：1，自引：1，他引：1  
Chemical interaction between molten basalt and felsic minerals of the continental crust (quartz, K-feldspar, and oligoclase) was examined in static and dynamic experiments at 1,200°–1,400° C. Under circumstances of continuous stirring at 1,400°, -quartz dissolves in tholeiite melt at a rate of 3.3×10–6 g/s per cm2 of contact area; at 1,300°, the solution rate is 1.5×10–6 g/cm{cm2}s. The feldspars are molten at the experimental conditions, and interact with contacting basalt melt by diffusion in the liquid state. This is a complex process characterized by rapid initial diffusion of alkalies to establish a distribution between felsic melt and basalt similar to that observed in cases of actual two-liquid equilibrium (both alkalies reach concentrations in the felsic melt 1.5–3 times those in the basalt). Alkali diffusion may be uphill or downhill, depending on which direction of net flux is required to produce a two-liquid type distribution. Once this distribution is attained, subsequent diffusion of all melt species is slow and apparently limited by the diffusivity of SiO2, which is 10–9-l0–10 cm2/s at 1,200° C. Interdiffusion experiments involving molten basalt and synthetic granite confirm the behavior illustrated by the feldspar/basalt results, and give similar SiO2 diffusivities.The solution rates and interdiffusion data can be used to model basalt contamination processes likely to occur in the continental crust. For the restricted case of solid quartzitic xenoliths, the uptake of SiO2 in a well-mixed basalt magma is quite fast: appreciable SiO2 contamination may occur over exposure times of only days to years. If basalt magma induces local melting of crustal rocks, the assimilation process becomes one of liquid-state interdiffusion. In this case, the varying diffusivities of ions and their differing preferences for silicic relative to basaltic melts can produce marked selective contamination effects. Selective contamination of ascending basaltic magmas is particularly likely in the case of K2O, which may be introduced in substantial amounts even when other elements remain unaffected. The Na2O content of mantle-derived magmas is buffered against contamination by crustal materials, and K2O is buffered against further increases once it reaches a level of 1–1.5 wt.%.  相似文献   

3.
The diffusion rate of 18O tracer atoms in anorthite (An97Ab03) under anhydrous conditions has been measured using SIMS techniques. The tracer source was 18O2 98.4% gas at 1 bar, in the temperature range 1300° C–850° C. The measured diffusion constants are D 0=1 –0.6 +1 ×10–9 m2s–1 Q=236±8 kJ mol–1 Comparison of these values with published data for 18O diffusion in anorthite under hydrothermal conditions shows that dry oxygen diffusivities are orders of magnitude lower than equivalent wet values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.  相似文献   

4.
The dehydration rate of hydrous rhyolitic glasses at 475–875 °C was measured by in situ infrared (IR) spectroscopy in order to determine the diffusion coefficient of water in rhyolitic glasses. The IR spectra of glass thin sections were obtained at 90-s intervals during 90 min at high temperatures, and the change in absorbance at 3550 cm–1 corresponding to total water was monitored. The diffusion coefficients obtained from dehydration rates of the rhyolitic glasses are considered to be averaged value over the water-concentration profile in the sample. The averaged apparent diffusion coefficients increase with the initial total water content from 0.20 m2 s–1 for 0.7 wt% to 0.37 m2 s–1 for 2.8 wt% at 700 °C. The apparent activation energy for the diffusion of total water decreases with increasing initial water content from 112 ± 6 kJ mol–1 for 0.7 wt% to 60 ± 17 kJ mol–1 for 4.1 wt%. Assuming a linear relation between the diffusion coefficient of total water and the total water content, the diffusion coefficients at each initial total water content were also determined. The diffusion coefficients of total water at the water contents of 0.7 and 1.9 wt% and at 0.1 MPa were best fitted by ln D=[(12.9 ± 0.8) – (111 500 ± 6400)/RT] and ln D=[(10.6 ± 0.4) – (86 800 ± 2800)/RT], respectively, and are in agreement with previous data (D in m2 s–1, T in K). The present in situ IR dehydration experiment is a rapid and effective method for the determination of water diffusivity at high temperatures.  相似文献   

5.
Self-diffusion of Si under anhydrous conditions at 1 atm has been measured in natural zircon. The source of diffusant for experiments was a mixture of ZrO2 and 30Si-enriched SiO2 in 1:1 molar proportions; experiments were run in crimped Pt capsules in 1-atm furnaces. 30Si profiles were measured with both Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis with the resonant nuclear reaction 30Si(p,γ)31P. For Si diffusion normal to c over the temperature range 1,350–1,550°C, we obtain an Arrhenius relation D = 5.8 exp(−702 ± 54 kJ mol−1/RT) m2 s−1 for the NRA measurements, which agrees within uncertainty with an Arrhenius relation determined from the RBS measurements [62 exp(−738 ± 61 kJ mol−1/RT) m2 s−1]. Diffusion of Si parallel to c appears slightly faster, but agrees within experimental uncertainty at most temperatures with diffusivities for Si normal to c. Diffusion of Si in zircon is similar to that of Ti, but about an order of magnitude faster than diffusion of Hf and two orders of magnitude faster than diffusion of U and Th. Si diffusion is, however, many orders of magnitude slower than oxygen diffusion under both dry and hydrothermal conditions, with the difference increasing with decreasing temperature because of the larger activation energy for Si diffusion. If we consider Hf as a proxy for Zr, given its similar charge and size, we can rank the diffusivities of the major constituents in zircon as follows: D Zr < D Si << D O, dry < D O, ‘wet’.  相似文献   

6.
 Diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800–1100° C, water contents of 0–7.3 wt%, and oxygen fugacities from the quartz-fayalite-magnetite buffer to air. Experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days. Electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses. Regression of the diffusion data in dry rhyolite melt gives Dsulfur=0.05·exp{−221±80RT}, which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as H2O, CO2 and Cl-. Diffusion of sulfur in melt with 7 wt% dissolved water is 1.5 to 2 orders of magnitude faster than diffusion in the anhydrous melt, depending on temperature. Sulfur is known to dissolve in silicate melts as at least two different species, S2− and S6+, the proportions of which vary with oxygen fugacity; despite this, oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions. This result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity. The most likely candidate for the diffusing species is the sulfide ion, S2−. Re-equilibration between S2− and S6+ in oxidized melts must generally be slow compared to S2− diffusion in order to explain the observed results. In a silicic melt undergoing degassing, sulfur will tend to be fractionated from other volatile species which diffuse more rapidly. This is consistent with analyses of tephra from the 1991 eruption of Mount Pinatubo, Philippines, and from other high-silica volcanic eruptions. Received: 26 April 1995 / Accepted: 1 November 1995  相似文献   

7.
Diffusion coefficients of Cr and Al in chromite spinel have been determined at pressures ranging from 3 to 7 GPa and temperatures ranging from 1,400 to 1,700°C by using the diffusion couple of natural single crystals of MgAl2O4 spinel and chromite. The interdiffusion coefficient of Cr–Al as a function of Cr# (=Cr/(Cr + Al)) was determined as D Cr–Al = D 0 exp {−(Q′ + PV*)/RT}, where D 0 = exp{(10.3 ± 0.08) × Cr#0.54±0.02} + (1170 ± 31.2) cm2/s, Q′ = 520 ± 81 kJ/mol at 3 GPa, and V* = 1.36 ± 0.25 cm3/mol at 1,600°C, which is applicable up to Cr# = 0.8. The estimation of the self-diffusion coefficients of Cr and Al from Cr–Al interdiffusion shows that the diffusivity of Cr is more than one order of magnitude smaller than that of Al. These results are in agreement with patterns of multipolar Cr–Al zoning observed in natural chromite spinel samples deformed by diffusion creep.  相似文献   

8.
Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite-H2O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400-600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e−5 exp (−123,382 J/RT) cm2/s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e−10 exp (−62484 J/RT) cm2/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e−6 exp (−156,528 J/RT) cm2/s for temperatures between 450 and 700 °C and D = 1.39e−14 exp (−34518 J/RT) cm2/s for temperatures between 50 and 400 °C, where the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively.Results from these new experiments have implications for isotopic exchange during natural UO2-water interactions. The exceptionally low δ18O values of natural uraninites (i.e. −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite-water and UO3-water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ18O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ18O values of ca. −18‰, despite petrographic and U-Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material.  相似文献   

9.
Halogen diffusion in a basaltic melt   总被引：2，自引：0，他引：2  
The diffusion of the halogens fluorine, chlorine and bromine was measured in a hawaiitic melt from Mt. Etna at 500 MPa and 1.0 GPa, 1250 to 1450 °C at anhydrous conditions; the diffusion of F and Cl in the melt was also studied with about 3 wt% of dissolved water. Experiments were performed using the diffusion-couple technique in a piston cylinder. Most experiments were performed with only one halogen diffusing between the halogen-enriched and halogen-poor halves of the diffusion couple, but a few experiments with a mixture of halogens (F, Cl and Br) were also performed in order to investigate the possibility of interactions between the halogens during diffusion. Fluorine and chlorine diffusivity show a very similar behavior, slightly diverging at low temperature. Bromine diffusion is a factor of about 2-5 lower than the other halogens in this study. Diffusion coefficients for fluorine range between 2.3 × 10−11 and 1.4 × 10−10 m2 s−1, for chlorine between 1.1 × 10−11 and 1.3 × 10−10 and for bromine between 9.4 × 10−12 and 6.8 × 10−11 m2 s−1. No pressure effect was detected at the conditions investigated. In experiments involving mixed halogens, the diffusivities appear to decrease slightly (by a factor of ∼3), and are more uniform among the three elements. However, activation energies for diffusion do not appear to differ between experiments with individual halogens or when they are all mixed together. The effect of water increases the diffusion coefficients of F and Cl by no more than a factor of 3 compared to the anhydrous melt (DF = 4.0 × 10−11 to 1.6 × 10−10 m2 s−1; DCl = 3.0 × 10−11 to 1.9 × 10−10 m2 s−1). Comparing our results to the diffusion coefficients of other volatiles in nominally dry basaltic melts, halogen diffusivities are about one order of magnitude lower than H2O, similar to CO2, and a factor of ∼5 higher than S. The contrasting volatile diffusivities may affect the variable extent of volatile degassing upon melt depressurization and vesiculation, and can help our understanding of the compositions of rapidly grown magmatic bubbles.  相似文献   

10.
Diffusion couples made from homogeneous gem quality natural pyrope and almandine garnets were annealed within graphite capsules under anhydrous conditions at 22–40 kbar, 1057–1400 °C in a piston-cylinder apparatus. The concentration profiles that developed in each couple were modeled to retrieve the self diffusion coefficients [D(I)] of the divalent cations Fe, Mg, Mn and Ca. Because of their usually low concentrations and lack of sufficient compositional change across the interface of the diffusion couples, only a few reliable data can be obtained for D(Ca) and D(Mn) from these experiments. However, nine sets of D(Fe) and D(Mg) data were retrieved in the above P-T range, and cast in the form of Arrhenian relation, D=D 0exp{−[Q(1 bar)+PΔV +]/RT}. The values of the activation energy (Q) and activation volume (ΔV +) depend on whether f O2 is constrained by graphite in the system C-O or held constant. For the first case, we have for Fe:Q(1 bar)=65,532±10,111 cal/mol, D 0=3.50 (±2.30)×10−5 cm2/s, ΔV +=5.6(±2.9) cm3/mol, and for Mg:Q(1 bar)=60,760±8,257 cal/mol, D 0=4.66 (±2.48)×10−5 cm2/s, ΔV +=5.3(±3.0) cm3/mol. Here the ΔV + values have been taken from Chakraborty and Ganguly (1992). For the condition of constant f O2, the Q values are ∼9 kcal lower and ΔV + values are ∼4.9 cm3/mol larger than the above values. Lower temperature extrapolation of the Arrhenian relation for D(Mg) is in good agreement with the Mg tracer diffusion data (D * Mg) of Chakraborty and Rubie (1996) and Cygan and Lasaga (1985) at 1 bar, 750–900 °C, when all data are normalized to the same pressure and to f O2 defined by graphite in the system C-O. The D * Mg data of Schwandt et al. (1995), on the other hand, are lower by more than an order of magnitude than the low temperature extrapolation of the present data, when all data are normalized to the same pressure and to f O2 defined by the graphite buffer. Comparison of the D(Fe), D(Mg) and D(Mn) data in the pyrope-almandine diffusion couple with those in the spessartine-almandine diffusion couple of Chakraborty and Ganguly (1992) shows that the self diffusion of Fe and Mn are significantly enhanced with the increase in Mn/Mg ratio; the enhancement effect on D(Mg) is, however, relatively small. Proper application of the self diffusion data to calculate interdiffusion coefficient or D matrix elements for the purpose of modeling of diffusion processes in natural garnets must take into account these compositional effects on D(I) along with the effects of thermodynamic nonideality, f O2, and pressure. Received: 8 May 1997 / Accepted: 2 October 1997  相似文献   

11.
Despite the growing interest for Li and B as geochemical tracers, especially for material transfer from subducting slabs to overlying peridotites, little is known about the behaviour of these two elements during partial melting of mantle sources. In particular, mineral/melt partition coefficients for B and to a lesser extent Li are still a matter of debate. In this work, we re-equilibrated a synthetic basalt doped with ~10 ppm B and ~6 ppm Li with an olivine powder from a spinel lherzolite xenolith at 1 GPa–1,330°C, and we analyzed Li and B in the run products by secondary ion mass spectrometry (SIMS). In our experiment, B behaved as a highly incompatible element with mineral/melt partition coefficients of the order of 10−2 (D ol/melt = 0.008 (0.004–0.013); D opx/melt = 0.024 (0.015–0.033); D cpx/melt = 0.041 (0.021–0.061)), and Li as a moderately incompatible element (D ol/melt = 0.427 (0.418–0.436); D opx/melt = 0.211 (0.167–0.256); D cpx/melt = 0.246 (0.229–0.264)). Our partition coefficients for Li are in good agreement with previous determinations. In the case of B, our partition coefficients are equal within error to those reported by Brenan et al. (1998) for all the mineral phases analyzed, but are lower than other coefficients from literature for some of the phases (up to 5 times for cpx). Our measurements complement the data set of Ds for modelling partial melting of the upper mantle and basalt generation, and confirm that, in this context, B is more incompatible than previously anticipated.  相似文献   

12.
Milke et al. (Contrib Mineral Petrol 142:15–26, 2001) studied the diffusion of Si, Mg and O in synthetic polycrystalline enstatite reaction rims. The reaction rims were grown at 1,000°C and 1 GPa at the contacts between forsterite grains with normal isotopic compositions and a quartz matrix extremely enriched in 18O and 29Si. The enstatite reaction rim grew from the original quartz-forsterite interface in both directions producing an inner portion, which replaced forsterite and an outer portion, which replaced quartz. Here we present new support for this statement, as the two portions of the rim are clearly distinguished based on crystal orientation mapping using electron backscatter diffraction (EBSD). Milke et al. (Contrib Mineral Petrol 142:15–26, 2001) used the formalism of LeClaire (J Appl Phys 14:351–356, 1963) to derive the coefficient of silicon grain boundary diffusion from stable isotope profiles across the reaction rims. LeClaires formalism is designed for grain boundary tracer diffusion into an infinite half space with fixed geometry. A fixed geometry is an undesired limitation in the context of rim growth. We suggest an alternative model, which accounts for simultaneous layer growth and superimposed silicon and oxygen self diffusion. The effective silicon bulk diffusivity obtained from our model is approximately equal within both portions of the enstatite reaction rim: D Si,En eff =1.0–4.3×10–16 m2 s–1. The effective oxygen diffusion is relatively slow in the inner portion of the reaction rim, D O,En eff =0.8–1.4×10–16 m2 s–1, and comparatively fast, D O,En eff =5.9–11.6×10–16 m2 s–1, in its outer portion. Microstructural evidence suggests that transient porosity and small amounts of fluid were concentrated at the quartz-enstatite interface during rim growth. This leads us to suspect that the presence of an aqueous fluid accelerated oxygen diffusion in the outer portion of the reaction rim. In contrast, silica diffusion does not appear to have been affected by the spatial variation in the availability of an aqueous fluid.
  相似文献   

13.
Chemical diffusion of Pb has been measured in K-feldspar (Or93) and plagioclase of 4 compositions ranging from An23 to An93 under anhydrous, 0.101 MPa conditions. The source of diffusant for the experiments was a mixture of PbS powder and ground feldspar of the same composition as the sample. Rutherford Backscattering (RBS) was used to measure Pb diffusion profiles. Over the temperature range 700–1050°C, the following Arrhenius relations were obtained (diffusivities in m2s-1):Oligoclase (An23): Diffusion normal to (001): log D = ( – 6.84 ± 0.59) – [(261 ± 13 kJ mol –1)/2.303RT]Diffusion normal to (010): log D = ( – 3.40 ± 0.50) – [(335 ± 11 kJ mol –1)/2.303RT]Andesine (An43): Diffusion normal to (001): log D = ( – 6.73 ± 0.54) – [(266 ± 12 kJ mol –1)/2.303RT] Diffusion normal to (010) appears to be only slightly slower than diffusion normal to (001) in andesine.Labradorite (An67): Diffusion normal to (001): log D = ( – 7.16 ± 0.61) – [(267 ± 13 kJ mol –1)/2.303RT] Diffusion normal to (010) is slower by 0.7 log units on average.Anorthite Diffusion normal to (010): log D = ( – 5.43 ± 0.48) – [(327 ± 11 kJ mol –1)/2.303RT]K-feldspar (Or93): Diffusion normal to (001): log D = ( – 4.74 ± 0.52) – [(309 ± 16 kJ mol –1)/2.303RT] Diffusion normal to (010): log D = ( – 5.99 ± 0.51) – [(302 ± 11 kJ mol –1)/2.303RT]In calcic plagioclase, Pb uptake is correlated with a reduction of Ca, indicating the involvement of PbCa exchange in chemical diffusion. Decreases of Na and K concentrations in sodic plagioclase and K-feldspar, respectively, are correlated with Pb uptake and increase in Al concentration (measured by resonant nuclear reaction analysis), suggesting a coupled process for Pb exchange in these feldspars. These results have important implications for common Pb corrections and Pb isotope systematics. Pb diffusion in apatite is faster than in the investigated feldspar compositions, and Pb diffusion rates in titanite are comparable to both K-feldspar and labradorite. Given these diffusion data and typical effective diffusion radii for feldspars and accessory minerals, we may conclude that feldspars used in common Pb corrections are in general less inclined to experience diffusion-controlled Pb isotope exchange than minerals used in U-Pb dating that require a common Pb correction.  相似文献   

14.
Colin H. Donaldson 《Lithos》1975,8(2):163-174
Concentration gradients in glass adjacent to skeletal olivines in a DSDP basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170°C, the diffusion coefficient of Mg2+ ions in the basalt is 4·5.10?9 cm2/s. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates of 2–6.10?7 cm/s. This is too slow for olivine to have grown in situ during quenching. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.  相似文献   

15.
Kinetics of hydrogen extraction were investigated by FTIR spectroscopy on a Dora Maira pyrope single crystal. Annealing experiments were performed at ambient pressure, between 1073 and 1323 K, and under two different redox conditions (pO2=0.21 atm and pO2 10–16 atm). During hydrogen extraction the two principal OH absorption bands behave independently. The OHa triplet, centred on 3651 cm–1, decreases at least five times faster than the OHb band at 3602 cm–1. This suggests the presence of two distinct H defects, each with its own kinetics, which are slightly dependent on oxygen partial pressure:Both kinetics are slower than previous data (Wang et al. 1996), but activation energies are similar. At the same time as the OHa bands decrease, an exchange of hydrogen from OHa to OHb occurs. The extent of this transfer can be modelled by assuming it is directly proportional to the fraction of OHa band which has been removed, independent of temperature. This suggests that OHb defects could be produced by partial dehydrogenation of OHa defects. Activation energies and values of diffusion coefficients are very close to those found by Kohlstedt and Mackwell (1998) for the diffusion of metal vacancies in olivine. Thus it is very likely that cation vacancies control the kinetics of dehydrogenation reactions in pyrope.  相似文献   

16.
Diffusivities for calcium, iron, magnesium, manganese and aluminum have been measured for St. John's olivine undergoing cation exchange with synthetic basaltic melts. The variety of temperature, pressure and fO2 conditions under which the diffusivities were measured complement the equilibrium-partitioning study of calcium in olivine-bearing basalts by Jurewicz and Watson, 1988. Olivine was found to be anisotropic with respect to the diffusion of calcium, iron, magnesium and manganese. This anisotropy is a weak function of temperature, but strongly dependent upon oxygen fugacity.Because diffusion is independent of olivine composition over the small range of compositions used in this study, it could be shown that the absolute values of the diffusion coefficients were also functions of temperature and fO2. At near-atmospheric total pressure and an oxygen fugacity of 10–8atm, D Fe>D Mn>D Ca and D MgD Mn for a range of geologically reasonable temperatures. These relative diffusivities were shown to change with oxygen fugacity. The power-law dependence of diffusion on oxygen partial-pressure was determined for each cation and the results are consistent with the range of values given by Stocker (1978) and by other workers.For Ca and Fe, the effect of hydrostatic pressure on diffusion appears to be weak, at least for transport parallel to the c crystallographic direction. Unfortunately, no true activation volumes (or other pressure-related parameters) could be computed because the oxygen fugacity was not held constant over changes in pressure, and because accurate post-experiment reconstruction of sample orientation was not possible. Al was found to enter high-pressure olivines at concentrations of up to 0.14 weight percent, thus allowing aluminum diffusion to be characterized. The diffusivity of aluminum is, within error, the same as iron at 20 kb at 1430° C at the ambient fO2 of our piston-cylinder cells. This correspondence suggests that diffusion of Al may depend on transport of either Fe or of Fe +3 defects. While the results of these experiments are generally consistent with results published elsewhere, there are important inconsistencies. Tracer diffusion and interdiffusion in pure, ordered, olivine endmembers (e.g., tephroite and forsterite) showed significantly higher activation energies. This discrepancy could reflect the role of Fe+3 defects in diffusion; however, it may also suggest that order-disorder phenomena may be significant factors influencing diffusion in analog systems.The results of this study are applied to four petrologic problems: (1) calculation of rates of equilibration for olivine xenocrysts; (2) calculation of closure temperatures for the CaO/MgO olivine/basalt geothermometer (Jurewicz and Watson 1988); (3) delineation of an intrinsic-/O2 geobarometer; and (4) investigation of the dependence of olivine dissolution upon crystallographic orientation. In addition, it is demonstrated that diffusion-exchange experiments are useful for studying the dominant point-defect mechanisms for cation diffusion.Currently, a visiting scientist with Air Force Wright Aeronautical Laboratories Materials Laboratory (MLLM), Wright-Patterson AFB, OH 45433  相似文献   

17.
It has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours. The presence of phenocrysts larger than 0.5 mm increased considerably the efficiency of mixing. Flow patterns in the experimental system were visualized using Pt spheres, which indicated that convection occurs in basalt melt, but not in dacite melt. The Reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10–3 and 10–6, respectively. Mixing proceeds initially by mechanical mixing of the two magmas in a large scale, but later by coupling interfacial convection and mutual diffusion. Thus, depending on the depth where vesiculation and following disruption of the magma occurs, banded pumice, homogeneous pumice and homogeneous andesite lava are erupted. The observed textures of mixed rocks of Plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model.  相似文献   

18.
The partitioning of samarium and thulium between garnets and melts in the systems Mg3Al2-Si3O12-H2O and Ca3Al2Si3O12-H2O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D Sm (D Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10–13 cm2 s–1 at 1,300 ° C and 2 × 10–12 cm2 s–1 at 1,500 ° C, and for grossular, 8.3 × 10–12 cm2 s–1 at 1,200 ° C and 4.6 × 10–11 cm2 s–1 at 1,300 ° C. The equilibrium values of D Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass.Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm3Al5O12 in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) DREE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions.The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg3Al2Si3O12-H2O at low Sm concentrations. These change D Sm in the defect region, demonstrating their role in the production of vacancies.Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature.  相似文献   

19.
The Adam-Gibbs equations describing relaxation in silicate melts are applied to diffusion of trace components of multicomponent liquids. The Adam-Gibbs theory is used as a starting point to derive an explicit relation between viscosity and diffusion including non-Arrhenian temperature dependence. The general form of the equation is Diη = Aiexp{Δ(scEi)/TSc}, where D is diffusivity, η is melt viscosity, T is absolute temperature, Δ(scEi) is the difference between the products of activation energies and local configurational entropies for viscous and diffusive relaxation, Ai is a constant that depends on the characteristics of the diffusing solute particles, and Sc is configurational entropy of the melt. The general equation will be impractical for most predictive purposes due to the paucity of configurational entropy data for silicate melts. Under most magmatic conditions the proposed non-Arrhenian behaviour can be neglected, allowing the general equation to be simplified to a generalized form of the Eyring equation to describe diffusion of solutes that interact weakly with the melt structure: Diη/T = Qiexp{ΔEi/RT}, where Qi and ΔEi depend on the characteristics of the solute and the melt structure. If the diffusing solute interacts strongly with the melt structure or is a network-forming cation itself, then ΔEi = 0, and the relation between viscosity and diffusion has the functional form of the classic Eyring and Stokes-Einstein equations; Diη/T = Qi. If the diffusing solute can make diffusive jumps without requiring cooperative rearrangement of the melt structure, the diffusivity is entirely decoupled from melt viscosity and should be Arrhenian, i.e., Di = Qiexp{Bi/T}. A dataset of 594 published diffusivities in melts ranging from the system CAS through diopside, basalt, andesite, anhydrous rhyolite, hydrous rhyolite, and peralkaline rhyolite to albite, orthoclase, and jadeite is compared with the model equations. Alkali diffusion is completely decoupled from melt viscosity but is related to melt structure. Network-modifying cations with field strength Zi2/r between 1 and 10 interact weakly with the melt network and can be modelled with the extended form of the Eyring equation. Diffusivities of cations with high field strength have activation energies essentially equal to that of viscous flow and can be modelled with a simple reciprocal Eyring-type dependence on viscosity. The values of Qi, ΔEi and Bi for each cation are different and can be related to the cation charge and radius as well as the composition of the melt through the parameters Zi2/r, M/O, and Al/(Na + K + H). I present empirical fit parameters to the model equations that permit prediction of cation diffusivities given only charge and radius of the cation and temperature, composition and viscosity of the melt, for the entire range of temperatures accessible to magmas near to or above their liquidus, for magmas ranging in composition from basalt through andesite to hydrous or anhydrous rhyolite. Pressure effects are implicitly accounted for by corrections to melt viscosity. Ninety percent of diffusivities predicted by the models are within 0.6 log units of the measured values.  相似文献   

20.
Huaiwei Ni  Youxue Zhang   《Chemical Geology》2008,250(1-4):68-78
Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H2O diffusion is not constrained satisfactorily. We investigated H2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H2Ot) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H2O diffusion in rhyolitic melt is controlled by the mobility of molecular H2O (H2Om), the diffusivity of H2Om (DH2Om) at H2Ot ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by
DH2Om=D0exp(aX),
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