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This study is aimed at determining the diffusion coefficient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl
system, in which the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and
Ca were measured at the temperatures of 750 – 1400°C, pressures of 0.001 × 108 – 2 × 108 Pa, and initial H2O contents of 0 wt% –6.9 wt% in the granitic melts. The diffusion coefficients of Fe and Mg were difficult to resolve. In
all experiments a NaCl melt was present as well. In the absence of H2O, the diffusion of net-work modifiers follows an Arrhanious equation at 1 × 105 Pa: lgDca=−3. 88−5140/T, lgDk =−3. 79−4040/T, and lgDNa, =−4.99−3350/T, where D is in cm2 /s andT is in K. The diffusion coefficients of Ca, Na, K, and Fe increase non-linearly with increasing H2O content in the melt. The presence of about 2 wt% H2O m the melt will lead to a dramatical increase in diffusivity, but higher H2O content has only a minor effect. This change is probably the result of a change in the melt structure when H2O is present. The diffusion coefficients measured in this study are significantly different from those in previous works.
This may be understood in terms of the “transient two-liquid equilibrium” theory. Element interdiffusion depends not only
on its concentration, but also on its activity co-efficient gradient, which is reflected by the distribution coefficient,
of the two contacting melts. 相似文献
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Sylvie Demouchy Stephen J. Mackwell David L. Kohlstedt 《Contributions to Mineralogy and Petrology》2007,154(3):279-289
Interdiffusion of Fe and Mg in (Mg,Fe)O has been investigated experimentally under hydrous conditions. Single crystals of
MgO in contact with (Mg0.73Fe0.27)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
Mg1−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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Sylvie DemouchyEmail: |
3.
Fe-Mg interdiffusivities in (Fe,Mg)O magnesiowüstite have been measured in experiments conducted at pressures of 7-35 GPa and temperatures of 1573-1973 K using a Kawai-type high-pressure apparatus. The diffusion profiles were measured across the interface between MgO and (Fe0.5,Mg0.5)O samples by electron microprobe analysis, and the Fe-Mg interdiffusivities were determined as DFe-Mg=D0exp{−E*(1+PV*Mg/E*Mg)/RT}, where D0=4.1(+16.1−3.3)×10−7 m2/s, E*=(1−CMg)E*Fe+CMgE*Mg (activation energy for the concentration of Mg, where E*Fe=113(±74) kJ/mol and E*Mg=226(±32) kJ/mol), the activation volume V*Mg=1.8(±1.2)×10−6 m3/mol. By extrapolating these results to the P-T conditions of the core-mantle boundary, we conclude that the interdiffusivity of Fe-Mg in magnesiowüstite at the bottom of the lower mantle is at least one order of magnitude larger than that at the top of the lower mantle. 相似文献
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