首页|Rebuilding the theory of isotope fractionation for evaporation of silicate melts under vacuum condition
Rebuilding the theory of isotope fractionation for evaporation of silicate melts under vacuum condition
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维普
Rebuilding the theory of isotope fractionation for evaporation of silicate melts under vacuum condition
Isotope effects are pivotal in understanding sili-cate melt evaporation and planetary accretion processes.Based on the Hertz-Knudsen equation,the current theory often fails to predict observed isotope fractionations of labo-ratory experiments due to its oversimplified assumptions.Here,we point out that the Hertz-Knudsen-equation-based theory is incomplete for silicate melt evaporation cases and can only be used for situations where the vaporized spe-cies is identical to the one in the melt.We propose a new model designed for silicate melt evaporation under vacuum.Our model considers multiple steps including mass trans-fer,chemical reaction,and nucleation.Our derivations reveal a kinetic isotopic fractionation factor(KIFF or α)αour model=[m(1species)/m(2species)]0.5,where m(species)is the mass of the reactant of reaction/nucleation-limiting step or species of diffusion-limiting step and superscript 1 and 2 represent light and heavy isotopes,respectively.This model can effectively reproduce most reported KIFFs of laboratory experiments for various elements,i.e.,Mg,Si,K,Rb,Fe,Ca,and Ti.And,the KIFF-mixing model referring that an over-all rate of evaporation can be determined by two steps jointly can account for the effects of low PH2 pressure,composition,and temperature.In addition,we find that chemical reac-tions,diffusion,and nucleation can control the overall rate of evaporation of silicate melts by using the fitting slope in ln(-lnf)versus ln(t).Notably,our model allows for the theo-retical calculations of parameters like activation energy(Ea),providing a novel approach to studying compositional and environmental effects on evaporation processes,and shed-ding light on the formation and evolution of the proto-solar and Earth-Moon systems.
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