Rebuilding the theory of isotope fractionation for evaporation of silicate melts under vacuum condition  

作　　者：Jie Wang Yun Liu 

机构地区：[1]Research Center for Planetary Science,College of Earth and Planetary Sciences,Chengdu University of Technology,Chengdu 610059,China [2]State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China [3]CAS Center for Excellence in Comparative Planetology,Hefei 230026,China

出　　处：《Acta Geochimica》2024年第4期661-676,共16页地球化学学报（英文）

基　　金：supported by Chinese NSF project(42,130,114);the strategic priority research program(B)of CAS(XDB41000000);the pre-research Project on Civil Aerospace Technologies No.D020202 funded by Chinese National Space Administration(CNSA)and Guizhou Provincial 2021 Science and Technology Subsidies(No.GZ2021SIG).

摘　　要：Isotope eff ects are pivotal in understanding silicate melt evaporation and planetary accretion processes.Based on the Hertz-Knudsen equation,the current theory often fails to predict observed isotope fractionations of laboratory 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 species 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 transfer,chemical reaction,and nucleation.Our derivations reveal a kinetic isotopic fractionation factor(KIFF orα)αour model=[m(^(1)species)/m(^(2)species)]^(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 eff ectively 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 overall rate of evaporation can be determined by two steps jointly can account for the eff ects of low P_(H2)pressure,composition,and temperature.In addition,we find that chemical reactions,diffusion,and nucleation can control the overall rate of evaporation of silicate melts by using the fitting slope in ln(−ln f)versus ln(t).Notably,our model allows for the theoretical calculations of parameters like activation energy(E_(a)),providing a novel approach to studying compositional and environmental eff ects on evaporation processes,and shedding light on the formation and evolution of the proto-solar and Earth-Moon systems.

关 键 词：EVAPORATION Kinetic isotopic fractionation Chemical kinetics Hertz-Knudsen equation CAIS 

分 类 号：P18[天文地球—天文学]