Theoretical calculation of equilibrium Mg isotope fractionation between silicate melt and its vapor

• 作者: Haiyang Luo Huiming Bao Yuhong Yang Yun Liu
• 来源期刊: 地球化学学报(英文）
• 关键词: Equilibrium Mg isotope fractionation Force constant Structural optimization RPFR
• 年，卷（期）: 2018,（5）
• 页码范围: 655-662
• 页数: 7页
• 语种: 英文
• DOI: 10.1007/s11631-018-0287-2

摘要

Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium–aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (a) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt's isotopic compositions. However, equilib rium α is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO3 and Mg2SiO4 melt–vapor systems based on first-principles molecular dynamics and the high-temperature approximation of the Bigeleisen–Mayer equation. We found that, at 2500 K, δ25Mg values in the MgSiO3 and Mg2SiO4 melts were 0.141 ± 0.004 and 0.143 ± 0.003‰ more positive than in their respective vapors. The corresponding δ26Mg values were 0.270 ± 0.008 and 0.274 ± 0.006‰ more positive than in vapors, respectively. The general α–T equations describing the equilibrium Mg α in MgSiO3 and Mg2SiO4 melt–vapor systems were: respectively, where m is the mass of light isotope 24Mg and m' is the mass of the heavier isotope, 25Mg or 26Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution.