First-principles investigation of the concentration effect on equilibrium fractionation of Ca isotop

• 作者: Yahui Song Yonghui Li Wenzhong Wang Zhongqing Wu
• 来源期刊: 地球化学学报(英文）
• 关键词: Ca isotope First-principles calculations Forsterite Concentration effect Equilibrium isotope fractionation
• 年，卷（期）: 2019,（4）
• 页码范围: 497-507
• 页数: 10页
• 语种: 英文
• DOI: 10.1007/s11631-019-00346-w

摘要

Previous theoretical studies have found that the concentration variations within a certain range have a prominent effect on inter-mineral equilibrium isotope fractionation (103lnα). Based on the density functional theory, we investigated how the average Ca–O bond length and the reduced partition function ratios (103 lnβ) and 103lnα of 44Ca/40Ca in forsterite (Fo) are affected by its Ca concentration. Our results show that Ca–O bond length in forsterite ranges from 2.327 to 2.267 Å with the Ca/ (Ca + Mg) varying between a narrow range limited by an upper limit of 1/8 and a lower limit of 1/64. However, outside this narrow range, i.e., Ca/(Ca + Mg) is lower than 1/64 or higher than 1/8, Ca–O bond length becomes insensitive to Ca concentration and maintains to be a constant. Because the 103lnβ is negatively correlated with Ca–O bond length, the 103lnβ significantly increases with decreasing Ca/(Ca + Mg) when 1/64 < Ca/(Ca + Mg) < 2/16. As a consequence, the 103lnα between forsterite and other minerals also strongly depend on the Ca content in forsterite. Combining previous studies with our results, the heavier Ca isotopes enrichment sequence in minerals is: forsterite > orthopyroxene > clinopyroxene > calcite ≈ diopside > dolomite > aragonite. Olivine and pyroxenes are enriched in heavier Ca isotope compared to carbonates. The 103lnα between forsterite with a Ca/(Ca + Mg) of 1/64 and clinopyroxene (Ca/Mg = 1/1, i.e., diopside) is up to ~ 0.64‰ at 1200 K. The large 103lnαFo-diopside relative to the current analytical precision for Ca isotope measurements suggests that the dependence of 103lnαFo-diopside on temperature can be used as a ther mometer, similar to the one based on the 103lnα of 44Ca/40Ca between orthopyroxene and diopside. These two Ca isotope thermometers both have a precision approximate to that of elemental thermometers and provide independent constraints on temperature.