Pressure and concentration effects on intermineral calcium isotope fractionation involving garnet
Pressure and concentration effects on intermineral calcium isotope fractionation involving garnet
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DOI：
10.1016/j.chemgeo.2022.120722
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发表时间：
2022-01
期刊：
影响因子：
3.9
通讯作者：
Yonghui Li;ZhongqingWu;Shichun Huang;Wenzhong Wang
中科院分区：
文献类型：
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作者：
Yonghui Li;ZhongqingWu;Shichun Huang;Wenzhong Wang
为了更好地理解钙（Ca）同位素在区分地球和其他岩石行星的火成岩和变质过程中的行为，我们采用第一性原理计算方法研究了压力和Ca/Fe浓度对石榴石中平均Casingle键O键长和约化配分函数比44 Ca/40 Ca（103 ln 44/40 Ca β）的影响。计算结果表明：（1）随着Ca浓度的增加，平均Casingle键O键长增加，103 ln 44/40 Ca β减小（1000 K时Ca/（Fe + Ca + Mg）从1/24到12/12约为0.41‰）;与斜方辉石和镁橄榄石的Casingle键O键长和103 ln 44/40 Ca β仅在较窄范围内对Ca浓度敏感不同，石榴石的Casingle键O键长和103 ln 44/40 Ca β在较宽范围内对Ca浓度敏感。(2)随Fe含量的增加，平均Casingle键O键长增加，103 ln 44/40 Ca β减小（1000 K时减小0.12‰，FeO含量为12.4wt%）。(3)在1000 K时，压力从0增加到3GPa，平均Casingle键O键长减小~0.014 μ m，103 ln 44/40 Ca β增大0.1‰-0.2‰。(4)利用压力和Ca/Fe浓度对103 ln 44/40 Ca α石榴石-单斜辉石影响的新结果，评价了天然样品中石榴石和单斜辉石Ca同位素组成的差异。大多数样品已经达到平衡，而有些样品超过了我们的预测，这可能反映了主要的影响或动力学分馏。
To better understand the behaviors of calcium (Ca) isotopes during igneous and metamorphic processes that differentiate the Earth and other rocky planets, we investigated the effects of pressure and Ca/Fe concentration on the average Casingle bondO bond length and the reduced partition function ratio of44Ca/40Ca (103ln44/40Caβ) in garnet using first-principles calculations. Our calculations show that (1) the average Casingle bondO bond length increases and 103ln44/40Caβ decreasing (approximately 0.41‰ at 1000 K from a Ca/(Fe + Ca + Mg) of 1/24 to 12/12) with increasing Ca concentrations. In contrast to orthopyroxene and forsterite, whose Casingle bondO bond lengths and 103ln44/40Caβ are sensitive to Ca concentrations only in narrow range, the garnet Casingle bondO bond length and 103ln44/40Caβ vary with Ca concentrations in wide ranges. (2) The average Casingle bondO bond length increases and 103ln44/40Caβ decreases with increasing Fe concentrations (by 0.12‰ at 1000 K, FeO content of 12.4 wt%). (3) The average Casingle bondO bond length decreases by ~0.014 Å, and 103ln44/40Caβ increases by 0.1‰–0.2‰ at 1000 K when the pressure increases from 0 to 3 GPa. (4) However, the pressure effect on 103ln44/40Caαgarnet–clinopyroxeneis insignificant.Using our new results of the effects of pressure and Ca/Fe concentration on 103ln44/40Caαgarnet–clinopyroxene, we evaluated the observed difference in Ca isotope compositions between garnet and clinopyroxene in natural samples. Most have reached equilibrium, whereas some samples exceed our prediction, which might reflect the majorite effect or kinetic fractionation.