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首页> 外文期刊>Contributions to Mineralogy and Petrology >Experimental and computational study of trace element distribution between orthopyroxene and anhydrous silicate melt: substitution mechanisms and the effect of iron
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Experimental and computational study of trace element distribution between orthopyroxene and anhydrous silicate melt: substitution mechanisms and the effect of iron

机译：邻二甲苯和无水硅酸盐熔体中微量元素分布的实验和计算研究：取代机理和铁的作用

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摘要

Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to l,420°C in the system CaO-MgO-Al_2O_3-SiO_2 and subsystem CaO-MgO-SiO_2- We provide experimental partition coefficients for a wide range of trace elements (large ion lithophile: Li, Be, B, K, Rb, Sr, Cs, Ba, Th, U; rare earth elements, REE: La, Ce, Nd, Sm, Y, Yb, Lu; high field strength: Zr, Nb, Hf, Ta, Ti; transition metals: Sc, V, Cr, Co) for use in petrogenetic modelling. REE partition coefficients increase from D_La~(Opx-melt) approx 0.0005 to D_(Lu)~(Opx-melt) approx 0.109, D values for highly charged elements vary from D_Th~(Opx-melt) approx 0.0066 through D_(Nb)~(Opx-melt) approx 0.0033 and D_U~(Opx-melt) approx 0.0066 to D_(Ti)~(Opx-melt) approx 0.058, and are all virtually independent of temperature. Cr and Co are the only compatible trace elements at the studied conditions. To elucidate charge-balancing mechanisms for incorporation of REE into Opx and to assess the possible influence of Fe on Opx-melt partitioning, we compare our experimental results with computer simulations. In these simulations, we examine major and minor trace element incorporation into the end-members enstatite (Mg_2Si_2O_6) and ferrosilite (Fe_2Si_2O_6). Calculated solution energies show that R~(2+) cations are more soluble in Opx than R~(3+) cations of similar size, consistent with experimental partitioning data. In addition, simulations show charge balancing of R~(3+) cations by coupled substitution with Li~+ on the M l site that is energetically favoured over coupled substitution involving Al-Si exchange on the tetrahedrally coordinated site. We derived best-fit values for ideal ionic radii r_0, maximum partition coefficients D_0, and apparent Young's moduli E for substitutions onto the Opx M l and M 2 sites. Experimental r_0 values for R~(3+) substitutions are 0.66-0.67 A for Ml and 0.82-0.87 A for M2. Simulations for enstatite result in r_0 = 0.71-0.73 A for Ml and approx 0.79-0.87 A for M2. Ferrosilite r_0 values are systematically larger by approx 0.05 A for both Ml and M2. The latter is opposite to experimental literature data, which appear to show a slight decrease in r_0~(M2) in the presence of Fe. Additional systematic studies in Fe-bearing systems are required to resolve this inconsistency and to develop predictive Opx-melt partitioning models for use in terrestrial and lunar magmatic differentiation models.
机译：尽管在地幔和月球幔的各种岩浆分化过程中都存在邻苯二酚（Opx），但其对熔体痕量元素含量的影响尚未得到很好的量化。我们介绍了Opx和无水硅酸盐熔体之间痕量元素分配的组合实验和计算研究结果。 CaO-MgO-Al_2O_3-SiO_2和CaO-MgO-SiO_2子系统在大气中，大气压力和1,326至1,420°C的温度下进行了实验。我们提供了多种痕量元素的实验分配系数（大离子亲石剂：Li，Be，B，K，Rb，Sr，Cs，Ba，Th，U;稀土元素; REE：La，Ce，Nd，Sm，Y，Yb，Lu;高场强：Zr， Nb，Hf，Ta，Ti;过渡金属：Sc，V，Cr，Co），用于成岩建模。 REE分配系数从D_La〜（Opx-melt）约0.0005增加到D_（Lu）〜（Opx-melt）约0.109，高电荷元素的D值从D_Th〜（Opx-melt）约0.0066到D_（Nb） 〜（像素熔化）约0.0033和D_U〜（像素熔化）约0.0066至D_（Ti）〜（像素熔化）约0.058，并且几乎都与温度无关。在研究条件下，Cr和Co是唯一相容的微量元素。为了阐明将REE掺入Opx中的电荷平衡机制，并评估Fe对Opx熔体分配的可能影响，我们将实验结果与计算机模拟进行了比较。在这些模拟中，我们检查了主要和次要痕量元素掺入末端成员顽辉石（Mg_2Si_2O_6）和硅铁矿（Fe_2Si_2O_6）中的情况。计算得出的溶液能表明，R〜（2+）阳离子比相似大小的R〜（3+）阳离子更易溶于Opx，与实验分配数据一致。另外，模拟显示通过在M l位点上用Li〜+进行偶合取代，R〜（3+）阳离子的电荷平衡，在能量上优于在四面体配位位点上涉及Al-Si交换的偶合取代。我们得出了理想离子半径r_0，最大分配系数D_0和视在杨氏模量E的最佳拟合值，以替代Opx M 1和M 2位置。 R 1（3+）取代的实验r_0值对于M1为0.66-0.67 A，对于M2为0.82-0.87A。顽辉石的模拟得出M1的r_0 = 0.71-0.73 A，M2的约为0.79-0.87A。 M1和M2的硅铁矿r_0值都系统地增大约0.05A。后者与实验文献数据相反，后者在铁的存在下似乎显示r_0〜（M2）略有下降。需要对含铁系统进行更多的系统研究，以解决这种矛盾并开发可预测的Opx​​-熔体分配模型，以用于陆地和月球岩浆分化模型。

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