A predictive model for rare earth element partitioning between clinopyroxene and anhydrous silicate melt
A predictive model for rare earth element partitioning between clinopyroxene and anhydrous silicate melt
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DOI：
10.1007/s004100050330
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发表时间：
1997-10
影响因子：
3.5
通讯作者：
B. Wood;J. Blundy
中科院分区：
文献类型：
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作者：
B. Wood;J. Blundy
本文提出了一个定量模型来描述稀土元素（REE）和钇在单斜辉石和无水硅酸盐熔体之间的分配随压力（P）、温度（T）和体相组成（X）的变化。该模型基于Brice（1975）方程，该方程将元素i（Di）的分配系数与元素o（Do）的分配系数相关联，其中后者与感兴趣的晶体学位点（在这种情况下为单斜辉石M2位点）具有相同的离子半径：NA是阿伏伽德罗数，EM 2是该位点的杨氏模量，R是气体常数，T是K。通过拟合Brice方程得到的EM 2值实验稀土分配系数模式是在良好的协议与那些从众所周知的体积模量，金属-氧距离和阳离子电荷之间的相关性。用此关系式约束3+阳离子的EM ~ 2，然后用Brice方程拟合同时测定3个或3个以上REE分配系数的实验数据，得到82个Doandro值。后者被认为是一个简单的和晶体化学合理的功能的单斜辉石组成。我们发现，对于任何单斜辉石熔体对ifD为一个中间的稀土元素（如Sm或Gd）是已知的，那么Brice方程可以用来predictDs为所有其他的稀土元素，与不确定性类似的实际测量中所涉及的。通过对稀土元素晶体相和熔体相的热力学描述，对该模型进行了推广，估算了假想稀土元素REEMgAlSiO 6和Na0.5REE0.5MgSi2O6的熔融自由能（ΔGf）。对于熔体，我们发现六氧熔体组分（CaMgSi_2O_6，NaAlSi_2O_6，Mg_3Si_（1.5）O_6等）在广泛的天然成分中以恒定的活度系数混合。将Δ Gfin代入Brice模型，得到了Do ~（3+）与单斜辉石M_1位Mg原子分数、熔体Mg数、PandT的关系式。任何REE的D都可以用Brice公式从Do ~（3+）计算。用此方法计算的454个DREE点中，92%以上与实验值在0.63-1.59之间。该方法可推广到在给定P（≤ 6 GPa）和T（12002038 K）条件下，计算任意稀土元素的D值，其误差在仅考虑晶体和熔体成分时的0.60-1.66倍之内。该模型具有广泛的适用性，地球化学建模的所有自然过程，涉及单斜辉石，例如减压地幔熔融，使第一次考虑到响应于变化的压力，温度和相组成的分配系数的变化。
We present a quantitative model to describe the partitioning of rare earth elements (REE) and Y between clinopyroxene and anhydrous silicate melt as a function of pressure (P), temperature (T) and bulk composition (X). The model is based on the Brice (1975) equation, which relates the partition coefficient of elementi(Di) to that of elemento(Do) where the latter has the same ionic radiusroas the crystallographic site of interest, in this case the clinopyroxene M2 site:NAis Avogadro's number,EM2is the Young's Modulus of the site,Ris the gas constant andTis in K. Values ofEM2obtained by fitting the Brice equation to experimental REE partition coefficient patterns are in good agreement with those obtained from the well-known correlation between bulk modulus, metal-oxygen distance and cation charge. Using this relationship to constrainEM2for 3+ cations and then fitting the Brice equation to those experimental data where 3 or more REE partition coefficients had been simultaneously measured we obtained 82 values ofDoandro. The latter was found to be a simple and crystallochemically reasonable function of clinopyroxene composition. We show that for any clinopyroxene-melt pair ifDfor one middle REE (e.g. Sm or Gd) is known then the Brice equation can be used to predictDs for all the other REE, with uncertainties similar to those involved in the actual measurements. The model was generalised using thermodynamic descriptions of REE components in crystal and melt phases to estimate the free energy of fusion (ΔGf) of the fictive REE components REEMgAlSiO6and Na0.5REE0.5MgSi2O6. For the melt we find that 6-oxygen melt components (CaMgSi2O6,NaAlSi2O6, Mg3Si1.5O6etc.) mix with constant activity coefficient over a wide range of natural compositions. Propagating ΔGfinto the Brice model we obtain an expression forDo3+in terms of the atomic fraction of Mg on the clinopyroxene M1 site, theMg-number of the melt,PandT. TheDfor any REE can be calculated fromDo3+using the Brice equation. Over 92% ofDREE(454 points) calculated in this way lie within a factor 0.63–1.59 of the experimental value. The approach can be extended to calculateDfor any REE at a givenP(≤6GPa) andT(12002038K) to within 0.60–1.66 times the true value given only the crystal and/or melt composition. The model has widespread applicability to geochemical modelling of all natural processes involving clinopyroxene, e.g. decompression mantle melting, enabling for the first time account to be taken of variations in partition coefficient in response to changing pressure, temperature and phase composition.