Distinct site preferences for heavy and light REE in amphibole and the prediction of Amph/LDREE
Distinct site preferences for heavy and light REE in amphibole and the prediction of Amph/LDREE
复制标题
DOI：
10.1007/s004100050580
复制
发表时间：
1999-10
影响因子：
3.5
通讯作者：
P. Bottazzi;M. Tiepolo;R. Vannucci;A. Zanetti;R. Brumm;S. Foley;R. Oberti
中科院分区：
文献类型：
--
作者：
P. Bottazzi;M. Tiepolo;R. Vannucci;A. Zanetti;R. Brumm;S. Foley;R. Oberti
新的实验角闪石/熔体分配系数从各种地质上相关的角闪石(红柱石、钾闪石和钾闪石)和熔体成分中获得，这些熔体成分是在上地幔研究感兴趣的条件下获得的，并与X射线单晶结构精修的结果相结合。在稀土元素在[8]M4完全有序化的假设下，用Blundy和Wood(1994)的晶格弹性应变模型计算的理想阳离子半径(R0)与[8]Ca~(2+)的结构修正和离子半径计算结果相差很大。重稀土元素也可能强烈偏离另一种稀土元素所定义的抛物线趋势。根据角闪石中主量元素择位的晶体化学知识和M4空穴中存在两个配位不同的位置(M4为Ca和Na，M4‘为Fe2+和Mg)，提出了一个新的稀土元素掺入模型。LREE顺序位于[8]M4位，而HREE偏向于角闪石中配位较低的M4‘位，并可能进入M2八面体，至少在富钙岩中是这样。这一更复杂的模型与观察到的Amph/Ld一致，并放弃了通常的假设，即稀土在M4位置表现为均质群和有序性。REE3+掺入的多种晶体化学机制的可用性解释了为什么测量和估计的Amph/LDHREE可能相差一个数量级。当稀土元素进入同一空穴中的两个不同位置时，根据一条曲线进行的拟合可能看起来是正确的，但得到的r0值偏向于主要位置的值，而且杨氏模数被低估了。当稀土元素被掺入不同洞穴的多个地点时，观测到的模式不能被简化为单一曲线，单地点拟合将严重低估重稀土的分配系数。因此，简单地假设稀土元素占据角闪石结构中的一个位置，可能会对基于弹性-应变理论的预测模型产生重大偏差。我们的组合方法允许精细位置偏好和矿物的宏观性质之间的联系，并为矿物/熔体分配提供更可靠的预测模型。在确定了可能的地点分配后，根据准确确定的Amph/LDREE构建的Onuma曲线的形状现在允许即使在没有地点种群的情况下也能识别角闪石中REE结合的活跃机制。稀土元素偏向于大小和配位比钙小的多面体，否定了钙作为稀土元素“载体”的一般观点。
New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at[8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of[8]Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe2+and Mg), we propose a new model for REE incorporation. LREE order at the[8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observedAmph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE3+incorporation explains why measured and estimatedAmph/LDHREEmay differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r0are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determinedAmph/LDREEnow allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE.