Speciation of palladium and ruthenium in S-bearing, sulfide-undersaturated basaltic melt - implications for enrichment mechanisms of noble metals in magmatic sulfide melt

尽管经过多年的研究，铂族元素（PGE）从硅酸盐熔体到岩浆硫化物熔体的转移过程还没有得到很好的理解。一个参数是硫化物/硅酸盐分配系数D，其是定量给定元素优先于硅酸盐熔体对硫化物的亲和力的浓度比。另一个经常被引用的参数是R因子。R是硅酸盐熔体体积的量度，硫化物熔体必须与该体积平衡以收集其PGE。在R因子概念中隐含的想法是，只有当使出溶的硫化物熔体有机会与最大量的硅酸盐熔体接触时，高硫化物/硅酸盐Ds才是有效的。在这里，我们将探索R因子模型的替代方案。通过研究S2-对Pd和Ru在苦味酸硅酸盐熔体中溶解度的影响，我们可以量化硅酸盐熔体中的PGE在多大程度上优先选择S2-而不是O2-。如果存在对S2-配体的偏好，则硫化物熔体的出溶将使硅酸盐中的所有PGE-S分子与硫化物螯合，而不管R如何。PGE在硫化物中的富集反映了与S2-缔合的PGE的比例，它将不受R因子模型所描述的物理条件的影响。

Despite years of research, the processes governing the transfer of platinumgroup elements (PGE) from a silicate melt to a magmatic sulfide melt are not well understood. One parameter is the sulfide/silicate partition coefficient D, a concentration ratio that quantifies the affinity of a given element to sulfide in preference to silicate melt. Another parameter often invoked is the R factor. R is a measure of the silicate melt volume with which a sulfide melt must equilibrate in order to collect its PGE. Implicit in the R factor concept is the idea that high sulfide/silicate Ds can only be effective if the exsolved sulfide melt is given opportunity to come in contact with maximum amounts of silicate melt. Here, we will explore alternatives to the R factor model. By investigating the effect of S2- on the solubility of Pd and Ru in picritic silicate melt, we may quantify to what extent the PGE in silicate melt prefer the S2- ligand over O2-. If a preference exists toward the S2- ligand, exsolution of a sulfide melt will sequester all PGE-S molecules in silicate to the sulfide regardless of R. PGE enrichment in the sulfide then reflects the proportion of PGE associated with S2-, and it will be independent by physical conditions laid out by R factor model.