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

尽管进行了多年的研究，但控制铂族元素(PGE)从硅酸盐熔体转移到岩浆硫化物熔体的过程尚不清楚。一个参数是硫化物/硅酸盐分配系数D，这是一个浓度比，它量化了给定元素对硫化物的亲和力，而不是硅酸盐熔体的亲和力。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才能收集其PGE。在R系数概念中隐含着这样一种想法，即只有当释放出的硫化物熔体有机会与最大数量的硅酸盐熔体接触时，高硫化物/硅酸盐熔体才能有效。在这里，我们将探索R因子模型的替代方案。通过研究S~(2-)对Pd、Ru在苦味硅酸盐熔体中溶解度的影响，我们可以定量地确定PGE在多大程度上优先于S_2~-而不是O_2~-。如果存在对S2-配体的偏好，硫化物熔体的出溶将把硅酸盐中的所有前列腺素E-S分子隔离到硫化物中，而不管R。PGE在硫化物中的丰度反映了与S_2-有关的PGE的比例，它将不受R因子模型所确定的物理条件的影响。

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.