内蒙古柯单山蛇绿岩地幔橄榄岩铂族元素( PGEs)的分布特征及分异机制探讨

摘要

Platinum-group elements (PCEs) abundances of mantle peridotites from the Kedanshan ophiolite, Inner Mongolia, were determined by using a Carius tube sample digestion and the MC-ICP-MS system. Compared with primitive mantle and typical mantle peridotites in the world, Kedanshan mantle peridotites are enriched in Pt and Pd, but depleted in Ir and Ru, with high Pd/Ir ratio. The PGEs distribution patterns have very steep positive slope, unlike the PCEs distribution patterns in the mantle peridotites of partial melting relict origin which usually are negative-slop patterns or flat patterns. Ir and Ru have a positive correlation with MgO, which might reflect the consumption of sulfide during partial melting, but have no relations with sulfide melt/silicate melt Nernst partition coefficients. Enriched Pt and Pd indicate that these samples cannot be the residuum of simple partial melting, but have a more complex origin (e. G. Melt percolation). Similar with the behavior of Pt and Pd enriched in the abyssal peridotites(AP) and the subcontinental lithospheric mantle (SCLM) harzburgites, the kedanshan mantle peridotites can be explained by sulfide precipitation during melt percolation. The melt may come from ' filtered' sulfide during magma evolution.%采用Carius管结合MC-ICPMS法分析了内蒙古柯单山蛇绿岩地幔橄榄岩中Ir、Ru、Pt和Pd的含量,与典型的地幔橄榄岩进行对比研究,发现柯单山地幔橄榄岩中Ir和Ru明显亏损,Pt和Pd强烈富集,具有极高的Pd/Ir值,PGEs地幔标准化配分模式具有较陡的正斜率,明显不同于通常观测到的代表部分熔融残留相中铂族元素配分模式(负斜率或平坦型).柯单山地幔橄榄岩的Ir和Ru与MgO呈正相关关系,表明Ir和Ru的亏损可能与部分熔融过程中硫化物的消耗程度有关,而与PGEs在硫化物/硅酸盐间的能斯特分配系数没有直接关系；Pt、Pd的富集表明本区的地幔橄榄岩不仅仅是经历过部分熔融的残余,而与来自深海的橄榄岩和大陆岩石圈地幔( SCLM)中的方辉橄榄岩相似,因此推测,本区地幔橄榄岩在部分熔融后又经历了富Pd的熔/流体交代,而熔/流体的来源可能是在岩浆分异演化过程中“熔离”出来的硫化物.