西太平洋采薇海山群富钴结壳铂族元素地球化学特征与来源

Geochemistry and source of platinum group elements in cobalt-rich crusts from Caiwei Seamounts in the western Pacific

为探讨富钴结壳铂族元素的来源,以西太平洋采薇海山群富钴结壳样品为研究对象,利用X射线衍射法、等离子体发射光谱法和质谱法,对富钴结壳样品进行了矿物组成、主量元素和铂族元素含量分析。研究表明,富钴结壳的主要结晶矿物为水羟锰矿,次要矿物有石英、斜长石、钾长石和碳氟磷灰石,同时含有大量非晶态铁氧/氢氧化物。富钴结壳中Mn和Fe含量最高,且明显富集铂族元素,铂族总量为142~1352ng/g,其中Pt为114~1268ng/g,占铂族总量的80%以上。老壳层的铂族元素含量高于新壳层,老壳层发生了磷酸盐化作用。富钴结壳的铂族元素之间发生了分异作用,Pd组(PPGE)含量高于Ir组(IPGE)。铂族元素配分模式显示Pt正异常和Pd负异常,具有Pt富集而Pd亏损特征,推测富钴结壳的铂族元素来源于洋壳蚀变过程中海山玄武岩和海水之间的水−岩反应。富钴结壳的铂族元素与CaO、P_(2)O_(5)、Ba和Cu正相关,推测铂族元素赋存于磷酸盐矿物相中。富钴结壳生长剖面从上到下,由新到老,铂族元素含量呈逐渐增加趋势,海水氧化性环境和高海洋生产力可增进磷酸盐化作用,从而进一步促进了铂族元素的富集。该研究对于揭示富钴结壳铂族元素的来源具有一定的参考价值。

To explore the source of platinum group elements(PGE)in cobalt-rich crusts,the samples from Caiwei Seamounts in western Pacific were chosen as the research object,for which XRD,ICP-OES and ICP-MS were used to analyze the mineral composition,major elements contents and PGE contents in cobalt-rich crusts.The results showed that,the main crystalline minerals were vernadites in cobalt-rich crusts,and the minor minerals included quartz,plagioclase,potassium feldspar and carbon fluoride apatite.Also many amorphous ferric minerals were contained in cobalt-rich crusts.In addition,Mn and Fe contents were the highest in cobalt-rich crusts,and PGE were enriched in cobalt-rich crusts.PGE contents were 142−1352 ng/g,and Pt contents were 114−1268 ng/g,in which Pt accounted for more than 80%.PGE contents in the old layers were higher than that in the new layers.And the phosphatization appeared in the old crust layers.Moreover,there was an obvious contrast in PGE of cobalt-rich crusts,Pd group(PPGE)contents were more than Ir group(IPGE).PGE diagrams showed the positive Pt anomalies and negative Pd anomalies.And Pt was enriched and Pd was poor in cobalt-rich crusts.Meanwhile,PGE in cobaltrich crusts were probably derived from the reaction between seamount basalt and seawater during the oceanic shell erosion process.PGE had positive correlation with CaO,P_(2)O_(5),Ba and Cu,so PGE were probably enriched in the phosphate phase.In addition,the growth profile of cobalt-rich crust was from top to bottom and from new to old,PGE contents increased gradually.Therefore,oxidative marine environment and high marine productivity had a positive impact on phosphatization,which further promoted the enrichment of PGE.This study provided a certain reference value for revealing the source of PGE in cobalt-rich crusts.