PACMANUS热液区Fe-Si-Mn羟基氧化物的成因及地球化学特征

分析了东马努斯海盆PACMANUS热液区Roger's Ruins和Roman Ruins两个热液点之间拖网取得的Fe-Si-Mn羟基氧化物样品的主量、微量和稀土元素含量。Fe-Si-Mn羟基氧化物样品的微量、稀土元素含量很低,Co/Zn比值较小,生长速率非常快,表明Fe-Si-Mn羟基氧化物样品具有明显的热液成因。大部分样品具有明显的正Ce异常(Ce/Ce*值为1.021~1.769),其Ce的正异常范围(lg 3Ce/(2La+Nd),0.008~0.229)低于典型水成成因的Ce的正异常范围(lg 3Ce/(2La+Nd),0.352~0.637),说明样品受到海水的影响较小。由于出现正Ce异常的样品主要集中在Fe羟基氧化物中,因此,稀土元素的正Ce异常主要受到Fe羟基氧化物的吸附作用和后期成岩作用的影响。TiO2的含量以及Al/(Al+Fe+Mn)比值明显低于火山碎屑物质的TiO2 的含量和Al/(Al+Fe+Mn)比值,表明样品中几乎没有火山碎屑物质的贡献。样品中高含量的Ba很可能来自散落于Fe-Si-Mn羟基氧化物样品中的重晶石以及钡镁锰矿。较高含量的Pb说明早期形成的硫化物并没有带走大量的Pb,相反这些Pb随热液流体喷出进入Fe-Mn羟基氧化物。

THE ORIGIN AND GEOCHEMICAL CHARACTERISTICS OF FE-SI-MN OXYHYDROXIDES AT PACMANUS HYDROTHERMAL FIELD

Four Fe-Si-Mn oxyhydroxides dredged from the area between hydrothermal sites of Roger's Ruins and Roman Ruins at the PACMANUS hydrothermal field in the Eastern Manus Basin were analyzed for major, trace and rare earth elements. The low trace metal and REE contents, low Co/Zn ratio and high growth rate indicate a hydrothermal origin. Most of the samples show obvious positive Ce anomalies, of which the ranges (lg 3Ce/(2La + Nd),0.008~0.229) are lower than the range of typical hydrogenic oxyhydroxides (lg 3Ce/(2La + Nd),0.352~0.637). Therefore, the contribution of seawater is very little. The samples with positive Ce anomalies are mainly composed of Fe-oxyhydroxides. It means that the positive Ce anomalies were mainly controlled by the adsorption by Fe-oxyhydroxides and the diagenesis in the later period. The content of TiO2, and Al/(Al +Fe + Mn) ratio were lower than the ratio of volcanic materials, which implies that the contribution of volcanic materials is little. The high Ba contents may be derived from the todorokite and barite scattering in the samples. The high Pb contents reflect that Pb was not incorporated into the sulfides formed in the early period. On the contrary, Pb was incorporated into Fe-Si-Mn oxyhydroxides along with the hydrothermal fluid.