西太平洋海底海山富钴结壳惰性气体同位素组成及其来源

采用高真空气体质谱系统测定了西太平洋麦哲伦海山富钴结壳不同层圈及其基岩的惰性气体丰度和同位素组成,结果显示:(1)西太平洋富钴结壳主要是水成成因,其中惰性气体来源不同,He 绝大多数来自宇宙尘(IDPs),少量来自陆源风成微粒;Ar 主要来自海水溶解的大气,少量来自陆源风成微粒或沉积岩建造水;Ne 和 Xe 主要来自海水中溶解大气, 少量来自宇宙尘;(2)在具三层结构的结壳中,亮煤层(致密层)的惰性气体同位素相对外层和疏松层有较大的不同,显示大洋磷酸岩化对早期沉积的结壳惰性气体组成有较大的影响,如导致~4He 的升高和~3He/~4He 的显著降低;(3)太平洋富钴结壳玄武岩基岩的~3He/~4He 非常低,为0.0095～0.074Ra,与本区磷块岩基岩(0.087Ra)相似,而远低于正常海底玄武岩的~3He/~4He 比值,显示这些基岩曾与富含放射性成因~4He 和 P 的上升洋流或沉积物中建造水发生过水/岩反应,这个过程将释放出较多的成矿元素,有利于富钴结壳的形成,海底海山玄武岩中较低的 He 同位素组成可作为富钴结壳的找矿标志之一。

Noble gases isotopic compositions and sources of cobalt-rich crusts from west Pacific Ocean seamounts.

Noble gases abundances and isotopic compositions of various layers in the three-layered Cobalt-rich ferromanganese crusts and their substrate rocks,which were collected from Magellan seamounts in west Pacific,were analyzed by using a high vacuum gas mass spectrum,and the results show that:1.Most of the ferromanganese crusts in the west Pacific Ocean were hydrogenous origin and the noble gases in the crusts derived from different sources.The He was sourced predominantly from extraterrestrial grains such as interplanetary dust particles(IDPs),with a little from aeolian continental dust grains in the seawater.The Ar derived mainly from the air dissolved in the ambient sea water,and partly from the aeolian dust grains or formation water in the submarine sediments.While the Xe and Ne were mostly sourced from dissolved air in the sea water,and partly from the interplanetary dust particles(IDPs).(2)In the three-layered crusts,the compact layers possess quite different noble gases isotopic compositions compared with the outer and porous layers,suggesting that marine phosphatization might have greatly modified the noble gases isotopic compositions in the old crusts, resulting in dramatic inerease of ~4He concentrations and decrease of ~3He/~4He.(3)~3He/~4He ratios in the basaltic substrates of the Cobalt-rich crusts are very low,varying between 0.0095Ra to 0.074Ra,which are similar to that of the phosphorite substrates (0.087Ra),but much lower than those of normal submarine basalts,suggesting that the basaltic substrates might have been strongly reacted with radiogenic ~4He and P enriching upwelling marine currents or formation water in the sediments.Large amount of ore-forming elements might have been released during basalt/water interaction and which might favor growth of the Cobalt-rich crusts.The relative low ~3He/~4He ratios in the basaltic substrates may be used as one of the important exploration criterions for the Cobalt-rich ferromanganese crusts.