冲绳海槽Jade热浪活动区块状硫化物的铅同位素组成及其地质意义

Lead isotopic compositions of massive sulfides from the Jade hydrothermal field in the Okinawa Trough and its geological implications

新测得Ｊａｄｅ热液活动区中５件块状硫化物样品的铅同位素组成，具有较小的变化范围，表现出较均一的铅同位素组成特征。在Ｐｂ－Ｐｂ图解上，块状硫化物的铅同位素数据构成线形排列，与该区沉积物和蚀交火山岩的铅同位素组成一致，而与该区新鲜火山岩相比具较高的放射成因铅，证实了该区海底块状硫化物中的铅是由沉积物与长英质火山岩来源铅共同构成的混合铅。不同热液活动区铅同位素组成对比研究表明，地质构造环境的不同是导致各热液活动区中热液沉积物铅同位素组成变化的主要原因。

New data on lead isotpe compositions of five massive sulfide samples from the Jade hydrothermal field in the Okinawa Trough were obtained, and the Pb isotope ratios of massive sulfides are homogeneous and define a small field (the range of ^(206)Pb/~204Pb ratios is from 18. 491 to 18. 609, with a mean of 18. 542, the range of ^(207)Pb/ ^(204)Pb ratios is from 15. 580 to 15. 684, with a mean of 15. 626, and the range of ^(208)Pb/^(204)Pb ratios is from 38. 570 to 38. 859, with a mean of 38. 702). In Pb-Pb diagrams, the Pb isotope ratios of massive sulfides form a linear array, that is similar to those of the sediment and altered volcanic rock samples in the Jade hydrothermal field. The lead isotopic compositions of Jade massive sulfides tend to be slightly more radiogenic than those of fresh felsic volcanic rocks, which shows that the sources of lead in the Jade massive sulfides is a mixture of sedimental lead plus a component derived from felsic volcanic rocks. Comparing the lead isotopic compositions of hydrothermal sediments from different hydrothermal fields, it is clear that the lead-isotopic compositions of hydrothermal sulfides from the hydrothermal fields not covered with sediments reflect the composition of the mantle source, whereas in the hydrothermal fields covered sediments, the lead isotopic compositions of hydrothermal sulfides are either a mixture of mantle and sedimentary lead or it may even be completely dominated by the latter, and the difference of geologic-tectonic settings in seafloor hydrothermal fields is the major reason to result in the variation of lead isotopic compositions in the hydrothermal sediments.