洋中脊和弧后盆地热液区热液流体B同位素组成的系统性差异

Systematic differences in boron isotope compositions between mid-ocean ridge and back-arc basin hydrothermal fluids

硼(B)是流体迁移元素,趋向于在热液流体中富集而成为常量元素。不同来源的B其同位素组成有着明显的区别。因此,B的含量及其同位素组成可标识热液流体(元素)的物质来源、水–岩反应程度及沉积物(元素)混入等重要过程,对海底热液活动及其成矿作用过程具有重要的示踪意义。迄今,对全球主要热液活动区热液流体中B的含量及同位素组成特征已做了大量的测试分析及研究工作,积累了丰富的资料和重要研究成果。但是,对不同地质背景(构造环境)条件下热液流体中B的含量及同位素组成特征尚缺乏系统性的对比分析,进而对造成不同环境热液流体中元素及其同位素组成的系统性差异的原因或机制尚缺乏深入的认识。本文在获取了洋中脊和弧后盆地主要热液活动区热液端元流体中B的含量及其同位素组成数据的基础上,定量估算了热液流体中B的主要来源,并对洋中脊和弧后盆地热液端元流体中B同位素组成的系统性差异进行了分析及成因探讨。结果表明,不同热液活动区热液端元流体的δ11B值都具有较大的变化范围,水–岩反应过程中不同来源B的混合是热液流体B同位素组成变化的主要原因。无沉积物覆盖的洋中脊和弧后盆地热液区热液流体中的B主要为海水与基底岩石来源B的混合,弧后盆地岩浆挥发性组分对热液系统的直接贡献及两种不同地质背景下基底岩石地球化学组成与水–岩反应程度的差异是其热液端元流体B同位素组成差异的主要原因。在有沉积物覆盖的弧后盆地热液区,热液流体中B的同位素组成与前两者之间存在显著差异,具有异常低的δ11B值,水–岩反应过程中沉积物来源B的加入是导致热液流体中δ11B值系统性降低的主要原因,沉积物的吸附作用也在一定程度上影响了热液流体的B同位素组成。有沉积物覆盖的洋中脊热液区热液流体同样受到了沉积物来源B加入的影响,具有较低的δ11B值,且相对于冲绳海槽受到了更强烈的沉积物吸附作用的影响。基于以上分析,并结合热液流体的Sr同位素组成特征,本文提出了洋中脊和弧后盆地这两大构造环境中热液流体B同位素组成系统性差异的成因模式。

Boron is a common element in vent fluids of seafloor hydrothermal fields.Due to its typical fluid-mobility and distinct isotope compositions in different reservoirs,boron has been largely applied to trace metal sources,water/rock interaction and sediment contribution to hydrothermal fluids.Up to date,the B contents and its isotope compositions in hydrothermal fluids from the major seafloor hydrothermal fields have been studied sufficiently,but the comparisons of B behavior in different geological settings are relatively limited and the reasons causing the sys-tematic differences in elements and isotope compositions of hydrothermal fluids are still unclear.In this paper,the sources of B in hydrothermal fluids from different geological settings were calculated based on B contents and its isotope compositions,meanwhile,the causes of systematic differences in B isotope compositions between midocean ridge and back-arc basin hydrothermal fluids were discussed.Results show that theδ11B values of hydrothermal end member fluids from different hydrothermal fields have large variations,which mainly results from the mixing of B from difference sources in different proportions.Boron in mid-ocean ridge and sediment-starve backarc basin hydrothermal fluids are mainly from the mixing of seawater and basement-derived B,the contribution of magmatic volatiles to sediment-starve back-arc basin hydrothermal systems is the major causes of B isotope composition differences in these two geological settings.Moreover,the differences in geochemical compositions of basement rocks and degrees of water/rock interaction also cause the differences in B isotope compositions to some extent.While theδ11B values of hydrothermal end member fluids from sediment-covered hydrothermal fields are extremely low and contribution of sediment-derived B to hydrothermal fluid is the major cause of this phenomenon.Moreover,sediment absorption also causes the B isotope variation to some extent.In sediment-covered mid-ocean ridge hydrothermal fields,the B isotope compositions of hydrothermal fluids are also influenced by the incorporation of sediment-derived B and have relatively lowerδ11B values.Compared with the Okinawa Trough,the degrees of sediment absorption in sediment-covered mid-ocean ridge hydrothermal fluids are more intensely.Based on these analysis,we put forward the mechanism causing systematic differences in B isotope compositions of hydrothermal fluids from mid-ocean ridge and back-arc basins.