西藏羊八井地热田硼同位素地球化学特征初步研究

基于国外大量硼同位素示踪的地热研究实例,在先期建立的MC-ICP-MS测定水中硼同位素分析方法基础上,以羊八井地热田为研究对象,进行了热田地热流体的硼同位素地球化学初步研究。研究表明,羊八井地热田区热储流体的硼同位素值为 -10.5‰～-9.1‰,为非海相来源; 结合区域地质特征,研究认为热储内的硼组分来源于蚀变花岗岩围岩,并且蚀变花岗岩的硼同位素特征可能与深层地热流体相似。研究表明羊八井浅层热储内硼元素的地球化学行为并非完全是保守元素,存在着一定的硼同位素分馏。在端元硼同位素特征差异较小的羊八井热田,为硼同位素二元混合模型示踪水体混合过程带来了相当的复杂性。本文从热田开采过程与采样时间、水-岩相互作用、气-液相分离以及结垢过程等方面分析了浅层热储内引起硼同位素分馏的可能因素,其中从整体来看,气-液相分离过程的影响相对较大;而在个别井位水-岩相互作用与结垢的影响可能相对较为显著。

Geochemical study on boron isotopes in the Yangbajing geothermal field, Tibet

Based on previous geothermal tracing researches by boron isotopes,and by utilizing the state-of-art MC-ICP-MS analytical technique for measuring boron isotope ratios,the mixing processes between deep geothermal water and surface cold water of the Yangbajing(YBJ)geothermal field in Tibet were determined.The δ¹¹B values of geothermal water range from -10.5‰ to -9.1‰,indicating the non-marine sources.And further we propose that B is derived from host altered granite according to regional geological features,and the host altered granite should have similar boron isotope ratios to the deep geothermal water.Shallow geothermal water samples suggest that there is fractionation of B isotopes in the YBJ shallow reservoir.Accordingly,some complicated factors will be introduced into the two end-member mixing model in the YBJ since there is only little difference of boron isotopes between two end-members.Therefore,we discussed in details that exploitation,water-rock interaction,separation between vapor and liquid phase and scaling are some possible factors for yielding B isotopic fractionation.