青海察尔汗盐湖碳酸盐的硼同位素地球化学特征

摘要

Boron isotopic composition (δ11 B) of evaporite in salt lake is indicative of palaeo-salinity change of brine,but there is little study about boron isotopic compositions of carbonate deposition in salt lake system.In this study,we collected authigenic carbonate samples from ISL1A drilling core for analysis of boron content and boron isotopic composition.Results show that the boron content and isotopic composition of the samples vary from 10.88× 10-6 ～265.66 ×10-6 and-1.28‰～+9.94‰ respectively,and they present a positive correlation.Analysis also reveals that the B isotope composition of brine increases with increasing of brine salinity of salt lake,which results in increasing of boron isotope composition of salt sediment.In the natural salt lake,inadequate sedimentary differentiation may result in carbonate deposition during evaporite precipitation,which possibly ran through all the stages of evolution of salt lake.Therefore,B isotopic composition of carbonate can be used to analyze salinity change of brine.Variation of boron isotope composition in carbonate from the ISL1A drilling core reflects ancient salinity changes and climate fluctuations in Qarhan salt lake since 41.8 ka.%盐湖蒸发岩的硼(B)同位素组成(δ11B)对卤水古盐度变化具有一定的指示意义,但关于盐湖体系碳酸盐沉积的B同位素组成研究较少.本文对察尔汗盐湖百米钻(ISL1A)岩芯碳酸盐进行了B含量及B同位素组成的精确测定.结果表明,所测样品的B含量在10.88×10-6～265.66×10-6之间,δ11 B值的变化范围为-1.28‰～+9.94‰,B含量和δ11 B值呈现较明显的正相关关系.分析表明,随着盐湖卤水盐度的增大,卤水的B同位素组成也逐渐增大,这导致盐类沉积物的B同位素组成也相应增大.在天然盐湖中,不充分的沉积分异作用使得蒸发岩析出时依然可能存在碳酸盐沉积,碳酸盐的析出可能贯穿盐湖演化的各个阶段,因此可以尝试利用碳酸盐的B同位素组成分析盐湖在整个演化过程中卤水的盐度变化情况.ISL1A钻孔碳酸盐B同位素组成的变化较好地反映了察尔汗盐湖在41.8ka以来古卤水盐度的变化及其在析盐过程中经历的气候干湿波动.