柴达木盆地碱山深钻的硼同位素地球化学特征

Boron Isotope Geochemistry of Janshan Deep Borehole in Qaidam Basin

沉积物硼(B)同位素组成可以反映其地质成因及经历的地质过程,因此在许多领域的研究中都有较为广泛的应用。通过对位于柴达木盆地碱山背斜顶部的SG-1b钻孔沉积物(7.3~1.6 Ma)水溶组分的B同位素研究,发现钻孔沉积物B含量在38.55~172.3μg/g之间,平均含量为87.6μg/g;δ11 B值的变化范围在3.61‰~16.26‰之间,平均值为10.65‰,B含量与δ11 B值具有一定的正相关关系。进一步分析表明,受到碱山背斜构造隆升以及晚新生代以来气候干旱化的影响,柴西古湖逐渐咸化萎缩,沉积环境以及碳酸盐含量、粘土矿物含量及其矿物组合等也在发生变化,B含量和δ11 B值自钻孔底部向上的逐步增加以及后期的急剧增加,与水溶离子含量以及矿物和粒度等的变化一致,这说明柴达木盆地晚中新世以来湖泊沉积物的B含量和δ11 B值可以很好地反映研究区气候和湖水的演化过程,共同指示了研究区自7.3 Ma以来气候的持续干旱化和湖水盐度的逐步增加,以及3.3Ma以来干旱化和湖水浓缩过程的加剧。

Boron(B)isotopic composition of the sediments can reflect its geological genetic environment and geological process,so it has been widely used in many fields.In this paper,we studied the B isotopes of the water-soluble components from the SG-1b borehole sediments located at the top of the Jianshan Anticline in the western Qaidam Basin.The results show that the B contents range from 38.55 to 172.3μg/g,with an average 87.6μg/g;and the variation of theδ11 B values is 3.61‰to 16.26‰,with an average 10.65‰.There is a positive correlation between the B content and theδ11 B value.Further analysis shows that,influenced by the uplift of the Jianshan anticline and the climate drought since the late Cenozoic,the paleolake in Qaidam Basin has gradually become salty and shrinking,and the sedimentary environment,carbonate content,clay mineral content and mineral assemblage have also changed.From the bottom of the borehole to the top,the gradual increase of B contents andδ11 B values is consistent with the results of water soluble ion contents,mineral assemblages and particle size.It shows that the B content andδ11 B value of lake sediments in Qaidam Basin since the Late Miocene can reflect the evolution process of climate and lake water in the study area,and the results indicate the continuous drought of climate and the gradual increase of salinity of lake water in the study area since 7.3 Ma,and the intensification of the process of drought and lake water salinization since 3.3 Ma.