论文部分内容阅读
本文报道了南岭地区两个花岗岩(大东山和千里山)的全岩以及主要造岩矿物(黑云母、斜长石和石英)的硼含量和硼同位素组成。结果显示,黑云母含有最高的硼含量,斜长石次之,石英中的硼含量则非常低。花岗岩中的硼可能主要以晶格替换方式赋存在黑云母和斜长石中,而石英中的微量硼则来自其包裹的流体包裹体。同时,黑云母具有最低的硼同位素组成,斜长石次之,而石英则具有最高的硼同位素组成。本次研究首次发现花岗岩中的主要造岩矿物间存在着较大的硼同位素分馏:大东山花岗岩中黑云母与斜长石之间存在着-9.3‰的分馏,黑云母与石英之间存在-9.9‰的分馏;千里山花岗岩中黑云母与斜长石之间存在着-6.6‰的分馏,黑云母与石英之间存在着-10.2‰的硼同位素分馏。结合镜下观察和氧同位素研究发现,造成不同矿物间这一大的硼同位素分馏的原因很可能是岩浆后期的热液蚀变作用。综合以往所发表的研究数据,得到含四次配位硼为主的硅酸盐矿物与中酸性热液流体之间的硼同位素分馏方程:1000lnα硅酸盐-流体=-11.19×(103/T[K])+5.09,该分馏是由于硼的四次配位和三次配位之间的转换引起的。
This paper reports the boron content and boron isotopic composition of the whole rocks and the major rock-forming minerals (biotite, plagioclase and quartz) from two granites (Dadongshan and Qianlishan) in the Nanling area. The results show that biotite contains the highest boron content, followed by plagioclase, and boron in quartz is very low. Boron in granite may be mainly hosted in biotite and plagioclase in a lattice-replacement manner, whereas the trace boron in quartz is derived from the fluid inclusions it encapsulates. At the same time, biotite has the lowest boron isotopic composition, followed by plagioclase, while quartz has the highest boron isotopic composition. The study found for the first time that there is a large boron isotope fractionation between the major rock-forming minerals in the granites: there is a -9.3% o fractionation between biotite and plagioclase in the Dadongshan granite, 9.9 ‰. There is a -6.6 ‰ fractionation between biotite and plagioclase in the Qianlishan granite, and a boron isotope fraction of -10.2 ‰ exists between the biotite and the quartz. Combined with the microscopic observation and oxygen isotope study, it is found that the reason for this large boron isotopic fractionation among different minerals is probably the hydrothermal alteration in the late magmatic stage. According to the published research data, the boron isotope fractionation equation between silicate minerals mainly containing four coordination boron and the acidic hydrothermal fluids is obtained: 1000lnα silicate fluid = -11.19 × (103 / T [K]) + 5.09. This fractionation is due to the conversion between the fourth and the third coordination of boron.