赣南五里亭岩体晶质铀矿中铀离子价态及岩浆氧逸度计算

Magmatic Oxygen Fagucity Estimation Based on U^(6+)/U^(4+) Value——A Case Study of Wuliting Granite Pluton

晶质铀矿是花岗岩中广泛产出的副矿物,其铀离子价态的变化及比例关系,对于岩浆氧逸度具有一定的指示意义。笔者利用硅酸盐熔浆中U^(6+)-U^(4+)的平衡关系,推导出花岗岩副矿物——晶质铀矿的氧逸度计热力学计算方程,以及氧逸度(f O2)、温度(T)和不同价态铀离子比值(U^(6+)/U^(4+))的对数关系。以赣南五里亭花岗岩体为例,借助X射线光电子能谱分析,获得U^(6+)与U^(4+)的比例关系(U^(6+)/U^(4+)=0.30～0.46);利用晶质铀矿氧逸度计热力学方程,计算出该岩体形成时的平均氧逸度(为FMQ+0.58)。在温度(T)与氧逸度(lg f O2)协变图中,该结果靠近矿物反应缓冲线(FMQ),显示岩浆为弱还原性。因此,晶质铀矿U^(6+)/U^(4+)氧逸度计算结果能够代表岩浆源区的氧逸度特征,可作为常规氧逸度计算方法的一种有效补充。

Uraninite is widely spread in granite and occurs mainly as accessory mineral.The valence state and content of uranium have an important significance for magmatic oxygen fagucity.This paper derives the thermodynamic equation of oxygen fagucity by the equilibrium relationship between U^6+and U^4+.It has a logarithmic relationship between oxygen fagucity(fO2),temperature(T),and the ratio of U^6+and U^4+(U^6+/U^4+).Taking the Wuliting granite in south Jiangxi Province for example,we gained the ratio of U^6+/U^4+by analyzing X-ray photoelectron spectroscopy.The oxygen fugacity was estimated based on the ratio(0.30~0.46)and the value was about FMQ+0.58,which is close to FMQ redox buffer,and present weak reducibility.Though the method construction and comparision,we think the estimation of oxygen fugacity by the U^6+/U^4+of uraninite can represent the oxygen fugacity characteristics of magma source,and is a supplement to the regular methods of oxygen fugacity estimation.