Conditions of accumulation and fractionation of zirconium and hafnium in alkaline-carbonatite systems
The patterns of the distribution and fractionation of strategic metals (Zr, Hf) in the Kugda intrusion (Polar Siberia) have been studied. The contents of these elements significantly exceed their concentrations in other rocks (Zr 246 ppm, Hf 7.4 ppm). A significant increase in Zr and Hf from early rocks (olivinite and melilite rocks) to later differentiation products, syenites with up to 570 ppm of Zr and 16 ppm of Hf, has been revealed. During the evolution of the Kuga magmatic system, notable
... tic system, notable fractionation of Zr and Hf occurred. The Zr/Hf ratios in the dike rock, similar in composition to the primary Kugda Massif magma, and the early intrusions are fairly close to that of chondrite (Zr/Hf = 37 ), while in the latest phases this ratio increases by almost 5-fold. Our study showed that the distribution coefficient of Hf (Kd = 0.58) in alkaline pyroxenes is noticeably higher than that of Zr (Kd = 0.40). Consequently, fractionation of this mineral leads to an increase in the Zr/Hf ratio in the residual liquids. Another mineral concentrating up to 400 ppm of Zr and up to 1520 ppm of Hf is perovskite, which has a very wide crystallization field in the rocks of the Kugda Massif, especially in the earliest olivinite. The data obtained showed that the Zr/Hf ratio in the perovskite of olivinite varies between 2327, that is, noticeably below both the chondritic and the primary magma values. Early crystallization of perovskite is the main reason for increasing the Zr/Hf ratio in melilitolites (up to 54). Thus, the main process of forming the Kugda Massif was continuous crystallization differentiation, accompanied by a noticeable fractionation of rock-forming and accessory minerals (pyroxene and perovskite).