Conditions of Accumulation and Fractionation of Zirconium and Hafnium in the Alkaline–Carbonatite Systems

Аннотация

Abstract—The distribution and fractionation of strategic metals (Zr, Hf) have been studied in the Kugda intrusion (Polar Siberia). The contents of these elements significantly exceed those 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, up to 570 ppm Zr and 16 ppm Hf, has been revealed. During the evolution of the Kugda magmatic system, the notable fractionation of Zr and Hf occurred. The Zr/Hf ratios in the dike rock, which is similar in composition to the primary magma of the Kugda massif, and in the early intrusions are fairly close to that of chondrite (Zr/Hf = 37 [1]), and shows almost a five-fold increase in the latest phases. Our study showed that the partition coefficient of Hf (Kd = 0.58) in alkaline pyroxenes is much higher than that of Zr (Kd = 0.40). Consequently, the fractionation of this mineral leads to an increase in the Zr/Hf ratio in residual liquids. Another mineral concentrating up to 400 ppm Zr and up to 15–20 ppm of Hf is perovskite, which has a very wide crystallization field in the rocks of the Kugda Massif, especially in the earliest olivinites. The data showed that the Zr/Hf ratio in the perovskite from olivinite varies between 23–27, which is noticeably below this value in both the chondrite and the primary magma. Early crystallization of perovskite is the main reason for increasing the Zr/Hf ratio in melilitolites (up to 54). Thus, the formation of the Kugda Massif was mainly controlled by crystallization differentiation accompanying by a significant fractionation of rock-forming and accessory minerals (pyroxene and perovskite).