Hf contents and Zr/Hf ratios in granitic zircons

Abstract

We present a statistical analysis of 2201 electron microprobe determinations of Hf contents and Zr/Hf ratios in zircon from 65 samples of granitic rocks. The modal Hf content and modal Zr/Hf ratio of zircon taken from the data are 1.43 wt% HfO₂ and 38.5 respectively. Estimated partition coefficients are 2520 for D_Zr and 2420 for D_Hf between zircon and granitic magma. The large and similar Zr and Hf partition coefficients indicate that the crystallization of zircon would deplete the melt in both Zr and Hf, but would not cause significant fractionation of Zr from Hf. On the basis of the geochemical behaviour of Hf, granitic zircons can be divided into three types (early, late and hydrothermal zircon). Compared to the early zircon in granitic rocks, late-crystallizing zircon and especially hydrothermal zircon are characterized by much higher Hf contents and lower Zr/Hf, which can be explained by a change in D_Hf with decreasing crystallization temperature. The behavior of Hf in granitic zircon may be used as geochemical indicator to track the origins and crystallization environments of the host magma.