Zirconium and its stable isotopes in igneous systems

Zirconium, as a high field strength element, has been widely used to trace the differentiation of terrestrial planets. It is also a major constituent in zircon, an important accessory mineral providing constraints on the history of the Earth. Recently, stable Zr isotopes have shown potential in tracing magma differentiation. To fuel the applications of stable Zr isotopes in tracing igneous processes, we review (1) the geochemical behaviors of Zr in igneous systems, which are fundamental for understanding Zr isotopes, and (2) mass-dependent Zr isotope fractionation in igneous rocks and zircons regarding its isotopic variability, mechanisms, geological implications, and potential applications. At the bulk rock scale, igneous rocks with SiO2 > 65 wt% show large Zr isotopic variations, which have been proposed to result from the segregation of isotopically light zircon from the melt. At the mineral scale, igneous zircons show Zr isotopic variations from homogeneous to highly heterogeneous. Zircon–melt equilibrium isotope fractionation (EIF) is likely small, with the preference of zircon for light Zr isotopes from the melt. In comparison, diffusive effects in melt can lead to larger isotope fractionation between zircon and melt, if zircon growth is limited by Zr diffusion in melt. On the other hand, in a diffusion-controlled regime, the growth of Zr-poor mineral may produce local Zr saturated and isotopically heavy melt areas; zircons crystallized from such areas can be isotopically heavier than the initial melt. Although the Zr isotope variability and fractionation mechanisms are complex, current studies suggest that (1) bulk-rock Zr isotopes can be used to trace the differentiation of felsic magmas, and that (2) zircon Zr isotopes have the potential to reveal zircon crystallization kinetics and magma dynamics. Integrated with other geochemical and isotopic tools, stable Zr isotopes may provide new insight into the dynamic history of diverse igneous systems through time.