Trace-element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills, Australia

Hadean (∼4.5-4.0 Ga) zircon from the Jack Hills, Western Australia, have variable normalized REE patterns. In particular, the oldest piece of Earth-zircon crystal W74/2-36 (dated at 4.4 Ga)-contains both type-1 and type-2 patterns on a 50 μm scale, a phenomenon not yet reported for unaltered magmatic zircon. In the context of documented magmatic and hydrothermal zircon compositions from constrained samples from the BPZP and the literature, the type-2 patterns in crystal W74/2-36 and other Jack Hills Hadean (JHH) zircon are interpreted as hydrothermally-altered magmatic compositions. An alteration scenario, constrained by isotope and trace-element data, as well as α-decay event calculations, involving fluid/zircon cation and oxygen isotope exchange within partially metamict zones and minor dissolution/reprecipitation, may have occurred episodically for some JHH zircon and at ∼4.27 Ga for zircon W74/2-36. Type-2 compositions in JHH zircon are interpreted to represent localized exchange with a light-REE-bearing, high δ18O (∼6-10‰ or higher) fluid. Thus, a complex explanation involving “permanent” liquid water oceans, large-scale water/rock interaction and plate tectonics in the very early Archean is not necessary as the zircon textures and compositions are simply explained by exchange between partially metamict zircon and a low volume ephemeral fluid.