Recycling of ca 4.35 Ga KREEP-Like Crust in Western Australia 4.1 Gy ago

We measured Hf and Pb isotope compositions for 63 single zircons from Jack Hills (JH), Western Australia, using solution chemistry and, respectively, MC-ICP-MS and ICP-MS techniques. The data were pooled with the solution chemistry data of Harrison et al. (2005) totaling 104 analyses. Bulk 207Pb*/206Pb* ages, including ion microprobe dates at >4.33 and 3.32 Ga, are anormally distributed between 3.8 and 4.2 Ga with a well-defined peak at 4.10±0.03 Ga. Thus, for our purposes, we treat the data as a single population that formed at 4.1 Ga, rather than as a series of events, although ion microprobe dating indicates the presence of a small >4.2 Ga component. Most of the analyzed zircons have Hf isotope compositions significantly less radiogenic than both chondrites and the depleted mantle at 4.1 Ga. The simplest interpretation is that the JH zircon granitoid hosts were melts of an enriched reservoir with sub-chondritic Lu/Hf. Single-stage Hf model ages, calculated by back-tracking the Hf isotopic composition at 4.1 Ga to either the chondritic or the depleted mantle reservoirs for a range of Lu/Hf ratios, allow insights into the age and composition of this pre-existing crust. The modeling shows that for 176Lu/177Hf > 0.015, such as in basaltic crust, the resulting histograms are flat and poorly defined with many unacceptable zircon ages older than 4.56 Ga. In contrast, for extremely low 176Lu/177Hf of 0.005-0.010, the calculated histograms define narrow well-defined peaks with maxima at 4.31 and 4.36 Ga for the chondritic and depleted mantle reservoirs, respectively, with all zircons younger than the age of the Earth. This indicates that the source rock of the JH granites was not the equivalent of modern MORB or oceanic plateau basalts, which have 176Lu/177Hf of 0.026 and 0.029, respectively. Neither was it likely to have been granitic, similar to rocks formed in subduction zones today, because the low inferred Lu/Hf ratio for the source rock of the JH granites is different from granites in their present form. Thus, remelting of either hydrous oceanic crust or granites with modern Lu/Hf characteristics does not seem to be how the JH granites formed. Rather, the early crust giving rise to the JH granites may have originated from either the residual liquids after magma ocean crystallization or from melts of the last magma ocean cumulates, both of which would have been extremely differentiated with very low Lu/Hf due to garnet having crystallized at depth. Interaction with the hydrosphere, known from oxygen isotopes in JH zircons to have existed in the Hadean, would have enhanced the hydrous nature of this crust so as to constitute a suitable reservoir for the JH granites. A Rayleigh fractionation model of the terrestrial magma ocean, assuming chondritic Lu/Hf of the magma and a partition coefficient for Lu of 4 between garnet and the liquid, demonstrates that removal of either 85% of a cumulate with 25% garnet or 99% of a cumulate with 10% garnet would have left a residual liquid with 176Lu/177Hf of 0.005. Once this extremely differentiated hydrous KREEP-like proto-crust was in place, the scene was set for plate tectonics to begin and thus modern-type granites to form. The JH zircon Hf model ages place this transition at a minimum of ~4.35 Ga. The loss ultimately of this early crust, either by impacts or foundering into the deep mantle, provides an explanation for the hidden reservoir required by Hf-Nd and 142Nd isotope systematics.