Fig. 2. Hf isotope evolution diagrams.

Shown in (A) are the initial εHf values for the seven individual NWA 7034 zircons calculated with their corresponding ²⁰⁷Pb-²⁰⁶Pb ages using a λ¹⁷⁶Lu value of 1.867±0.008×10^-11 year^-1 (ref. ³⁰) and Chondritic Uniform Reservoir parameters of ref. 18. The upper boundary of the forbidden region represents a reservoir with a ¹⁷⁶Lu/¹⁷⁷Hf = 0 and a formation age defined by the age of the Solar System at 4567 Ma³¹. In (B), we show the time evolution of basaltic and andesitic crustal reservoirs required to account for the average initial Hf isotope compositions of the three concordant 4475 Ma zircons (S24b4, S24b7 and S25b10) using ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.020 and 0.011 for the basaltic and andesitic crusts, respectively²³,²⁵. Considering the upper uncertainty of the zircon average εHf value (–1.35±0.22), it is not possible to account for the initial Hf isotope composition of these grains if they formed from the reworking of a basaltic crust since extraction ages older than the Solar System are required. In contrast, using a more evolved, andesite-like ¹⁷⁶Lu/¹⁷⁷Hf ratio returns a minimum extraction age of 4547 Ma. Using the mean of the concordant grains at face value and a ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.011 yields an extraction age of ${4562}_{-15}^{+5}$ Ma. Note that the time evolution of this reservoir can account for the Hf isotope composition of the younger ~4450 Ma and ~4430 Ma zircons. Indeed, a regression of the mean of the ~4475 Ma, ~4450 Ma and ~4430 Ma zircons yields a slope corresponding to an andesite-like ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.011. Uncertainty on the εHf values reflect the internal precision (2SE) or the external reproducibility of 22 ppm, whichever is larger. Uncertainty on the ²⁰⁷Pb/²⁰⁶Pb ages (2σ) are smaller than symbols.