Coupled strontium-calcium isotopes in Archean anorthosites reveal a late start for mantle depletion

Voluminous felsic continental crust is, as far as we know, unique to Earth, yet the timescales of its earliest growth remain debated. Archean mantle evolution is complementary to crustal growth but is largely unconstrained for strontium isotopes due widespread Rb-Sr disturbance. Here, we perform high spatial resolution radiogenic Sr and Ca isotope measurements in magmatic plagioclase megacrysts from 3.7-2.8 Ga Archean anorthosites and leucogabbros. We report mantle-like Ca isotope signatures and the most unradiogenic terrestrial ⁸⁷Sr/⁸⁶Sr ratio yet measured on Earth, in plagioclase (⁸⁷Sr/⁸⁶Sr_initial = 0.700050 ± 0.000017, 95% confidence interval) from the 3.73 Ga Manfred Complex of the Narryer Terrane, Western Australia. These data are consistent with ⁸⁷Sr/⁸⁶Sr homogenisation between the Earth and Moon (⁸⁷Sr/⁸⁶Sr_initial ≈ 0.699061) at ca. 4.515 Ga during the Moon-forming giant impact. The appearance of a depleted mantle signature in the terrestrial strontium record post-3.5 Ga suggests that extensive continental growth began relatively late.