Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots.

The oxygen fugacity (fO 2 ) of the lower cratonic lithosphere influences diamond formation, melting mechanisms, and lithospheric evolution, but its redox evolution over time is unclear. We apply Cu isotopes (δ 65 Cu) of ~ 1.4 Ga lamproites and < 0.59 Ga silica-undersaturated alkaline rocks from the lithosphere-asthenosphere boundary (LAB) of the North Atlantic Craton to characterize fO 2 and volatile speciation in their sources. The lamproites' low δ 65 Cu (-0.19 to -0.12‰) show that the LAB was metal-saturated with CH 4  + H 2 O as the dominant volatiles during the Mesoproterozoic. The mantle-like δ 65 Cu of the < 0.59 Ga alkaline rocks (0.03 to 0.15‰) indicate that the LAB was more oxidized, stabilizing CO 2  + H 2 O and destabilizing metals. The Neoproterozoic oxidation resulted in an increase of at least 2.5 log units in fO 2 at the LAB. Combined with previously reported high fO 2 in peridotites from the Slave, Kaapvaal, and Siberia cratonic roots, this oxidation might occur in cratonic roots globally.