Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots.

The oxygen fugacity (fO) 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 (δCu) 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 and volatile speciation in their sources. The lamproites' low δCu (-0.19 to -0.12‰) show that the LAB was metal-saturated with CH + HO as the dominant volatiles during the Mesoproterozoic. The mantle-like δCu of the < 0.59 Ga alkaline rocks (0.03 to 0.15‰) indicate that the LAB was more oxidized, stabilizing CO + HO and destabilizing metals. The Neoproterozoic oxidation resulted in an increase of at least 2.5 log units in fO at the LAB. Combined with previously reported high fO in peridotites from the Slave, Kaapvaal, and Siberia cratonic roots, this oxidation might occur in cratonic roots globally.