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 (delta Cu-65) of similar to 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 delta Cu-65 (-0.19 to -0.12 parts per thousand) show that the LAB was metal-saturated with CH4 + H2O as the dominant volatiles during the Mesoproterozoic. The mantle-like delta Cu-65 of the < 0.59 Ga alkaline rocks (0.03 to 0.15 parts per thousand) indicate that the LAB was more oxidized, stabilizing CO2 + H2O 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.