Deep, hot, ancient melting recorded by ultralow oxygen fugacity in peridotites

The oxygen fugacity (f_O2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude^1-3. Although much attention has been given to mechanisms that drive variations in mantle f_O2 between tectonic settings^1,3,4 and to comparisons of f_O2 between modern rocks and ancient-mantle-derived rocks^5-10, comparatively little has been done to understand the origins of the high variability in f_O2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of f_O2. Refractory peridotites from the Gakkel Ridge record f_O2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow f_O2 can be produced by large degrees of melting at high potential temperature (T_p), beginning in the garnet field and continuing into the spinel field-conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.