The oxygen fugacity (f_O2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude^1,2,3. 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^5,6,7,8,9,10, 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.

脊橄榄岩记录的对流上地幔氧逸度（f _O2 ）变化超过四个数量级 ^1,2,3 。尽管人们对驱动地幔 f _O2 在构造环境 ^1,3,4 之间变化的机制以及现代岩石和古代地幔之间 f _O2 的比较给予了很多关注衍生岩石 ^5,6,7,8,9,10 ，对于了解现代洋中脊环境中橄榄岩记录的f _O2 高变异性的起源，人们所做的工作相对较少。在这里，我们报告了来自 Gakkel 海岭和东太平洋海隆 (EPR) 的橄榄岩的岩相学和地球化学，包括 16 个新的 f _O2 高精度测定。 Gakkel 海脊记录的难熔橄榄岩 f _O2 比地幔平均值低四个数量级以上。通过热力学和矿物分配模型，我们表明超低 f _O2 的偏移可以通过高位温 (T _p ) 下的大量熔融产生，从石榴石矿场开始，继续进入尖晶石领域——古代科马提岩的生成条件满足，但现代玄武岩不满足。这并不意味着太古宙时代的环境对流上地幔的铁与亚铁比率低于今天，也不意味着热点石榴石场的现代熔化产生了减少的岩浆。相反，它意味着古老的、难熔的、超还原地幔筏继续在现代地幔中循环，而对现代山脊火山作用几乎没有贡献。