Light oxygen isotopic composition in deep mantle reveals oceanic crust subduction before 3.3 billion years ago

Compositional heterogeneity exists in Earth’s deep mantle, which can be caused by the subduction of oceanic slabs. How early this process started on Earth remains highly debated due to the scarcity of early Archean materials with pristine mantle compositional signatures. Here, using the oxygen isotope and elemental compositions of fresh olivine grains in the 3.27-Ga komatiites of the Weltevreden Formation in the Barberton Greenstone Belt in Southern Africa, we discovered two groups of samples with primitive olivine grains. Group I exhibits normal mantle-like δ 18 O values and high Fo contents (δ 18 O = 4.9–5.4‰; Fo = 93–95); Group II is characterized by lower δ 18 O values with slightly lower Fo contents (δ 18 O = 3.6–4.7‰; Fo = 91–93). These δ 18 O values correlate with other geochemical proxies of olivine-poor iron-rich pyroxenite sources, indicating that the Weltevreden komatiites were derived from two distinct mantle sources. The existence of the low-δ 18 O magmas can be best explained by recycling of the altered oceanic crust into deep mantle arguably by subduction, which started 3.3 billion years ago and is responsible for the deep mantle heterogeneity in early Earth.