How and when plate tectonics initiated remain uncertain. In part, this is because many signals that have been interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant-lid tectonics. One such signal involves early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt of southwestern Greenland, some ultramafic rocks have been interpreted as tectonically-exhumed mantle during Eoarchean subduction. To explore whether all Archean phaneritic ultramafic rocks originated as cumulate and/or komatiite – i.e., without requiring plate tectonics – we examined the petrology and geochemistry of such rocks in the Isua supracrustal belt and the Paleoarchean East Pilbara Terrane of northwestern Australia, with Pilbara ultramafic rocks interpreted as representative of rocks from non-plate tectonic settings. We found that Pilbara ultramafic samples have relict cumulate textures, relative enrichment of whole-rock Os, Ir, and Ru versus Pt and Pd, and spinel with variable TiO2, relatively consistent Cr#, and variable and low Mg#. Similar geochemical characteristics also occur in variably altered Isua ultramafic rocks. We show that Isua and Pilbara ultramafic rocks should have interacted with low Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magma generation or crystallization. Such melts cannot have interacted with a mantle wedge. Furthermore, altered mantle rocks and altered cumulates could have similar rock textures and whole-rock geochemistry such that they may not distinguish mantle from cumulate. Our findings suggest that depleted mantle interpretations are not consistent with geochemistry and/or rock textures obtained from Isua and Pilbara ultramafic rocks. Instead, cumulate textures of Pilbara samples, whole-rock Pt and Pd concentrations, and spinel geochemistry of Isua and Pilbara ultramafic rocks support cumulate origins and metasomatism involving co-genetic melts that formed in hot stagnant-lid settings. Collectively, these findings permit ≤ 3.2 Ga initiation of plate tectonics on Earth.