Abstract:
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 mantle rocks tectonically exhumed 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 being representative of rocks from non-plate tectonic settings. We found that Pilbara ultramafic samples have cumulate textures and relative enrichment of whole-rock Os, Ir, and Ru versus Pt. In comparison, polygonal textures and variable whole-rock Os, Ir, Ru and Pt patterns are identified in Isua ultramafic samples. Isua and Pilbara ultramafic samples have (1) mineral assemblages that can form at crustal conditions; (2) broadly similar whole-rock major element patterns; (3) weakly fractionated to unfractionated trace element patterns that are close to primitive mantle values; and (4) spinel with variable TiO2, relatively consistent Cr#, and variable and low Mg#. Many features of Isua or Pilbara ultramafic rocks are similar to depleted mantle rocks, except for spinel chemistry and cumulate textures. However, all features are consistent with cumulates. Collectively, these data permit ≤ 3.2 Ga initiation of plate tectonics on Earth.