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 interpreted as representative of rocks from non-plate tectonic settings. We found that serpentinized Pilbara ultramafic samples have relict cumulate textures, weakly fractionated to unfractionated trace element patterns that are close to primitive mantle values, relative enrichment of whole-rock Os, Ir, and Ru versus Pt, 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 mantle rocks and cumulates may have similar rock textures and whole-rock geochemistry, especially when their geochemistry is controlled by similar alteration processes (e.g., serpentinization, melt-assisted alterations). Therefore, depleted mantle interpretations are not required to explain the data obtained from Isua and Pilbara ultramafic rocks. Instead, cumulate textures of Pilbara samples and spinel geochemistry of Isua and Pilbara ultramafic rocks imply cumulate origins, which are consistent with their geochemistry indicating interactions between cumulus olivine and co-genetic melts that formed in hot stagnant-lid settings. Collectively, these findings permit ≤ 3.2 Ga initiation of plate tectonics on Earth.