Earth's Earliest Phaneritic Ultramafic Rocks: Mantle Slices or Crustal Cumulates?

When plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant-lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction-related, tectonically-exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non-plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt-enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite-spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole-rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low-Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically-exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co-genetic melts. Because such rocks could have formed in either plate or non-plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings. Plain Language Summary Earth's rigid outer shell is broken into rigid pieces that move relative to each other. These motions are generally understood to reflect plate tectonics. However, the origins of plate tectonics are poorly understood. This contribution focuses on an aspect of this problem: the lack of consensus concerning when plate tectonics started. We examine some of the oldest rocks on Earth that researchers have speculated record plate tectonic processes: ultramafic rocks from the >= 3.7 billion-years-old Isua supracrustal belt of southwestern Greenland. A leading hypothesis suggests that these rocks are slices of mantle emplaced by plate tectonic deformation. We test the viability of an alternative hypothesis that they may have crystallized from magmas at shallow crustal levels, which does not require a plate tectonics setting. Specifically, we compare new and published mineral and chemical features of the Isua ultramafic rocks with similar rocks from known crustal and mantle settings, including new data from similarly ancient terrane in northwestern Australia that is widely considered to be non-plate tectonic. Results show that each feature of the Isua ultramafic rocks is consistent with crustal crystallization. Therefore, these ultramafic rocks do not constrain early plate tectonics, which could have developed later.