Figure 11. Evolutionary diagrams for Isua and Pilbara ultramafic rocks. Ultramafic rocks from both terranes can be interpreted via similar hot stagnant-lid tectonic models. Ultramafic rocks are initially cumulates formed during cooling of magmas in hot stagnant-lid settings that feature voluminous volcanism. During solidification of these cumulates, REE-enriched but HSE-depleted melts interacted with cumulus phases. These cumulates were then variably deformed and/or metamorphosed during tectonic events that either represent (1) shortening, corresponding to volcanic burial, plate-breaking or plate tectonic subduction (panel a); or (2) intra-crustal diapirism corresponding to gravitational instability (panel b). Later, mostly static (talc/carbonate/serpentine) alterations further modified the petrology and geochemistry.
ConclusionsSome ultramafic rocks preserved in or near the Isua supracrustal belt have been interpreted as tectonically emplaced mantle peridotites that require >3.7 Ga onset of plate tectonics (e.g., Nutman et al., 2020; Van de Löcht et al., 2018). In contrast, this study shows that cumulates and mantle rocks may have similar primary rock textures, and whole-rock geochemistry and igneous mineral assemblages generated by olivine-dominated rocks interacting with melts. Differences between Isua and Pilbara ultramafic rocks may largely have resulted from different alterations and/or deformation experienced by these rocks, which are also consistent with crustal conditions (Waterton et al., 2022; Mueller et al., pre-print). In contrast, other characteristics of these rocks, such as certain types of spinel geochemistry (e.g., Fe-Ti trends in Cr#-Mg# space, Barnes and Roeder, 2001) as well as cumulate textures, appear to be unique to cumulates. Furthermore, melts that have interacted with Isua and Pilbara ultramafic rocks should be co-genetic melts generated in magma chambers or deep mantle, which cannot be explained by sub-arc mantle origins. Thus, we conclude that no features preserved in ultramafic rocks of the Isua supracrustal belt and East Pilbara Terrane are diagnostic of plate tectonic-related mantle slices, but instead are compatible with crustal cumulates. Again, it is important to note that these interpretations do not exclude plate tectonic origins for the formation of the Isua supracrustal belt (e.g., Van Kranendonk, 2010; Nutman et al., 2020), but they permit a hot stagnant-lid tectonic origin for this terrane, consistent with previous studies for the belt (Ramírez-Salazar et al., 2021; Webb et al., 2020; Zuo et al., 2021). Therefore, because the East Pilbara Terrane can also be explained in terms of a hot stagnant-lid setting (e.g., Collins et al., 1998; Van Kranendonk et al., 2007), no tectonic shift between the Eoarchean and Paleoarchean is required. Short episodes of local plate tectonic processes during the Eo- and Paleoarchean might be possible, as regional stagnant-lid processes may have coexisted with local plate tectonic processes in early terrestrial planets (e.g., Van Kranendonk, 2010; Yin, 2012a; Yin, 2012b). Nonetheless, our findings show that a ≤3.2 Ga initiation of plate tectonics is viable.