5.5. A model for emplacement, metamorphism and alteration of Eo- and Paleo-Archean phaneritic ultramafic rocks
Here we describe a common evolutionary pathway for ultramafic rocks of early Earth terranes in the context of the hot stagnant-lid tectonic regimes such as heat-pipe tectonics (Moore and Webb, 2013) and partial convective overturn tectonics (Collins et al., 1998), which have been proposed for such terranes. Ultramafic rocks of early Earth could have initially crystallized from high-magnesium, fluid-rich magmas, either as ultramafic volcanic flows [e.g., komatiites, Byerly et al. (2019)], intrusions, or crustal cumulates at the bases of lava flows or magma chambers (Fig. 11 ). Some of these rocks could have experienced interactions with co-genetic melts, such as HSE-depleted melts derived from the deep mantle (Fig. 11 ). Later, these ultramafic rocks could have been metamorphosed under crustal conditions (e.g., greenschist or amphibolite facies conditions) that may or may not have been associated with significant deformation and mineral phase transformation. In the case of the Isua supracrustal belt, amphibolite facies metamorphism was accompanied by deformation during, at the end of, or after heat-pipe cooling (e.g., Ramírez-Salazar et al., 2021; Webb et al., 2020; Zuo et al., 2021). These P-T conditions are capable of producing olivine + serpentine ± Ti-humite ± carbonate ± talc metamorphic assemblages (Fig. 11a ). Primary igneous textures in olivine-rich cumulates could have been preserved by concentrating most of the strain into other phases (e.g., Yao et al., 2019; Zuo et al., 2021). Alternatively, growth of metamorphic olivine from dehydration breakdown of strongly oriented serpentine minerals could also produce a B-type olivine CPO (e.g., Nagaya et al., 2014). In contrast, hot stagnant-lid volcanism during the Paleoarchean time would have been less rapid in terms of long-term deposition and burial rates versus the Eoarchean Isua supracrustal belt, and thus would have led to a relatively hot lithosphere for the East Pilbara Terrane (Moore and Webb, 2013; Webb et al., 2020), potentially permitting intra-crustal partial convection via gravitational instability (Fig. 11b ; Collins et al., 1998). The metamorphic conditions experienced by the exposed Pilbara rocks may have been lower, and deformation may have been weaker (e.g., Collins et al., 1998; Wiemer et al., 2018), especially in rocks located far from the margins of the granitoid bodies (e.g., François et al., 2014) such as the samples studied here (Fig. 1b ). Consequently, Pilbara ultramafic samples only preserve evidence for greenschist facies metamorphism without identifiable strain (Fig. 3 ). Post-deformational alterations (such as talc, carbonate, or serpentine alterations) might have further modified these ultramafic rocks as well as nearby supracrustal rocks in the following >3 billion years (Fig. 11 ).