5.2. Isua and Pilbara ultramafic rocks, similar or different?
Ultramafic rocks from the East Pilbara Terrane are generally interpreted as cumulates or high-Mg volcanic flows, which are consistent with the widely-accepted non-plate tectonic origin for this terrane (Collins et al., 1998; Hickman, 2021). Therefore, a comparison between Isua and Pilbara ultramafic rocks in terms of their mineralogy, rock textures and whole-rock/spinel geochemistry can be used to explore whether or not they have similar igneous origins, thereby testing the viability of non-plate tectonic models for the Isua supracrustal belt.
Isua and Pilbara ultramafic rocks have similar protolith mineralogy and relict olivine polygonal textures, but record different deformation patterns. Serpentine grains preserved in Pilbara samples (Fig. 3 ) appear to be undeformed pseudomorphs after primary olivine and pyroxene. Spinel is abundant in our Pilbara ultramafic samples (Fig. 3 ). Olivine grains preserved in the Isua lens A sample AW17725-4 have forsterite contents of ~87, slightly lower than published forsterite contents of ~88–92 for lens A meta-dunite samples (e.g., Szilas et al., 2015; Nutman et al., 2020). Olivine grains from lens A samples have been interpreted as primary igneous olivine (e.g., Szilas et al., 2015; Nutman et al., 2020). Other primary minerals observed in Isua ultramafic samples are pyroxene and spinel (Fig. 2 ; e.g., Szilas et al., 2015; Nutman et al., 2020; Van de Löcht et al., 2020). Therefore, Isua ultramafic samples potentially have similar protolith mineral assemblages (olivine + spinel ± pyroxene) to their Pilbara counterparts. Pyroxene appears to be a minor component in Pilbara ultramafic samples, and spinifex textures are not observed (Fig. 3 ), which do not support an extrusive komatiite origin for our Pilbara ultramafic samples. Instead, the poikilitic textures of Pilbara ultramafic rocks (Fig. 3c ) as preserved by the serpentine pseudomorphs can only be explained through the formation of olivine-rich cumulates (Wager and Brown, 1967). The polygonal textures of Pilbara ultramafic rocks (Fig. 3b ) likely developed via re-equilibration and recrystallization of cumulate olivine grains under crustal conditions (e.g., Hunter, 1996). Therefore, rock textures support the hypothesis that Pilbara ultramafic samples are cumulates. However, primary rock textures of most of our Isua ultramafic samples are lost due to alteration and/or deformation that post-dates olivine and late-stage serpentine minerals (Fig. 2 ). Only one sample (AW17725-4) from the meta-peridotite lens A preserves relict polygonal textures that feature abundant ~120° triple junctions of olivine grains (Fig. 2b ), consistent with findings in rocks sampled from nearby outcrops (e.g., Nutman et al., 1996) and Pilbara ultramafic samples (Fig. 3b ). Therefore, relict polygonal textures in Isua lens A samples could also result from mineral re-equilibration and recrystallization under crustal conditions (Hunter et al. 1996) rather than under mantle conditions (Nutman et al., 1996).