5.2. Isua and Pilbara ultramafic rocks, similar or different?
Ultramafic rocks from the East Pilbara Terrane were produced by non-plate tectonic processes (Collins et al., 1998; Hickman, 2021). Therefore, a comparison between Isua and Pilbara ultramafic rocks can be used to explore the viability of non-plate tectonic models for the Isua supracrustal belt. Serpentine grains preserved in Pilbara samples (Fig. 3 ) appear to be pseudomorphs after primary olivine and pyroxene. Spinel is abundant in our Pilbara ultramafic samples (Fig. 3 ). Olivine grains preserved in the 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). These olivine grains from lens A meta-dunite samples have been interpreted as primary igneous olivine (e.g., Szilas et al., 2015; Nutman et al., 2020). Other primary minerals observed in our and compiled Isua ultramafic samples are olivine, 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 by originating as 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 (Fig. 2a, 2c–d ). 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 ).
Alteration overprints observed in the Isua ultramafic rocks are different from those of Pilbara ultramafic samples. One Isua ultramafic sample from lens B (AW17724-2C) preserves Ti-humite phases that grew in equilibration with secondary highly forsteritic olivine (with Mg# of 95–98; Table S2 ), magnesite, serpentines (serp1 inFig. 2a ), and/or perhaps talc (Fig. 2a ) (cf. Nutman et al., 2020; Nutman et al., 2021; Guotana et al., 2021). This was followed by additional serpentinization as reflected by serpentine minerals (serp2 in Fig. 2a ) cross-cutting the pre-existing minerals, including olivine, magnesite and older serpentine grains (i.e., serp1) (Fig. 2a ). Other Isua ultramafic samples show variable degrees of serpentinization and talc/tremolite alteration (Fig. 2b–d ). In contrast, our Pilbara ultramafic samples are devoid of tremolite/talc carbonate alterations. Serpentinization occurs in Pilbara ultramafic samples but appears to be much more pervasive compared to Isua ultramafic samples (Fig. 3 ). Small (~20 μm in diameters) clinopyroxene inclusions in olivine or mantle-like olivine oxygen isotopes from Isua meta-dunite samples of lens A have been reported as evidence of melt-rock or fluid-rock alterations, respectively (Nutman et al., 2021), but the degrees of serpentinization in Pilbara samples prevented us from conducting similar analyses. Most opaque minerals in Isua ultramafic samples appear to be magnetite, whereas chromite is rare (Fig. 2 ; Nutman et al., 2021; Szilas et al., 2015). In contrast, chromite (sometimes rimmed by magnetite,Fig. 3a-b ) is common in Pilbara samples. Some of these differences in alteration styles may result from different regional metamorphic grades such that the Isua supracrustal belt might have experienced multiple metamorphic/metasomatic events under conditions up to upper amphibolite facies (e.g., Ramírez-Salazar et al., 2021), whereas supracrustal belts of the East Pilbara Terrane predominantly experienced greenschist facies conditions (e.g., Hickman, 2021).
Weak crystallographic preferred orientations (CPOs) of olivine crystals in Isua ultramafic samples from lenses A and B match the CPO pattern created by B-type slip (via dislocation creep) that is commonly found in hydrous mantle wedge settings (Kaczmerak et al., 2016 and references therein). The alteration degrees of Pilbara ultramafic samples again inhibit us from examining their olivine CPOs. The shapes of serpentine pseudomorphs in these Pilbara ultramafic samples show that cumulate textures are relatively well preserved, with no sign of strong deformation after complete serpentinization (Fig. 2e–g ).