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 ).