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).