Figure 11. Evolutionary diagrams for Isua and Pilbara
ultramafic rocks. Ultramafic rocks from both terranes can be interpreted
via similar hot stagnant-lid tectonic models. Ultramafic rocks are
initially cumulates formed during cooling of magmas in hot stagnant-lid
settings that feature voluminous volcanism. These cumulates were then
variably deformed and/or metamorphosed during tectonic events that
either represent (1) shortening, corresponding to volcanic burial,
plate-breaking or plate tectonic subduction (panel a); or (2)
intra-crustal diapirism corresponding to gravitational instability
(panel b). Later, mostly static (talc/carbonate/serpentine)
alterations further modified the petrology and geochemistry.
ConclusionsSome ultramafic rocks preserved in or near the Isua supracrustal belt
have been interpreted as tectonically emplaced mantle peridotites that
require >3.7 Ga onset of plate tectonics (e.g., Nutman et
al., 2020; Van de Löcht et al., 2018). In contrast, this study shows
that: (1) the polygonal rock textures of Isua ultramafic samples can
also be observed in Pilbara ultramafic rocks which show rock textures
of crustal cumulates; (2) the whole-rock major element, trace element
and HSE patterns of Isua ultramafic rocks are similar to those of
Pilbara ultramafic rocks and/or crustal cumulates; (3) the
co-existence of Ti-humite, magnesite, serpentine, olivine,
clinopyroxene and perhaps talc may be compatible with crustal
conditions; (4) the olivine oxygen isotopic signatures of Isua
ultramafic rocks can be explained by mantle-derived or metamorphic
fluid fluxing in a hot stagnant-lid setting or a plate tectonic
subduction setting; (5) the CPO inferred B-type olivine fabrics are
consistent with crustal cumulates; and (6) the spinel geochemistry of
Isua ultramafic rocks is only compatible with crustal cumulates. In
summary, many petrological and geochemical aspects (e.g., rock and
mineral textures, Ti-humite phases, and normalized HSE patterns) of
phaneritic ultramafic rocks in early Earth terranes on Earth could be
explained in the contexts of tectonically-emplaced mantle slices atop
of crustal rocks, but are also consistent with crustal cumulates (cf.
Nutman et al., 2021). In contrast, other characteristics of these
rocks, such as certain types of spinel geochemistry (e.g., Fe-Ti
trends in Cr#-Mg# space, Barnes and Roeder, 2001) as well as
cumulate textures, appear to be unique to cumulates. Thus, we conclude
that no features preserved in ultramafic rocks of the Isua
supracrustal belt and East Pilbara Terrane are diagnostic of plate
tectonic-related mantle slices, but instead are compatible with
crustal cumulates. We argue that differences between ultramafic rocks
within two terranes only reflect contrasting metamorphism,
deformation, and/or alteration conditions experienced by these rocks,
not necessarily different protoliths (cf. Friend and Nutman, 2011;
Nutman et al., 2020). Again, it is important to note that these
interpretations do not exclude plate tectonic origins for the
formation of the Isua supracrustal belt (e.g., Van Kranendonk, 2010;
Nutman et al., 2020), but they permit a hot stagnant-lid tectonic
origin for this terrane, consistent with previous studies for the belt
(Ramírez-Salazar et al., 2021; Webb et al., 2020; Zuo et al., 2021).
Therefore, because the East Pilbara Terrane (e.g., Collins et al.,
1998; Van Kranendonk et al., 2007) can also be explained in terms of a
hot stagnant-lid setting, no tectonic shift between the Eoarchean and
Paleoarchean is required. Short episodes of local plate tectonic
processes during the Eo- and Paleoarchean might be possible, as
regional stagnant-lid processes may have coexisted with local plate
tectonic processes in early terrestrial planets (e.g., Van Kranendonk,
2010; Yin, 2012a; Yin, 2012b). Nonetheless, our findings show that a
≤3.2 Ga initiation of plate tectonics is viable.