5.4. A model for emplacement, metamorphism and alteration of Eo-
and Paleo-Archean phaneritic ultramafic rocks
As both Isua and Pilbara ultramafic rocks can be interpreted by a hot
stagnant-lid tectonic regime such as heat-pipe tectonics (Moore and
Webb, 2013) and partial convective overturn tectonics (Collins et al.,
1998), we propose a common evolutionary pathway for ultramafic rocks of
early Earth terranes. Ultramafic rocks of early Earth could have
initially crystallized from high-magnesium, fluid-rich magmas, either as
ultramafic volcanic flows [e.g., komatiites, Byerly et al. (2019)],
intrusions, or crustal cumulates at the bases of lava flows or magma
chambers (Fig. 11 ). Later, these ultramafic rocks could have
been metamorphosed under crustal conditions (e.g., greenschist or
amphibolite facies conditions) that may or may not have been associated
with significant deformation. In the case of the Isua supracrustal belt,
amphibolite facies metamorphism was accompanied by deformation during,
at the end of, or after heat-pipe cooling (e.g., Ramírez-Salazar et al.,
2021; Webb et al., 2020; Zuo et al., 2021). These P-T conditions are
capable of producing olivine + serpentine Ti-humite + carbonate + talc
bearing assemblages over the ultramafic protoliths (Fig. 11a ).
Primary igneous textures in olivine-rich cumulates could have been
preserved by concentrating most of the strain into other phases (e.g.,
Yao et al., 2019; Zuo et al., 2021). Alternatively, growth of
metamorphic olivine from dehydration breakdown of strongly oriented
serpentine minerals could also produce a B-type olivine CPO (e.g.,
Nagaya et al., 2014a, 2014b; cf. Nozaka, 2014). In comparison, hot
stagnant-lid volcanism during the Paleoarchean time would have been less
rapid in terms of long-term deposition and burial rates versus the
Eoarchean Isua supracrustal belt, and thus would have led to a
relatively hot lithosphere for the East Pilbara Terrane (Moore and Webb,
2013; Webb et al., 2020), potentially permitting intra-crustal partial
convection via gravitational instability (Fig. 11b ; Collins et
al., 1998). The metamorphic conditions experienced by the exposed
Pilbara rocks may have been lower, and deformation may have been weaker
(e.g., Collins et al., 1998; Wiemer et al., 2018), especially in rocks
located far from the margins of the granitoid bodies (e.g., François et
al., 2014) such as the samples studied here (Fig. 1b ).
Consequently, Pilbara ultramafic samples only preserve evidence for
greenschist metamorphism without identifiable strain (Fig. 3 ).
Post-deformational alterations (such as talc, carbonate, or serpentine
alterations) might have further modified these ultramafic rocks as well
as nearby supracrustal rocks in the following >3 billion
years (Fig. 11 ).