Plain Language Summary:
Earth’s rigid outer shell is broken into pieces that move relative to
each other. These motions are generally understood according to the
theory of plate tectonics. However, the origins of plate tectonics are
not well understood. This contribution focuses on an aspect of this
problem, namely, the lack of consensus concerning when plate tectonics
started. We examine some of the most ancient evidence which has been
speculated to record plate tectonic processes: ultramafic rocks from the
≥3.7 billion-years-old Isua supracrustal belt of southwestern Greenland.
A leading hypothesis suggests that these are mantle (deep) rocks
emplaced by plate tectonic deformation. We test the viability of an
alternative hypothesis: that these rocks may have crystallized from
magmas at crustal (shallow) levels, a history that would not require
plate tectonics. Specifically, we compare new and published mineral and
chemical features of the Isua ultramafic rocks with similar rocks from
known crustal and mantle settings, including new data from a
northwestern Australia crustal site which is similar, yet widely
considered non-plate tectonic. Results show that each feature of the
Isua ultramafic rocks is consistent with crustal crystallization.
Therefore, these rocks do not constrain early plate tectonics, which
could have developed later.
Introduction:
When, how, and why plate tectonics began on Earth remain among the most
important unresolved questions in plate tectonic theory
(e.g., Bauer et al., 2020; Beall
et al., 2018; Brown and Johnson, 2018; Condie and Puetz, 2019; Hansen,
2007; Harrison, 2009; Korenaga, 2011; Nutman et al., 2020; Stern, 2008;
Tang et al., 2020). Investigations of plate tectonic initiation have
significant implications for questions associated with the evolution of
early terrestrial planets, including whether early Earth experienced any
pre-plate tectonic global geodynamics/cooling after the magma ocean
stage (e.g., Bédard, 2018; Collins
et al., 1998; Lenardic, 2018; Moore and Webb, 2013; O’Neill and
Debaille, 2014); and why other terrestrial planets in the solar system
appear to lack plate tectonic records (e.g.,
Moore et al., 2017; Stern et al.,
2017; cf. Yin, 2012a; Yin, 2012b).
Many proposed signals for the initiation or early operation of plate
tectonics on Earth are controversial due to the issue of non-uniqueness.
For instance, the origin of Hadean zircons from the Jack Hills of
western Australia have been contrastingly interpreted as (1) detrital
crystals from felsic magmas generated by ~4.3 Ga plate
subduction (Harrison, 2009;
Hopkins et al., 2008); (2) zircons
crystallized via impact heating and ejecta sheet burial
(Marchi et al., 2014) or (3) low
pressure melting of Hadean mafic crustal materials
(Reimink et al., 2020). Similarly,
researchers continue to debate whether the presence of Archean high
Al2O3/TiO2 mafic lavas
(also known as boninite or boninitic basalts) must indicate subduction
initiation as early as ~3.7 Ga (cf.
Pearce and Reagan, 2019;
Polat and Hofmann, 2003). Another
example is how a ~3.2 Ga shift in zircon Hf-isotope
signatures has been variably interpreted to indicate the onset of plate
tectonics (Næraa et al., 2012) or
enhanced mantle melting during a proposed mantle thermal peak
(Kirkland et al., 2021). Due to
these equivocal interpretations, the initiation of plate tectonics has
been suggested to be ≤3.2 Ga using geological records that are generally
considered unique to plate tectonics (e.g., paired metamorphic belts,
ultra-high pressure [UHP] terranes, and passive margins) (e.g.,
Brown and Johnson, 2018; Cawood et
al., 2018; Stern, 2008; cf. Bauer et al., 2020;
Foley et al., 2014; Harrison,
2009; Korenaga, 2011; Nutman et al., 2020). The ≤3.2 Ga onset of plate
tectonics requires early Earth tectonic evolution to be
non-uniformitarian, involving some form of single-plate stagnant-lid
tectonics (e.g., Bédard, 2018; Collins et al., 1998; Moore and Webb,
2013).
One proposed signal of early plate tectonics is the preservation of
phaneritic ultramafic rocks in Eo- and Paleoarchean terranes. However,
the issue of non-uniqueness also extends to their interpretations. In
the Eoarchean Isua supracrustal belt and adjacent meta-tonalite bodies
exposed in southwestern Greenland (Fig. 1a ), some dunites and
harzburgites have been interpreted to represent melt-depleted mantle
rocks that experienced UHP metamorphism, percolated by arc basalts, and
then emplaced on top of crustal rocks via subduction thrusting (e.g.,
Friend and Nutman, 2011; Nutman et
al., 2020; Van de Löcht et al.,
2018), similar to how modern ophiolitic ultramafic rocks formed in the
mantle and are preserved in collisional massifs (e.g.,
Boudier et al., 1988;
Lundeen, 1978;
Wal and Vissers, 1993). These
processes are not compatible with non-plate tectonic origins, where the
ultramafic rocks can only be cumulates or high-Mg extrusive rocks (e.g.,
komatiites) without UHP metamorphic overprints
(Webb et al. 2020;
Ramírez-Salazar et al. 2021).
Although
Szilas
et al. (2015) and Waterton et al.
(2022) argue that dunites and harzburgites in the Isua supracrustal belt
can be interpreted as crustal cumulates formed by fractionation of Isua
basalts, additional examinations are necessary to rule out depleted
mantle origins and thus plate tectonics as necessary for their igneous
and metamorphic petrogenesis. Namely, further investigations of the
igneous and metamorphic features of Isua ultramafic rocks, the origins
of their potential parent melts, and the natures of melt/fluid
components that have been interacted with them (Waterton et al. 2022)
are necessary outstanding tests. If Isua ultramafic rocks cannot be used
as unequivocal indicators of plate tectonics, then the preservation of
phaneritic ultramafic rocks in Eo- and Paleoarchean terranes may be all
attributed to processes consistent with non-uniformitarian, non-plate
tectonics.
This contribution explores the origins of Isua ultramafic rocks via
analysis of new and published geochemical and petrological findings,
including comparative analysis of key Isua samples and rocks of similar
lithology from settings considered representative of hot stagnant-lid
tectonics [In this study, we follow tectonic taxonomy from Lenardic
(2018)]. The Paleoarchean
geology of the East Pilbara Terrane of western Australia is widely
accepted as representing hot stagnant-lid tectonics
(Hickman, 2021;
Johnson et al., 2014;
Smithies et al., 2007, 2021;
Van Kranendonk et al., 2004,
2007); Pilbara ultramafic samples are investigated in this study
(Fig. 1b ) as examples of ultramafic rocks from non-plate
tectonic regimes. We also compare the petrology and geochemistry of Isua
ultramafic rocks with those of (1) ultramafic cumulate rocks; (2)
modelled ultramafic cumulate rocks; (3) melt-depleted mantle rocks from
plate tectonic settings; and (4) modelled melt-depleted mantle rocks. We
examine whether the generation of Isua and Pilbara ultramafic rocks is
compatible with the predictions of hot stagnant-lid tectonics. Our
findings help to evaluate whether plate tectonics is indeed required to
explain the Eoarchean assembly of the Isua supracrustal belt. A
complementary work (Mueller et al.
pre-print) further explores these tectonic questions via re-examination
of the pressure-temperature conditions experienced by Isua ultramafic
rocks.