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.