Figure 6. Major element geochemical characteristics of the Isua and Pilbara ultramafic samples in comparison with those of Phanerozoic cumulates, arc peridotites, >3.2 Ga ultramafic rocks (seeFig. 3 for data sources), and modelled liquid lines of descent. All data are presented using anhydrous values (i.e., all major element abundances are normalized to zero LOI and 100 wt.% total). The data in this figure show that Isua and Pilbara ultramafic rocks, Mg-rich cumulates and mantle peridotites have similar major element geochemical systematics. Data sources for the cumulates and MELTS modelling curves are from Chin et al. (2018), Mallik et al. (2020), and references therein. Specifically, cumulates from oceanic island settings (panel b) cannot be modelled due to limitations of MELTs programs on modelling ultrahigh pressure (>3 GPa) melting and enriched mantle sources, which are necessary for generating oceanic island basalts.
Mineral geochemistry
Olivine grains in Isua sample AW17724-2C (lens B) have extraordinarily high Mg# values of ~95 98 and NiO of ~0.39–0.63 wt.%. In contrast, olivine grains in Isua sample AW17725-4 (lens A) have Mg# values of ~87 and NiO of ~0.52 0.61 wt% (Table S2 ). Ti-humite phases in sample AW17724-2C have variable TiO2abundances of ~3.0 8.1 wt.%. All analyzed spinel grains in the Isua samples contain a high magnetite component (i.e., FeOt of ~90 wt.%) (Table S2 ).
Spinel of both chromite or magnetite compositions occur in the Pilbara samples. Specifically, chromite spinel grains have Cr2O3 of ~40 50 wt.%, TiO2 of 0.6 4.3 wt.%, and MgO of 5 12 wt.%. The Cr# [Cr/(Cr+Al)] values and Mg# values of chromite spinel grains are ~65 75 and ~17 46, respectively (Fig. 9 ; Table S2 ).
DiscussionWe analyzed phaneritic ultramafic rocks in the Eoarchean Isua supracrustal belt and the East Pilbara Terrane to explore their petrogenesis as a means of testing the viability of existing tectonic models. Specifically, we explore whether these rocks need to be explained as mantle peridotites that were emplaced in the crust in a subduction setting. Our new petrological and geochemical data from six ultramafic samples from the Isua supracrustal belt and three ultramafic samples from the East Pilbara Terrane show that (1) Isua and Pilbara samples have been variably altered and now contain several alteration minerals (e.g., serpentine, talc, carbonate) that replaced igneous ferromagnesian silicates (Figs. 2–3 ) ; (2) Pilbara ultramafic samples preserve poikilitic textures and polygonal textures (Fig. 3 ); one Isua sample (AW17725-4 from lens A) also preserves relict polygonal textures (Fig. 2b ); (3) trace element abundances in both Isua and Pilbara ultramafic samples range from depleted with respect to the primitive mantle values (0.1 times primitive mantle values) to enriched (10 times primitive mantle values) (Fig. 6a–b ); (4) two out of three Pilbara ultramafic samples show fractionated, relatively high concentrations of Os and Ir versus Pt, Pd, and Re in the primitive mantle-normalized diagram (Fig. 7c ), which are similar to those of Isua meta-peridotite lens samples (Waterton et al., 2022); and (5) chromite spinel in Pilbara ultramafic samples feature Cr# of ~65 75, and Mg# of ~17 46 (Fig. 9 ). In the following sections, we first discuss the potential impacts of alterations on petrology and geochemistry. Then, we show that new and compiled petrology, geochemistry, and microstructures of Isua and Pilbara ultramafic rocks are consistent with a cumulate origin, whereas an origin as thrust-emplaced mantle slices is not required. We then discuss the implications for testing early Earth tectonic models and the initiation of plate tectonics.