Figure 8. Highly siderophile element (HSE) (including platinum-group elements, PGEs: Os, Ir, Ru, Pt, and Pd) characteristics of the Pilbara samples, Isua ultramafic rocks, cumulates, volcanics and mantle peridotites. Panels a to c show primitive mantle (PM)-normalized Pt/Ir and Ru/Ir ratios [i.e., (Pt/Ir)PM and (Ru/Ir)PM] of new Pilbara samples in comparison with those of Isua ultramafic rocks (from the supracrustal belt and peridotite enclaves, see Figure 4 caption; panel a), mantle peridotites (panel b), volcanics (komatiites and basalts) and peridotitic cumulates (panel c). Peridotites from meta-tonalite enclaves south of the Isua supracrustal belt are divided by Van de Löcht et al. (2018) into two groups according to their HSE signatures: “group 2” peridotites have higher Pt, Pd and Re versus “group 1” peridotites. Panel d shows PM-normalized HSE patterns of new Pilbara samples and compiled rocks in spider diagrams. These plots show that HSE characteristics of Pilbara ultramafic rocks are similar to those of cumulate rocks, but are different from those of mantle peridotites. Furthermore, HSE patterns of ultramafic rocks from peridotite enclaves of meta-tonalites south of the Isua supracrustal belt are consistent with those of cumulates and do not require mantle peridotite origins (cf. Van de Löcht et al., 2018). Data sources: compiled cumulates involve samples from the Eoarchean Uharagssuit nunât layered intrusion of southwestern Greenland (Coggon et al., 2015) the Mesoarchean Nuasahi Massif of India (Khatun et al., 2014), the Mesoarchean Tartoq Group of southwestern Greenland (Szilas et al., 2014), the Mesoarchean Seqi Ultramafic Complex of southwestern Greenland (Szilas et al., 2018), and the Eoarchean Tussapp Ultramafic Complex of southwestern Greenland (McIntyre et al., 2019); compiled Isua ultramafic samples and basalts are from the Isua supracrustal belt (Szilas et al., 2015) or the peridotite enclaves in meta-tonalite south of the Isua supracrustal belt (Van de Löcht et al., 2018); komatiites are from the Paleoarchean Barberton Greenstone Belt of South Africa (Maier et al., 2003); arc peridotites experienced serpentinization and melt-rock interaction are from the Northwest Anatolian orogenic complex, Turkey (Aldanmaz and Koprubasi, 2006); fresh and variably melt-refertilized abyssal peridotites are from the collisional massifs in Italian Alps, Italy (Wang et al., 2013); abyssal peridotites that experienced serpentnization and melt-rock interaction are from the Troodos Ophiolite Complex of Cyprus (Büchl et al., 2002); sub-continental lithospheric mantle rocks that experienced melt-rock interactions are from the Bohemian Massif of the Czech Republic (Ackerman et al., 2009). Primitive mantle values: Becker et al. (2006).
Ultramafic samples from the Isua supracrustal belt have similar major and trace element geochemistry to the Pilbara ultramafic samples (see below; Figs. 4–9 ). Three (AW17724-2C, AW17724-4, and AW17725-4) Isua ultramafic samples from meta-peridotite lenses show similar compositions to three Pilbara ultramafic samples in MgO SiO2, MgO CaO, and MgO Al2O3 spaces (Fig. 5 ). Three Isua ultramafic samples collected from the Isua supracrustal belt outside of the lenses either have extraordinarily low MgO (AW17725-2B), high CaO (AW17724-1), or high Al2O3 (AW17725-2B and AW17806-1). Both Isua and Pilbara ultramafic samples show similar normalized trace element abundances (i.e., ~0.1 10 times PM). In PM-normalized diagrams, the Pilbara ultramafic samples show fractionated La-Sm trends [with (La/Eu)PM of ~1.9 2.4], and generally unfractionated heavy REE [with (Dy/Yb)PM of ~0.8 1.2] (Fig. 7a ). Such fractionation trends are consistent with some Isua ultramafic samples [note that all Isua samples have (La/Sm)PM of ~1.5 3.8 and (Dy/Yb)PM of ~0.3 1.2; Fig. 7a ]. The Th concentrations and Gd/Yb ratio are also similar (Isua versus Pilbara ultramafic rocks: ~0.04 0.13 versus ~0.10 0.19 ppm; ~0.5 1.9 versus 1.2 1.7, respectively; Fig. 7b ).
Pilbara ultramafic samples appear to have similar HSE patterns compared to some ultramafic samples from the Isua supracrustal belt [compiled from Szilas et al. (2015); Fig. 8a ]. All three Pilbara ultramafic samples have high PM-normalized concentrations of Os, Ir, and Ru (I-PGE) relative to Pt and positive Ru anomalies (note that Pd and Re could be mobilized during alterations, see section 5.1), highlighted by (Pt/Ir)PM of ~0.3 0.6 and (Ru/Ir)PM of ~2.0 3.5. Compiled ultramafic rocks of the Isua supracrustal belt (Szilas et al., 2015), including those from the dunite lenses, have much broader ranges of (Pt/Ir)PM (~0.5 26.1) and (Ru/Ir)PM (~0.6 18.2) values which largely encompass these of Pilbara ultramafic samples. In contrast, “group 1” peridotites from ultramafic enclaves in the meta-tonalite south of the Isua supracrustal belt (Fig. 8a ; Van de Löcht et al., 2018) have unfractionated to slightly fractionated Os-Ir-Ru elements [with (Ru/Ir)PM of ~0.6 2.0] and relatively low Pt and Pd versus I-PGE [with (Pt/Ir)PM of ~0.2 0.5].
Spinel crystals from the Pilbara ultramafic samples show similar chemistry to those of new and compiled ultramafic samples from the Isua supracrustal belt. Our Pilbara ultramafic samples preserve both chromite and magnetite. The chromite crystals in these samples show relatively constant Cr# (~60 80), highly variable TiO2 (~0.5 5.0 wt.%), and variable Mg# (~20 50). Only magnetite was found in our Isua ultramafic samples from the dunite lenses, which shows low TiO2 (<0.5 wt.%), high Cr# (>90), and low Mg# (<20) (Fig. 9 ). Compiled ultramafic samples from the dunite lenses of the Isua supracrustal belt contain both chromite and magnetite (Szilas et al., 2015). Most of the compiled chromite from these samples shows similar Mg# and Cr# values to the chromite from the Pilbara samples. Other chromite yields Mg# and Cr# trends towards the magnetite composition (Fig. 9 ). The compiled chromite also shows variable TiO2 (~0.2 2.4 wt.%).