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 primitive mantle-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 Ujaragssuit 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 Isua ultramafic samples (AW17724-2C, AW17724-4, and AW17725-4) 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 primitive mantle-normalized diagrams, the Pilbara ultramafic samples show fractionated LREE trends [with (La/Sm)PM of ~1.9 2.4], and generally unfractionated heavy REE [with (Gd/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 ~1.1 3.8 and (Gd/Yb)PM of ~0.3 1.6; Fig. 7a ]. The Th concentrations and Gd/Yb ratio also have significant overlaps (Isua versus Pilbara ultramafic rocks: ~0.04 1.13 versus ~0.10 0.19 ppm; ~0.4 2.1 versus 1.2 1.7, respectively; Fig. 7b ).
Pilbara ultramafic samples appear to have similar HSE patterns compared to the Isua meta-peridotite lens samples [compiled from Waterton et al. (2022); Fig. 8a ], highlighted by their overlapping (Pt/Ir)PM (~0.3 0.6 Pilbara vs ~0.2 0.9 Isua meta-peridotite lenses) and (Ru/Ir)PM (~2.0 3.5 Pilbara vs ~0.5 10.0 Isua meta-peridotite lenses) ratios. Compiled ultramafic rocks from other parts of the Isua supracrustal belt (Szilas et al., 2015) have much broader ranges of (Pt/Ir)PM (~0.5 26.1) and (Ru/Ir)PM (~0.6 18.2) values, which largely encompass the Pilbara ultramafic rocks but extended to much higher (Pt/Ir) PM. “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)PMof ~0.2 0.5], which are similar to some Isua lenses A and B samples featuring lower Ru enrichment than Pilbara ultramafic samples.
Spinel (chromite and magnetite) from the Pilbara ultramafic samples show similar chemistry to those of new and compiled ultramafic samples from the Isua supracrustal belt. Chromite yields relatively constant Cr# (~60 80), but variable Mg# (~20 50), and highly variable TiO2 (~0.5 5.0 wt.%). Only magnetite was found in our Isua ultramafic samples from the meta-peridotite lenses, which shows low TiO2(<0.5 wt.%), high Cr# (>90), and low Mg# (<20) (Fig. 9 ). Compiled ultramafic samples from the meta-peridotite 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.%).