Figure 4. Major element and loss-on-ignition (LOI) geochemical characteristics of Pilbara and Isua ultramafic samples in comparison with common primary and alteration minerals in ultramafic rocks. All major element concentrations are anhydrous values (i.e., normalized to zero LOI and 100 wt.% total). Panel a shows a ternary plot of SiO2, LOI, and SumOxides (MgO + Al2O3 + CaO + Na2O + K2O + TiO2 + Fe2O3T + MnO + P2O5) (modified from Deschamps et al., 2013). Panel b shows MgO/SiO2–Al2O3/SiO2space with Pilbara and Isua ultramafic samples, common primary and alteration minerals in ultramafic rocks, the terrestrial array of mantle peridotites (fitted by abyssal peridotites, AP), and MgO-loss or SiO2-gain alteration curves. The data in this figure show that the major element systematics of our Isua and Pilbara samples reflect various degrees of serpentinization without strong talc alteration, consistent with thin-section petrography (Figs. 2–3 ). Two samples (AW17725-2B, AW17806-1) which were collected from outcrops near the meta-tonalite have significantly elevated Al2O3. These high Al concentrations cannot be attributed to serpentinization and talc alteration. Panel b is modified from Malvoisin et al. (2015) which itself is a modified version of Jagoutz (1979). PM: primitive mantle; DM: depleted mantle. All mantle values are from McDonough and Sun (1995).
Pilbara ultramafic rocks have whole-rock SiO2 of ~43 46 wt.%, MgO of ~41 45 wt.%, CaO of 0.02 0.12 wt.%, Al2O3 of ~1.6 2.5 wt.%, FeOt of ~6.1 12.8 wt.%, Mg# of 85 93, and LOI of 12.3 12.9 wt.% (Figs. 4–6, Table S1 ). The trace element abundances in these samples are also 0.1 10 times those in the modelled primitive mantle. Pilbara samples show fractionated light to medium rare earth elements, with (La/Sm)PM ranging from 1.9 to 2.4 (Fig. 7 ). These samples also have weak negative Nb anomalies, and generally flat heavy rare earth element trends [with (Dy/Yb)PM of 0.8 1.1]. The Th concentrations and Dy/Yb ratios of Pilbara ultramafic samples range from 0.10 to 0.19 ppm and 1.2 1.7, respectively (Fig. 7b ). The PM-normalized (Becker et al., 2006) HSE patterns of the Pilbara samples exhibit similar fractionated patterns characterized by Os Ir depletion over Ru [(Ru/Ir)PM = 2.0 3.5] and Pt depletion over Os-Ir [(Pt/Ir)PM =0.3 0.6], whereas Pd and Re contents are highly variable (Fig. 8 ). One Pilbara sample (AW52514-4A) shows significantly higher Pd abundance (close to the primitive mantle value) than the rest of the samples. The present-day187Os/188Os values range between ~0.1094 and 0.1166. Rhenium contents are high in two samples (0.13 ppb in AL52614 and 0.35 ppb in AW52514), resulting in superchondritic 187Re/188Os (0.53 and 0.86, respectively) and together with unrealistic low initial187Os/188Os values (<0.078) calculated at ~3.4 Ga.