5.4. Revised cumulate origins to explain the REE-HSE signatures of Isua and Pilbara ultramafic rocks
The primitive mantle-like, generally weakly fractionated REE patterns of Isua ultramafic rocks (Fig. 7 ) have been explained by binary mixing of up to 20% Isua tholeiitic basalts (the high Al2O3/TiO2 series) and cumulus forsteritic (Fo90-92) olivine, in addition to minor orthopyroxene accumulation and/or metamorphic alterations (see Fig. 6 of Waterton et al. 2022). However, because the limited HSE data for Isua tholeiitic basalts show high Pt and Pd abundances (~11 and ~6 ppb, respectively;Fig. 10a ; Szilas et al. 2015), new petrogenesis considerations are necessary for using the mixing model to explain the low Pt and Pd contents (<0.8 ppb) observed in Isua meta-peridotite lens samples (Fig. 10a ; Waterton et al., 2022). Below, we show that Isua ultramafic rocks could have interacted with: (1) basaltic melts experienced sulphide removal in magma chambers; (2) basaltic melts formed by deep mantle melting; and/or (3) more evolved (e.g., andesitic) melts that are co-genetic with the cumulates. These interpretations are also applicable to Pilbara ultramafic rocks because their HSE and REE geochemistry are similar to Isua ultramafic rocks (Figs. 7–8, 10 ).