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 ).