Figure 10: Primitive-mantle normalized REE (McDonough and Sun,
1995) and HSE (Becker et al., 2006) patterns of Isua (panel a) and
Pilbara (panel b) ultramafic rocks (this study; Szilas et al., 2015;
Waterton et al., 2022), compared to modelled and compiled mineral and/or
whole-rock compositions that may explain the REE–HSE systematics of
Isua and Pilbara ultramafic rocks. We use binary mixing models between a
relatively REE, Pt and Pd depleted component and a relatively REE, Pt
and Pd enriched component trying to reproduce the trace element
characteristics of Isua and Pilbara ultramafic rocks. For REEs, we mix
modelled cumulate olivines (Waterton et al., 2022) with potentially
co-genetic melts, as olivines in Isua dunites, which show relatively
enriched LREEs and Eu-anomalies (Friend and Nutman, 2011), may not
retain their igneous trace element abundances (Waterton et al., 2022).
For HSEs, due to the lack of HSE data for modelled cumulate olivines
(Waterton et al., 2022), we mix the most HSE depleted ultramafic sample
in Isua or Pilbara with potentially co-genetic melts. Because the HSE
data for Pilbara andesites are not available, we use the HSE composition
of andesite studied by Day et al.
(2013). Mixing models show that Isua and Pilbara ultramafic rocks can be
best explained by mixing between cumulates and co-genetic HSE depleted
melts. Such HSE depleted melts can be formed by sulphide removal or
fractional crystallization in magma chambers, or deep mantle melting.
Additional data sources: average REE values of Isua tholeiitic basalts
are taken from the least altered samples measured by Polat and Hofmann
(2003); HSE compositions of Isua tholeiitic basalts are taken from a
single sample measured by Szilas et al. (2015); average Pilbara
tholeiitic basalts and andesites are from the ~3.35 Ga
Euro Basalt Formation and/or the ~3.43 Ga Panorama
Formation of the East Pilbara Terrane (Smithies et al. 2007);
continental flood basalts after sulphide removal are from
Lightfoot et al. (2005).
Isua ultramafic rocks may have mixed with coeval tholeiitic basalts with
generally unfractionated REE patterns (e.g., Van de Löcht et al. 2020;
Waterton et al. 2022) but also low Pt and Pd contents (potentially ≤1
ppb). Although basaltic melts typically have primitive mantle-like or
higher Pt and Pd due to their preferential partitioning to melts during
mantle melting (e.g., Bockrath et
al. 2004; Fiorentini et al. 2010;
Brenan et al., 2016), because HSEs
are extremely siderophile (Lesher
et al. 2001; Fiorentini et al. 2010), sulphide removal during
crystallization (e.g., Lightfoot et al. 2005) and retaining sulphide as
residues in the mantle sources (e.g., Fiorentini et al. 2011) are two
mechanisms that can lower the Pt and Pd abundances of the co-genetic
melts. In Isua, some meta-basalt layers were found to contain sulphide
minerals which crystallized coevally with the basalts
(Appel, 1979). Therefore, at least
some Isua basalts may have undergone sulphide exhaustion and
consequently have sufficiently low HSE concentrations (see Fig.
10a for low HSE contents of basalts after sulphide removal; Lightfoot
et al. 2005). Furthermore, because S is less soluble in melts under
higher pressures (O’Neill and
Mavrogenes, 2002), mantle partial melts with low HSE abundances indicate
deep mantle sources (e.g., low HSE komattites studied by Fiorentini et
al., 2011). Therefore, the potential presence of low Pt and Pd Isua
tholeiitic basalts may be explained by deep mantle melting, which is not
compatible with a plate tectonic mantle wedge origin (e.g., Figure 8 of
Nutman et al., 2013a), but would be consistent with a recent geochemical
model for Isua basalts that imply a heat-pipe origin
(Rollinson, 2021).
Alternatively, Isua ultramafic rocks may have interacted with other
co-exisitng melt / rock types instead of the tholeiitic basalts. TTGs
can be excluded because these crust partial melting products (e.g.,
Nagel et al. 2012) would have
radiogenic 187Os/188Os signatures
(e.g., Gannoun et al. 2016), which is inconsistent with the overall
unradiogenic 187Os/188Os values of
Isua meta-peridotite lens samples (Waterton et al. 2022). Instead,
mixing with evolved melts formed by fractional crystallization of
basaltic magmas can explain the geochemistry of Isua and Pilbara
ultramafic rocks (Fig. 10b ). For example, in the East Pilbara
Terrane, minor andesites can be found in thick (ultra)mafic to felsic
volcanic successions (Smithies et al. 2007), which may be co-genetic
with lava fractional crystallization. These andesites
(SiO2 ≈ 54 to 61 wt.%) have flat to mildly fractionated
REE patterns (Fig. 10b ). Simple binary mixing models show that
10% to 15% mixing between cumulate olivine (Waterton et al. 2022) and
a component with Pilbara andesite REE compositions can successfully
reproduce REE systematics of Pilbara ultramafic rocks, especially their
mildly fractionated LREE and negative Eu anomalies (Fig. 10b )
which cannot be accounted only by mixing with potentially coeval Pilbara
tholeiitic basalts (Fig. 10b ; cf.
Smithies et al. 2009 showing that
some ~3.5 Ga Pilbara basalts yield enriched and
fractionated LREE patterns). Although no HSE data are available for
these andesites, limited data obtained so far for melts with similar
andesitic composition (Day et al. 2013) indicate generally low HSE
contents in such melt type (e.g., Pt and Pd <3.1 ppb). Thus,
we envision that REE/HSE abundances of Pilbara ultramafic rocks can be
best explained by mixing with Pilbara andesites (Fig. 10b ).
Although such evolved components are not currently observed in the Isua
supracrustal belt, they are speculated to exist (Szilas et al. 2015) and
together with the observation that they may have been volumetrically
minor (as in Pilbara), they might be unexposed or remain undetected due
to ~90% thinning by regional deformation (e.g., Webb et
al., 2020). Indeed, several Isua ultramafic samples have enriched,
fractionated REEs and negative Eu anomalies (Figs. 7, 10a ),
indicating the presence of evolved components that may have compositions
similar to Pilbara andesites.
To conclude, in contrast to Waterton et al. (2022) who only interpret
that Isua ultramafic rocks formed by cumulates mixing with unspecified
co-genetic tholeiitic basaltic magmas, we specify that such melts must
be enriched in REEs but depleted in HSEs (especially Pt and Pd).
Possible candidates are (1) tholeiitic basalts that derived from deep
mantle and/or experienced sulphide removal in magma chambers; and (2)
evolved melts derived from fractional crystallization of basaltic
magmas. Because Isua and Pilbara ultramafic rocks show similar REE and
HSE systematics (Figs. 7, 8 ), mixing REE-enriched but
HSE-depleted melts with cumulus olivine ± chromite ± orthopyroxene can
explain the formation of both Isua and Pilbara ultramafic rocks, along
with potentially other Eoarchean ultramafic cumulates which show similar
REE and HSE geochemistry (Coggon et al. 2015; McIntyre et al. 2019).