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.%).