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 PM-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 Uharagssuit 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 (AW17724-2C, AW17724-4, and
AW17725-4) Isua ultramafic samples 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 PM-normalized diagrams, the Pilbara ultramafic samples show
fractionated La-Sm trends [with (La/Eu)PM of
~1.9– 2.4], and generally unfractionated
heavy REE [with (Dy/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
~1.5– 3.8 and (Dy/Yb)PM of
~0.3– 1.2; Fig. 7a ]. The Th
concentrations and Gd/Yb ratio are also similar (Isua versus Pilbara
ultramafic rocks: ~0.04– 0.13 versus
~0.10– 0.19 ppm;
~0.5– 1.9 versus 1.2– 1.7,
respectively; Fig. 7b ).
Pilbara ultramafic samples appear to have similar HSE patterns compared
to some ultramafic samples from the Isua supracrustal belt [compiled
from Szilas et al. (2015); Fig. 8a ]. All three Pilbara
ultramafic samples have high PM-normalized concentrations of Os, Ir, and
Ru (I-PGE) relative to Pt and positive Ru anomalies (note that Pd and Re
could be mobilized during alterations, see section 5.1), highlighted by
(Pt/Ir)PM of ~0.3– 0.6 and
(Ru/Ir)PM of ~2.0– 3.5.
Compiled ultramafic rocks of the Isua supracrustal belt (Szilas et al.,
2015), including those from the dunite lenses, have much broader ranges
of (Pt/Ir)PM (~0.5– 26.1) and
(Ru/Ir)PM (~0.6– 18.2) values
which largely encompass these of Pilbara ultramafic samples. In
contrast, “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)PM of
~0.2– 0.5].
Spinel crystals from the Pilbara ultramafic samples show similar
chemistry to those of new and compiled ultramafic samples from the Isua
supracrustal belt. Our Pilbara ultramafic samples preserve both chromite
and magnetite. The chromite crystals in these samples show relatively
constant Cr# (~60– 80), highly variable
TiO2 (~0.5– 5.0 wt.%), and
variable Mg# (~20– 50). Only magnetite was
found in our Isua ultramafic samples from the dunite lenses, which shows
low TiO2 (<0.5 wt.%), high Cr#
(>90), and low Mg# (<20) (Fig. 9 ).
Compiled ultramafic samples from the dunite 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.%).