Figure 6. Major element geochemical characteristics of the Isua
and Pilbara ultramafic samples in comparison with those of Phanerozoic
cumulates, arc peridotites, >3.2 Ga ultramafic rocks (seeFig. 3 for data sources), and modelled liquid lines of descent.
All data are presented using anhydrous values (i.e., all major element
abundances are normalized to zero LOI and 100 wt.% total). The data in
this figure show that Isua and Pilbara ultramafic rocks, Mg-rich
cumulates and mantle peridotites have similar major element geochemical
systematics. Data sources for the cumulates and MELTS modelling curves
are from Chin et al. (2018), Mallik et al. (2020), and references
therein.
Mineral geochemistry
Olivine grains in Isua sample AW17724-2C (lens B) have extraordinarily
high Mg# values of ~95– 98 and NiO of
~0.39–0.63 wt.%. In contrast, olivine grains in Isua
sample AW17725-4 (lens A) have Mg# values of ~87 and
NiO of ~0.52– 0.61 wt% (Table S2 ).
Ti-humite phases in sample AW17724-2C have variable TiO2abundances of ~3.0– 8.1 wt.%. All analyzed
spinel grains in the Isua samples contain a high magnetite component
(i.e., FeOt of ~90 wt.%) (Table S2 ).
Spinel crystals of both chromite or magnetite compositions occur in the
Pilbara samples. Specifically, chromite spinel grains have
Cr2O3 of
~40– 50 wt.%, TiO2 of
0.6– 4.3 wt.%, and MgO of 5– 12 wt.%. The Cr#
[Cr/(Cr+Al)] values and Mg# values of chromite spinel grains are
~65– 75 and ~17– 46,
respectively (Fig. 9 ; Table S2 ).
DiscussionWe analyzed phaneritic ultramafic rocks in the Eoarchean Isua
supracrustal belt and the East Pilbara Terrane to explore their
petrogenesis as a means of testing the viability of existing tectonic
models. Specifically, we explore whether these rocks need to be
explained as mantle peridotites that emplaced in the crust in a
subduction setting. Our new petrological and geochemical data from six
ultramafic samples from the Isua supracrustal belt and three
ultramafic samples from the East Pilbara Terrane show that (1) Isua
and Pilbara samples have been variably altered and now contain several
alteration minerals (e.g., serpentine, talc, carbonate) that replaced
igneous ferromagnesian silicates; (2) Ti-humite phases preserved in
one Isua ultramafic sample (AW17724-2C from lens B) appear to be in
close association with magnesite + olivine + talc + serpentine
(Fig. 2 ); (3) Pilbara ultramafic samples preserve poikilitic
textures and polygonal textures (Fig. 3 ); one Isua sample
(AW17725-4 from lens A) also preserves relict polygonal textures; (4)
trace element abundances in both Isua and Pilbara ultramafic samples
range from the highly depleted, with respect to the PM values (0.1
times PM values), to highly enriched (10 times PM values)
(Fig. 6a–b ); (5) two out of three Pilbara ultramafic samples
show fractionated, relatively high concentrations of Os and Ir versus
Pt, Pd, and Re in the PM-normalized diagram (Fig. 7c ); and
(6) chromite spinel in Pilbara ultramafic samples feature Cr# of
~65– 75, and Mg# of
~17– 46 (Fig. 9 ). In the following
sections, we first discuss the potential impacts of alterations on
petrology and geochemistry. Then, we show that new and compiled
petrology, geochemistry, and microstructures of Isua and Pilbara
ultramafic rocks are consistent with a cumulate origin, whereas an
origin as thrust-emplaced mantle slices is not required. We then
discuss the implications for testing early Earth tectonic models and
the initiation of plate tectonics.