Figure 10 : Schreinemarkers diagrams showing the relevant talc +
magnesite forming reaction and the stability field of these phases in
temperature (T) versus X(CO2) space at two different
pressures. Calculations were performed with Perple_X (Connolly, 2005)
(version 6.9.0) and an updated version of the internally consistent
Holland and Powell (2011) dataset (ds63 update). The stability fields of
Ti-humite phases cannot be calculated due to a lack of relevant
thermodynamic data. Nonetheless, co-existing phases of Ti-humite (i.e.,
magnesite, forsteritic olivine and talc) in Isua sample AW17724-2C are
consistent with metamorphism under crustal conditions and 500–650 °C in
the presence of CO2. This is potentially consistent with
the regional amphibolite metamorphism affecting the Isua supracrustal
belt (Ramírez-Salazar et al., 2021). Abbreviations: en: enstatite; fo:
forsterite; mag: magnesite; qz: quartz; per: periclase; tlc: talc.
Whole-rock and trace element characteristics of ultramafic rocks may
help to discern crustal igneous rocks from tectonic mantle slices. This
is because crustal igneous rocks should have geochemical signatures
corresponding to mantle melts, their crystal precipitates assimilation
effects and/or post-cumulus modifications, whereas mantle slices should
show geochemical evidence reflecting melt-depletion and re-enrichment
under mantle conditions. To facilitate petrogenetic interpretation for
rocks with potentially complex alteration histories (see section 5.1),
we compare Isua and Pilbara ultramafic rocks with (1) modelled
melt-refertilized abyssal peridotites generated by combining batch
melting models (calculated using pMELTS; Ghiorso et al., 2002) and
mixing models (Chin et al., 2014); (2) variably altered (including melt
infiltrated and fluid infiltrated) arc peridotites; (3) variably altered
abyssal peridotites; (4) cumulates (which largely experienced
serpentinization) from Archean terranes and Phanerzoic settings; (5)
modelled cumulates of hydrous and anhydrous Phanerzoic settings
(calculated using alphaMELTS; Ghiorso and Sack, 1995; Smith and Asimow,
2005) as presented in Chin et al. (2018); and (6) komatiites (seeFigs 5–9 captions for references from which the literature
data were compiled ).
We find that both Isua and Pilbara ultramafic rocks show similar
whole-rock major element geochemistry to the compiled mantle peridotites
(e.g., melt-refertilized forearc peridotites from mantle wedges and
abyssal peridotites from mid-ocean ridges; Fig. 5 ) and the most
Mg-rich cumulates that represent <10% fractional
crystallization products of basaltic melts (Fig. 6 ). Although
the Isua and Pilbara ultramafic rocks generally have systematically less
CaO compared with mantle peridotites, possibly due to low to zero
abundances of clinopyroxene (Fig. 5 ), this can also be
explained by alteration effects (see section 5.1). In fact, small
clinopyroxene inclusions occur in olivine grains of some Isua ultramafic
rocks from lens A, which have been explained as indicative of olivine
participation coupled with clinopyroxene dissolution during reactions
between mantle peridotites and ascending melts (Nutman et al., 2021).
However, we note that clinopyroxene undersaturation and olivine
saturation is possible across a range of
pressure-temperature-composition combinations (Chen and Zhang, 2009 and
references therein) and could happen under crustal conditions during
magma crystallization in the presence of water, crustal assimilation
and/or magma recharge (e.g., Kelemen, 1990; Gordeychik et al., 2018).
Because Al2O3 is unlikely to be
significantly mobilized during fluid-assisted alterations (see section
5.1), the negatively correlated MgO and
Al2O3 found in these Isua and Pilbara
ultramafic samples should reflect primary igneous features or
melt-assisted interactions (note that MgO concentrations for most
samples may not be significantly altered, Fig. 4b ). We
therefore interpret that the observed major element geochemical
systematics reflect either (1) depleted mantle peridotites that were
variably altered by percolating melts (e.g., Friend and Nutman, 2011;
Nutman et al., 2021; Van de Löcht et al., 2020) or (2) Mg-rich cumulates
that were variably contaminated by co-existing, more evolved melts
(e.g., Szilas et al., 2015).
Compared to Isua and Pilbara ultramafic samples, most depleted mantle
rocks from plate tectonic settings generally show much stronger
depletions in many trace elements versus primitive mantle values
(Fig. 7 ), as highlighted by their (La/Sm)PM,
(Gd/Yb)PM, and Th values, possibly because of strong
melt depletion as expected for tectonically-emplaced mantle residues.
Elevation of trace element concentrations is expected during some
alterations (see section 5.1), but fluid-assisted alterations (including
serpentinization and talc carbonate alteration) alone cannot produce the
observed trace element geochemistry of Isua and Pilbara ultramafic
samples (Fig. 8a–b ). Instead, some mantle rocks (modified by
melt-rock interactions), and also cumulate rocks, have comparable trace
element geochemistry (Fig. 7 ). Indeed, similarities between
trace element patterns of Isua
ultramafic rocks and those of nearby basalts (Friend and Nutman, 2011;
Szilas et al., 2015; Van de Löcht et al., 2020) were explained by (1)
reactions between mantle resitites and melts (Friend and Nutman, 2011;
Nutman et al., 2021; Van de Löcht et al., 2020) or (2) reactions between
melt components (e.g., evolving basaltic melts) and cumulus minerals
(Szilas et al., 2015). Therefore, the observed generally flat,
primitive-mantle-like trace element characteristics found in new and
compiled Isua and Pilbara samples are consistent with both depleted
mantle residue or cumulate origins.
Relatively high primitive mantle-normalized Os, Ir, and Ru versus Pt, Pd
and Re [e.g., (Pt/Ir)PM < 1] is often used
to discriminate depleted mantle rocks because Pt, Pd and Re behave as
moderately incompatible elements during mantle melting (e.g., Bockrath
et al., 2004; Wang et al., 2013). However, we note that during mantle
melting, I-PGEs typically show similar compatibility [such that
(Ru/Ir)PM ≈ 1], whereas in the studied Pilbara
samples, significant fractionation among I-PGEs [i.e., >2
(Ru/Ir)PM values] are observed (Fig. 8 ).
Although mantle rocks which suffered extensive melt-rock interactions
could also have strongly fractionated HSEs (e.g., Büchl et al., 2002;
Ackerman et al., 2009), their overall HSE trends [including
(Pt/Ir)PM and (Ru/Ir)PM values] are
dissimilar to those of Pilbara samples (Fig. 8b ). In contrast,
some chromite-bearing peridotite cumulates from layered intrusions show
similar HSE fractionation trends versus Pilbara samples (Fig.
8c ). Therefore, HSE patterns of the Pilbara samples are consistent with
the proposed cumulate origin of these rocks depicted from their rock
textures (see above, Fig. 3 ) and spinel geochemistry (see
below, Fig. 9 ). The elevated Pd and/or Re as well as
unrealistic Re-Os systematics in some Pilbara samples may result from an
addition of Fe-sulphides during later alteration events (see section 5;
Lorand and Luguet, 2016). These Fe-sulphides then would have been
altered to magnetite (which commonly occurs in alteration veins or
triple junction points of serpentine clusters of Pilbara samples,Fig. 3b ) over billions of years. In comparison, ultramafic
rocks from the Isua supracrustal belt (including dunites from the
lenses; Fig. 8a ; Szilas et al., 2015) and meta-tonalite
enclaves south of the Isua supracrustal belt (Fig. 8a , Van de
Löcht et al., 2018) show a range of HSE patterns, highlighted by their
<0.5 to >10 (Ru/Ir)PM and
(Pt/Ir)PM values. Significantly elevated
(Ru/Ir)PM and (Pt/Ir)PM values may be
explained by interactions with melts; such interactions could happen to
both cumulates and mantle peridotites (e.g., Ackerman et al., 2009;
Gannoun et al., 2016; Szilas et al., 2015). Although some Isua
ultramafic rocks have (Ru/Ir)PM close to 1 and
(Pt/Ir)PM less than 1 [e.g., “group 1” peridotites
sampled from meta-tonalite enclaves (Van de Löcht et al., 2018) and a
portion of ultramafic rocks studied by Szilas et al. (2015)], which
are similar to depleted abyssal mantle rocks (Wang et al., 2013), such
patterns also occur in cumulates (Fig. 8c ) (McIntyre et al.,
2019; Szilas et al., 2014; 2018). McIntyre et al. (2019) argue that
these specific HSE patterns may be alternatively explained by
preferential partitioning of I-PGEs into cumulus phases such as olivine
and chromite. Therefore, HSEs may not be as discriminative as previously
thought in terms of recognizing depleted mantle rocks. Accordingly, HSE
patterns of Pilbara and Isua ultramafic rocks, including those from
meta-tonalite enclaves south of the Isua supracrustal belt, can be
explained by cumulate origins (± modifications potentially by melts).
Some spinel crystals with ~100 Cr# and
~0 Mg# values in the new and compiled Isua ultramafic
rocks reflect metamorphic modifications of primary chromite into
magnetite (Fig. 3b ; Barnes and Roeder, 2001). However, igneous
petrogenesis can be interpreted from primary chromite grains of both
Isua and Pilbara ultramafic samples. New and compiled spinel data
[from Szilas et al. (2015) which were obtained from rocks of lenses A
and B] of these rocks match the Fe– Ti trend in the
Mg#– Cr# space (Fig. 9b ). Such a trend can be
produced by equilibration of spinel phases during fractional
crystallization (Barnes and Roeder, 2001), and thus can be found in
cumulates (Fig. 9b ). In contrast, due to equilibration with
olivine, mantle spinel typically has high Mg# and varied Cr# (i.e.,
the Cr– Al trend in Fig. 9b , Barnes and Roeder, 2001)
as well as low TiO2 (Fig. 9a ) (e.g., Tamura and
Arai, 2006). Although fluid/melt assisted alterations could impact
spinel geochemistry in mantle rocks, expected changes include Cr#
reduction and Mg# increase along the Cr– Al trend (El Dien et
al., 2019), which are not consistent with the observed spinel
geochemistry. Therefore, we conclude that some chromite spinel crystals
from Isua (Szilas et al., 2015) and Pilbara ultramafic rocks are not
similar to spinel hosted in mantle rocks, but rather indicate cumulate
origins (cf. Nutman et al., 2021).
Olivine oxygen isotopes of some dunites from the Isua supracrustal rocks
are interpreted to be mantle-like and indicative of fluid metasomatism
(likely from recycling hydrated crust) in the mantle wedge (Nutman et
al., 2021). This material exchange between surface and mantle is thought
to be exclusive to plate tectonic subduction settings (Nutman et al.,
2020; Nutman et al., 2021). However, such material exchange is also
possible for hot stagnant-lid settings, accomplished by recycling of the
buried or dripping hydrated crustal materials (see tectonic models
introduced in section 2; also, Moore and Webb, 2013; Smithies et al.,
2007). Indeed, mantle-like oxygen isotopes are observed in zircons from
some tonalites (originally lower crust partial melts) of the East
Pilbara Terrane (where its geometry and structures largely suggest
non-plate tectonic origins, see section 2.2; Smithies et al., 2021).
This finding implies a fluid-rich early mantle, buffered by fluxing from
the recycled crust, that was capable of introducing mantle-like oxygen
isotope signatures to early crust and magmas (Smithies et al., 2021).
Therefore, mantle-like oxygen isotopic signatures found in some dunites
from the Isua supracrustal belt need not be explained by plate tectonic
subduction (cf. Nutman et al., 2021).
In summary, although several features of Isua or Pilbara ultramafic
samples are commonly associated with depleted mantle rocks (e.g., the
B-type olivine fabrics and Ti-humite preserved in Isua ultramafic
samples), these features are not inconsistent with cumulate origins. In
addition, the cumulate textures of Pilbara ultramafic samples and the
spinel geochemical characteristics of both Isua and Pilbara ultramafic
samples are inconsistent with tectonically-emplaced depleted mantle, but
instead are compatible with cumulate origins (Figs. 4–9 ). As
such, both Isua and Pilbara ultramafic samples can be interpreted as
crustal cumulates. Because crustal cumulates are produced by fractional
crystallization of melts, these rocks are consistent with both plate
tectonics and hot stagnant-lid tectonics. Thus, plate tectonics is not
required to explain the petrogenesis of Isua ultramafic rocks (cf.
Nutman et al., 2020; Nutman et al., 2021).