5.3. Do Isua ultramafic rocks require formation in mantle?
In this section, we compare our findings of Isua and Pilbara ultramafic
rocks with those of similarly altered compiled and modelled cumulates
and mantle peridotites to establish whether any feature of Isua
ultramafic rocks needs to be explained uniquely via plate
tectonic-related mantle slices. First, many geochemical parameters
commonly used to study modern mantle rocks (e.g., Mg/Si-Al/Si plots)
cannot be used to differentiate Eoarchean olivine-rich cumulates from
depleted mantle residues (see also Waterton et al., 2022). This is
because olivine-rich cumulates, which can be modelled via
<10% fractional crystallization of basaltic melts
(Fig. 6 ; Mallik et al. 2020), are similar to variably depleted
and altered mantle rocks in terms of many whole-rock major element
systematics (e.g., MgO, SiO2,
Al2O3, FeOt; Figs. 5, 6 ).
Although the Isua and Pilbara ultramafic rocks, and other Eoarchean
ultramafic cumulates generally have systematically lower CaO contents in
comparison to fertile mantle peridotites due to absence or very low
clinopyroxene abundance (Fig. 5 ), these characteristics can be
also related to alteration effects (see section 5.1). In fact, small
clinopyroxene inclusions occur in olivine grains of some Isua ultramafic
rocks from lens A. This has been explained to indicate olivine growths
coupled with clinopyroxene dissolution during reactions between mantle
peridotites and ascending melts (Nutman et al., 2021a). However,
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).
Similarly, primitive mantle-normalized trace element patterns cannot
distinguish mantle rocks from ultramafic cumulates. Isua and Pilbara
ultramafic rocks show flat or mildly fractionated trace element
abundances 0.1 to 10 times of primitive mantle values with
~1.1 to ~3.8 (La/Sm)PMand ~0.3 to ~1.7
(Gd/Yb)PM (Fig. 7 ). In contrast, depleted
mantle rocks that did not undergone fluid and melt metasomatism have
highly depleted trace element patterns (e.g., LREE lower than 0.01 times
PM) and systematically low (Gd/Yb)PM (generally
<0.3; Fig. 7A ). The level of trace element enrichment
in Isua and Pilbara ultramafic rocks, especially as shown by Th-Gd/Yb
systematics (Fig. 7B ), can only be explained by cumulates or
depleted mantle experienced melt-rock interactions. Previous studies
(Friend and Nutman, 2011; Van de Löcht et al. 2020; Waterton et al.
2022) show that Isua ultramafic rocks could have interacted with
basaltic melts, although the melts cannot be represented by the Isua
high Al2O3/TiO2 basalts
(boninitic) due to their largely radiogenic Os isotopic signatures in
comparison to the Isua ultramafic rocks (Waterton et al. 2022). As noted
above, melt-rock interaction is common in both crustal magma chambers
and upper mantle; therefore, the existence of such processes neither
discriminate petrogenetic origins nor tectonic models (cf. Friend and
Nutman, 2011; Van de Löcht et al. 2020).
The Nb depletion relative to Th and La, which exists in both Isua
ultramafic rocks and Isua basalts (Fig. 7 ; Polat and Hofmann,
2003), has been extensively cited as indicating arc-like signatures for
these rocks. In this Eoarchean subduction interpretation, Nb (and Ta)
depletion would indicate fractionation effects of fluids and rutile
associated with eclogitized slabs (e.g.,
Münker, 1998; Keppler, 1996).
However, this signature is not unique to volcanic arcs, particular with
respect to the early Earth. For example, basalts, felsic volcanics,
TTGs, and ultramafic rocks (Fig. 7C) of the East Pilbara
Terrane also have strong Nb depletion (e.g., Martin et al. 2005;
Smithies et al. 2007), but this terrane is widely thought to be
plume-generated in a non-plate tectonic setting (e.g., Van Kranendonk et
al. 2007). Furthermore, rutile as well as fluids form readily via
metamorphic dehydration in the lower parts of the thickened lithosphere
of non-plate tectonic settings via metamorphism (Johnson et al., 2017),
particularly in a heat-pipe lithosphere featuring cold geotherm (a la
Moore and Webb, 2013). Recycling of such lower crust materials (which is
possible in plate and non-plate tectonic settings via mechanisms like
delamination, sagduction and/or downwards advection) and subsequent
fluid fluxing and melting could generate igneous rocks with Nb-Ta
depletion. Alternatively, Nb depletion may be a secondary signature
formed via fluid metasomatism under amphibolite facies conditions
(Guice et al. 2018). Vigorous
fluid activities and material exchanges between mantle and crust in
non-plate tectonic settings can also explain the mantle-like oxygen
isotopes found in some Isua olivines (Nutman et al. 2021a). Indeed,
mantle-like oxygen isotopes are observed in zircons from some TTGs
(originally lower crust partial melts) of the East Pilbara Terrane
(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).
In terms of HSE patterns, we agree with Waterton et al. (2022) and
Szilas et al. (2015) that HSE signatures of Isua ultramafic are
consistent with cumulate origins. This argument is further strengthened
by the HSE patterns of our cumulate-textured Pilbara ultramafic rocks,
which are similar to those of Isua lenses A and B samples in terms of
positive Ru anomalies relative to Ir and Pt and relative depletion of Pt
and Pd versus Ir (Fig. 8 ; Waterton et al. 2022). Similar
patterns can be found in other Eoarchean cumulates dominated by olivine
and chromite (e.g., Coggon et al., 2015; McIntyre et al., 2019).
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 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 ). Chromite
crystals of Isua and Pilbara samples also have variable
TiO2 (up to ~2 and ~5
wt.%, respectively) 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 (typically <1 wt.%;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 with 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 (new data; Fig. 9 ) are not similar to spinel
hosted in mantle rocks, but rather indicate cumulate origins (cf. Nutman
et al., 2021a).
Although the B-type olivine fabrics (Kaczmarek et al., 2016) have been
interpreted to reflect mantle environments, they are also consistent
with cumulate origins. Waterton et al. (2022) pointed out that B-type
fabrics in Isua dunites may be formed via magmatic or metamorphic
processes (e.g., Chin et al.,
2020; Holtzman et al., 2003;
Nagaya et al., 2014;
Yao et al., 2019) rather than via
deformation in mantle wedge (cf. Kaczmarek et al. 2016). We found
additional evidence that may support this interpretation: olivine in
Isua lens B samples are considered to be dehydration products of
antigorite-breakdown (e.g., Guotana et al. 2022). Alignment of olivine
shape long-axes in lens B (see Fig. 1D of Nutman et al. 2021a) generally
parallel to the regional lineation directions (mostly trending
southeast; Zuo et al. 2021). If olivine long-axes correspond to their
[001] crystal directions as suggested by Kaczmerak et al. (2016),
then olivine [001] is generally parallel with lineation directions,
which are also antigorite (010) directions in deformed serpentinites
(e.g., Nagaya et al. 2017). Such
crystal axis relationships are consistent with a metamorphic origin of
olivine B-type fabrics, in which topotactic growth of olivine occurred
with olivine [001] axes parallel to antigorite (010) (Nagaya et al.
2014). Therefore, with current rock and mineral textural data from Isua
ultramafic rocks, mantle wedge conditions are not required, and cumulate
origins are viable.
Finally, the presence of Ti-humite in Isua ultramafic rocks have been
interpreted to reflect low-temperature, UHP (i.e., <500 °C,
>2.6 GPa) metamorphism (Friend and Nutman, 2011; Nutman et
al., 2020; Guotana et al., 2022) primarily using the petrogenetic grid
generated from experiments (i.e.,
Shen et al. 2015). However, our
complementary work (Mueller et al., pre-print) shows that the results of
Shen et al. (2015) cannot be directly applied to Isua ultramafic rocks.
This is because Shen et al. (2015) experimented on a
CO2-free chemical system, but Isua ultramafic rocks
preserve carbonate phases (Fig. 2a ) that appear to be a
reaction product of an olivine-breakdown reaction, equally producing
antigorite and Ti-humite (Mueller et al., pre-print). Conversely,
Mueller et al. (pre-print) show that Ti-humite could have been formed
under much lower pressures, such as the amphibolite facies conditions
recorded by the other parts of the belt (Ramírez-Salazar et al. 2021).
Therefore, the Isua supracrustal belt may not have experienced (U)HP
metamorphism, obviating the need for plate tectonic subduction (Waterton
et al., 2022; cf. Friend and Nutman, 2011; Nutman et al., 2020; Guotana
et al., 2022).
In summary, although several features of Isua or Pilbara ultramafic
samples are commonly associated with plate tectonic processes (e.g., the
B-type olivine fabrics, mantle-like oxygen isotopes, Nb depletion, and
Ti-humite preserved in Isua ultramafic samples), these features are not
inconsistent with rock formation and metamorphism under crustal
conditions. 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 that experienced alterations
under crustal conditions. 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 and
Pilbara ultramafic rocks (cf. Nutman et al., 2020, 2021a).