5.3. Are plate tectonic mantle slices necessary for explaining
Isua ultramafic rocks?
In this section, we expand our comparison between Isua and Pilbara
ultramafic rocks to other ultramafic rocks and comparing our findings
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. Although the polygonal textures of Isua ultramafic rocks (e.g.,
Nutman et al., 1996) and the B-type olivine fabrics (Kaczmarek et al.,
2016) have been interpreted to reflect mantle environments, these rock
textures are also consistent with cumulate origins. First, the polygonal
textures need not reflect equilibration under mantle conditions (cf.
Nutman et al., 1996) as these fabrics occur in Pilbara ultramafic
samples and other olivine-rich cumulates as potential products of
recrystallization (e.g., Hunter, 1996). Moreover, a CPO pattern of
B-type olivine fabrics does not need to be produced by deformation via
dislocation creep in hydrated mantle wedge environments (cf. Kaczmarek
et al., 2016). Formation of B-type fabrics in primary or secondary
olivine grains is possible under crustal conditions, where olivine
deformation may be accomplished by a range of dislocation slip systems
or other growth and/or deformation mechanisms (e.g., dissolution creep)
(Chin et al., 2020; Holtzman et al., 2003; Liu et al., 2018; Nagaya et
al., 2014a, 2014b; Wheeler et al., 2001; Yao et al., 2019). In
particular, a B-type CPO pattern can be produced in igneous olivine
grains via the formation of a shape-preferred orientation of olivine
crystals with the presence of melts and a stress field (e.g., during the
compaction of a cumulate mush) (e.g., Yao et al., 2019; Chin et al.,
2020; Holtzman et al., 2003). Such a CPO pattern can also be found in
secondary olivine grains overgrowing the strongly oriented serpentine
matrix, where olivine growth is associated with prograde metamorphism
not necessarily under mantle conditions (Nagaya et al., 2014a; 2014b;
cf. Nozaka, 2014). Therefore, with current rock and mineral textural
data from Isua ultramafic rocks, mantle wedge conditions are not
required, and cumulate origins are viable.
Igneous and metamorphic conditions reflected by mineral assemblages of
Isua ultramafic rocks are important for constraining their origins.
Primary mineral assemblages of the Isua ultramafic rocks (i.e., olivine
+ spinel ± pyroxene) are consistent with both mantle and cumulate
origins. Recently, Nutman et al. (2020) interpreted the metamorphic
assemblages, including occurrences of Ti-humite phases to reflect
low-temperature (<500 ◦C), UHP
(>2.6 GPa) metamorphism, which can only occur in sub-arc
mantle environments (Friend and Nutman, 2011; Nutman et al., 2020).
However, there are several features in the Isua ultramafic rocks that
suggest that Ti-humite could be stable at substantially lower pressures,
possibly as shallow as crustal conditions consistent with regional
amphibolite facies metamorphism. First, the experiment of Shen et al.
(2015) have been performed with a mineral assemblage and bulk rock
composition that is significantly different from those of the Isua
rocks. Most importantly, these experiments did not include the effects
of CO2 or halogens. Although Ti-humite in Isua
ultramafic samples have low halogen concentrations (e.g., Table S2;
Guotana et al., 2021), the effect of carbonate phases cannot be ignored.
In Isua sample AW17724-2C, magnesite is commonly found together with
olivine and Ti-humite phases. Magnesite may have been in equilibration
with olivine, Ti-humite phases and talc instead of representing a later
alteration mineral (e.g., Fig. 2a ; cf. Guotana et al., 2021).
Therefore, the carbonate-free experimental results from Shen et al.
(2015) are not directly applicable to Isua ultramafic rocks (cf. Nutman
et al., 2020). We note that the presence of carbonates or high
XCO2 conditions could significantly lower the pressure
required for the formation of Ti-humite phases. For example,
both Ti-clinohumite and Ti-chondrodite have been reported in marbles
that experienced contact metamorphism at amphibolite facies conditions,
where carbonates have been interpreted to play an essential role in
reactions forming Ti-humite phases (e.g., Ehlers and Hoinkes, 1987).
The observed mineral assemblage (i.e., olivine + serpentine ± Ti-humite
± magnesite±talc) can be used to constrain the metamorphic conditions
even though no reliable thermodynamic data for Ti-humite phases are
available. Ignoring Ca, Al, Ti and F, i.e., in a simplified
MgO–SiO2–H2O–CO2system, the observed reaction of forsterite + CO2 =
magnesite + talc as well as the antigorite forming reaction is limited
to a temperature range of 500–650 °C at 1 GPa and 2 GPa, respectively
(Fig. 9 ). However, we note that decreasing pressure increases
the range of fluid composition (XCO2) in which the
reaction and thus the observed mineral assemblage can occur. Moreover,
the temperature range is in strong agreement with the crustal-level
metamorphic conditions determined for the supracrustal rocks of the belt
(Ramirez-Salazar et al., 2021). Based on these findings, the possible
formation pressures of Ti-humite phases in Isua ultramafic rocks could
be far lower than previously interpreted (Nutman et al., 2020),
potentially matching amphibolite facies conditions (e.g., Ehlers and
Hoinkes, 1987) that are recorded across the whole Eoarchean Isua
supracrustal belt (Ramírez-Salazar et al., 2021; Rollinson, 2002;
Gauthiez-Putallaz et al., 2020). Interestingly, both Ti-chondrodite and
Ti-clinohumite (which are also associated with magnesite + olivine +
serpentine) were found in one other sample by Dymek et al. (1988a)
collected from an outcrop within the Isua supracrustal belt located
~5 km south of the two meta-peridotite ultramafic lenses
(Fig. 1 ) (Nutman and Friend, 2009). Based on the
chondrite-normalized REE pattern of this sample and geochemical and
petrological evidence from seven other samples from this outcrop (Dymek
et al., 1988b), Dymek et al. (1988a) concluded that this outcrop does
not represent a mantle slice. Hence, this Ti-humite bearing sample from
Dymek et al. (1988a) can, in turn, potentially be evidence of crustal
origins of Ti-humite phases in ultramafic rocks of the Isua supracrustal
belt.