Figure 7. Trace element characteristics for Isua and Pilbara
ultramafic samples in comparison with compiled cumulates and variably
altered mantle peridotites. a, Primitive mantle (PM) normalized
Gd/Yb and La/Sm ratios [i.e., (Gd/Yb)PM and
(La/Sm)PM] of investigated samples and compiled rocks.b, Th and Gd/Yb ratios of investigated samples and compiled
rocks. c, PM-normalized spider diagrams showing trace element
patterns of investigated samples and compiled rocks (see Figure
S4 for spider diagrams with sample locations). These diagrams show that
new and compiled data for ultramafic rocks from the Isua supracrustal
belt have similar trace element characteristics to dunite enclaves from
the south Isua meta-tonalites, Pilbara ultramafic samples and ultramafic
cumulates. Only some abyssal peridotites which experienced
serpentinization and melt-rock interactions have comparable trace
element patterns. Other mantle peridotites have lower Th, Gd/Yb,
(Gd/Yb)PM, and/or (La/Sm)PM values.
Data sources: compiled cumulates
involve samples from the Permian Lubei intrusion of NW China (Chen et
a., 2018), the late Proterozoic Ntaka Ultramafic Complex of Tanzania
(Barnes et al., 2016), the Mesoarchean Nuasahi Massif of India (Khatun
et al., 2014), the Mesoarchean Tartoq Group of SW Greenland (Szilas et
al., 2014), the Mesoarchean Seqi Ultramafic Complex of SW Greenland
(Szilas et al., 2018), and the Eoarchean Tussapp Ultramafic Complex of
SW Greenland (McIntyre et al., 2019); compiled Eoarchean ultramafic
samples are rocks from the Isua supracrustal belt (Szilas et al., 2015)
and the enclaves in meta-tonalite south of the Isua supracrustal belt
(Van de Löcht et al., 2020); fresh arc peridotites are from the
Kamchatka arc (Ionov, 2010); arc peridotites that experienced
serpentnization, talc/tremolite alteration, and/or melt-rock
interactions are from the Loma Caribe peridotite body of Dominican
Republic (Marchesi et al., 2016) and the Izu-Bonin-Mariana forearc
(Parkinson and Pearce, 1998); abyssal peridotites that experienced
serpentinization are from the Oman ophiolite (Hanghøj et al., 2010);
variably altered abyssal peridotites from the Mid-Atlantic Ridge are
summarized by Paulick et al. (2006). Primitive mantle values are from
McDonough and Sun (1995).
Assessment of alteration impactsPetrological and geochemical information obtained from Isua and
Pilbara ultramafic rocks represents the combined effects of
petrogenetic processes and alterations. Below we discuss potential
types and impacts of alteration on the petrology and geochemistry of
these rocks.
High-grade (e.g., granulite facies) metamorphism can lead to partial
melting. The partial melting process and subsequent melt-rock
interactions could strongly disturb the geochemistry and mineral
assemblages of affected rocks. However, the Isua supracrustal belt and
the area from which Pilbara ultramafic samples were collected
(Fig. 1 ) have only experienced amphibolite facies
metamorphism (or lower conditions) (e.g., Hickman, 2021;
Ramírez-Salazar et al., 2021). In general, for an anhydrous system
larger than a hand specimen, amphibolite facies or lower metamorphism
is usually considered isochemical. However, both Isua and Pilbara
samples show evidence of hydrothermal alterations, as indicated by the
dominance of serpentine minerals (Figs. 2–3 ). Therefore,
some whole-rock geochemical changes are possible (see below). In
addition, at mineral scales, some chemical changes during metamorphism
are possible. For example, Cr-spinel could be altered to magnetite
during metamorphism (e.g., Barnes and Roeder, 2001). Therefore, care
must be taken when interpreting petrogenesis using spinel data.
Fluid assisted alterations could result in changes in mineral
assemblages and element concentrations, especially for fluid-mobile
elements (e.g., K, Ca, Si, Rb, Ba and Sr, etc.). LOI contents
(Fig. 4a ), and the presence of serpentine, talc, and/or
magnesite (Figs. 2–3 ) in Isua and Pilbara ultramafic samples
show that these rocks have experienced variable degrees of
serpentinization and carbonitization (including talc -alteration;
although some magnesite crystals could be primary minerals
crystallized from fluid-rich magmas, see Fig. 2a for
magnesite crystals as olivine inclusions; also Smithies et al., 2021).
A ternary plot of anhydrous SiO2, LOI, and other
oxides (e.g., MgO, TiO2, see Fig. 4 caption)
shows that serpentinization is the dominant controlling factor for
their geochemistry as these samples plot near the serpentine mineral
composition. Effects of other alterations on major element
concentrations and LOI (e.g., Deschamps et al., 2013; Paulick et al.,
2006) in most samples seem to be secondary with the exception of
sample AW17724-1 which has a high anhydrous CaO concentration (10.4
wt.%). Hydrothermal fluids may mobilize some elements such as Mg, Si,
and trace elements including REEs (e.g., Deschamps et al., 2013;
Malvoisin et al., 2015; Paulick et al., 2006). Nonetheless, the
potential MgO and SiO2 loss/gain could be
insignificant (i.e., <10%) compared to other factors (e.g.,
melt depletion, melt-rock interaction, or talc-alteration, seeFig. 4b ; Snow and Dick, 1995). Some HSEs like Os, Ir, Ru and
Pt are largely immobile during fluid assisted alterations, but Pd and
Re could be mobile (e.g., Büchl et al., 2002; Deschamps et al., 2013).
Spinel Al and Cr concentrations can be increased or reduced during
fluid-rock interaction, respectively (e.g., El Dien et al., 2019).
Melt-rock interaction is commonly observed in mantle rocks (e.g.,
Ackerman et al., 2009; Büchl et al., 2002; Deschamps et al., 2013;
Niu, 2004; Paulick et al., 2006) where ascending melts react with wall
rocks. This process is similar to reactions between cumulate phases
and trapped/evolving melts during crystallization or post-cumulus
processes (e.g., Borghini and Rampone, 2007; Goodrich et al., 2001;
Wager and Brown, 1967). In general, melt-rock interaction can alter
the geochemistry of affected rocks towards those of melts at
increasing rock/melt ratios (e.g., Kelemen et al., 1992; Paulick et
al., 2006). For peridotites interacting with basalts or more evolved
melts, the elevation of elements that are relatively enriched in melts
(e.g., Si, Ca, Th, Al, Fe, Ti, LREE, Pt, Pd, and Re) is significant
(Figs. 4–7 ; e.g., Deschamps et al., 2013; Hanghøj et al.,
2010). Other effects include changes in mineral modes and/or mineral
geochemistries (e.g., olivine Mg# reduction; spinel Cr-loss and
Al-gain) (e.g., El Dien et al., 2019; Niu and Hekinian, 1997). Two
Isua supracrustal belt ultramafic samples (AW17725-2B, AW17806-1)
which were sampled near the meta-tonalite bodies have the highest
Al2O3 and lowest MgO concentrations
among all samples (Figs. 4b, 5 ), which may be explained by
reactions with relatively Al2O3 rich
components (fluids and/or melts).
In summary, fluid/melt rock interaction might partly control the
observed geochemistry and petrology of studied Isua and Pilbara
samples. Therefore, for petrogenetic interpretation, we compare the
observed geochemistry and petrology of Isua and Pilbara ultramafic
samples with those of cumulates and mantle peridotites that
potentially experienced similar alterations (including
serpentinization, talc/tremolite alteration, and melt-rock
interaction).