Figure 7. Trace element characteristics for Isua and Pilbara
ultramafic samples in comparison with compiled cumulates and variably
altered mantle peridotites. a, Primitive mantle 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, Primitive mantle-normalized spider diagrams showing
trace element patterns of investigated samples and compiled rocks (seeFigure S4 for spider diagrams grouped by sample locations).
These diagrams show that new and compiled data for ultramafic rocks from
the Isua supracrustal belt have similar trace element characteristics to
meta-peridotite enclaves from the south Isua meta-tonalites, Pilbara
ultramafic samples and other Eoarchean 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 al., 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 southwestern Greenland
(Szilas et al., 2014), the
Mesoarchean Seqi Ultramafic Complex of southwestern Greenland
(Szilas et al., 2018), and the
Eoarchean Tussapp Ultramafic Complex of southwestern 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. Partial melting process and subsequent melt-rock interactions
can strongly disturb the geochemistry and mineral assemblages of
affected rocks. However, the Isua supracrustal belt and the
supracrustal rocks in the East Pilbara Terrane (Fig. 1 ) have
only experienced amphibolite facies metamorphism (or lower conditions)
(e.g., Hickman, 2021; Ramírez-Salazar et al., 2021; Mueller et al.,
pre-print). Both Isua and Pilbara samples show evidence of
hydrothermal alterations, as indicated by the dominance of serpentine
minerals (Figs. 2–3 ). Therefore, modifications of whole-rock
geochemical budgets need to be taken into account (see below). In
addition, at mineral scale, 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 also taken when interpreting
petrogenesis using spinel data.
Fluid assisted alterations could result in changes in mineral
assemblages and whole-rock/mineral element concentrations including
REEs, but the impacts on fluid-mobile elements (e.g., K, Ca, Si, Rb,
Ba and Sr, etc) would be most significant (e.g., Deschamps et al.,
2013; Malvoisin et al., 2015; Paulick et al., 2006). Moderate to high
LOI contents (~5–21 wt.%; 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, carbonitization,
and/or talc-alteration. A ternary plot of anhydrous
SiO2, LOI, and other major element oxides (e.g., MgO,
Al2O3, Fig. 4a ) shows that
serpentinization is the dominant controlling factor of major element
geochemistry as these samples plot near the serpentine mineral
composition. Nonetheless, the potential MgO and SiO2loss/gain due to serpentinization may be smaller than 5 wt.% for all
Isua and Pilbara samples except for two Isua samples AW17725-2B and
AW17806-1 (Fig. 4b ). These two samples show strongly
disturbed MgO and SiO2 as well as significantly
enriched Al2O3, which cannot be
accounted by serpentinization but may be related to melt-rock
interactions (see below). Effects of other alterations on major
element concentrations and LOI (e.g., Deschamps et al., 2013; Paulick
et al., 2006) in most samples appear to be secondary with the
exception of sample AW17724-1, which has a high anhydrous CaO
concentration (10.4 wt.%). Elevation of CaO in Isua ultramafic rocks
has been interpreted as recording calcite addition during
carbonitization (Waterton et al. 2022). Although some trace elements
like LREEs and Th can be affected by fluid assisted alterations, it is
hard to evaluate such effects for Isua and Pilbara samples because
trace element systematics can also be strongly affected by primary
melt origin and evolution processes such as partial melting, melt
fractionation etc. (see below and Fig. 7 ; e.g., Paulick et
al. 2006). Some HSEs like Os, Ir, Ru and Pt are relatively immobile
during fluid assisted alterations, but Pd and Re could be relatively
mobile (e.g., Barnes and Liu,
2011; Büchl et al., 2002;
Deschamps et al., 2013; Gannoun
et al. 2016). 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 melt/rock 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, REEs, 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).
In summary, fluid/melt rock interaction might in part control the
observed geochemistry and petrology of studied Isua and Pilbara
samples. Thus, 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, carbonitization, talc/tremolite alteration, and
melt-rock interaction).