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).