Figure 9. Geochemical signatures of spinel in Pilbara and Isua samples, plotted with compiled fields for ultramafic cumulates and mantle peridotites. Panel a shows Cr# values [Cr/(Cr+Al)] and TiO2 concentrations of spinel. Panel bshows Mg# [Mg/(Mg+Fe)] and Cr# values of spinel. The FeTi trend of spinel (representing equilibration during fractional crystallization) and the CrAl trend of spinel (representing equilibration in mantle) are plotted for comparison. These plots indicate that spinel from Isua and Pilbara samples are similar to those of cumulates, but are different from those of mantle peridotites. Data sources: compiled spinel from Isua ultramafic rocks: Szilas et al. (2015); the spinel field of cumulates is fit by spinel data from the Uralian-Alaskan type intrusions (Abdallah et al., 2019; Garuti et al., 2003; Himmelberg and Loney, 1995; Krause et al., 2011; Thakurta et al., 2008), the Mesoarchean Seqi Ultramafic Complex of southwestern Greenland (Szilas et al., 2018) and data compiled in Barnes and Roeder (2001); the spinel field of arc peridotites is fit by spinel data in Ionov (2010), Parkinson and Pearce (1998) and Tamura and Arai (2006); the spinel field of abyssal peridotites is fitted by spinel data in Khedr et al. (2014), Standish et al. (2002) and Tamura and Arai (2006). Spinel Fe-Tl, Cr-Al and metamorphic trends are from Barnes and Roeder (2001).
In summary, rock textures found in Isua ultramafic rocks (i.e., polygonal textures) also occur in Pilbara ultramafic samples. Pilbara ultramafic rocks potentially have similar primary mineral assemblages (olivine + spinel ± pyroxene) compared to those of the Isua ultramafic rocks, although their alteration overprints differ. Ultramafic rocks from the Isua supracrustal belt show broadly similar HSE characteristics versus the Pilbara ultramafic rocks, although peridotites from enclaves in the meta-tonalite body south of the Isua supracrustal belt exhibit lower (Pt/Ir)PM values. The rocks have broadly similar geochemical characteristics in other whole-rock major and trace elements and spinel geochemistry. With our sample set, it is not possible to know whether Isua and Pilbara ultramafic rocks share similar olivine microstructures or oxygen isotopic systematics.