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
Fe–Ti trend of spinel (representing equilibration during
fractional crystallization) and the Cr–Al 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.