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, polygonal 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 and deformational overprints differ.
Ultramafic rocks from the Isua supracrustal belt show broadly similar
HSE characteristics to those found in the Pilbara ultramafic rocks,
although peridotites from enclaves in the meta-tonalite body south of
the Isua supracrustal belt exhibit lower (Pt/Ir)PMvalues. The rocks have broadly similar geochemical characteristics in
other whole-rock major and trace elements and spinel geochemistry. On
the other hand, with our serpentinized Pilbara sample set, possibly
similar olivine microstructures or oxygen isotopic systematics cannot be
evaluated in detail.