Whole-rock major and trace element characteristics
Whole-rock major and trace element characteristics are often used to
indicate the petrogenetic conditions, such as the degree of melt
depletion, and the sources of ultramafic rocks (e.g., Niu and Hekinian,
1997; Van de Löcht et al., 2020), although effects of alteration (as
observed in section 4.1) must be considered. Isua ultramafic rocks have
SiO2 of ~38– 49 wt.%, MgO of
~31– 47 wt.%, CaO of
~0.03– 10.49 wt.%,
Al2O3 of
~0.5– 5.0 wt.%, FeOt of
~6.2– 10.7 wt.%, Mg# [i.e., Mg/(Mg+Fe)]
of 84– 93, and loss-on-ignition (LOI) of
~5– 21 wt.% (all major oxide concentrations
are anhydrous values, i.e., normalized to zero LOI and 100 wt.% total;Figs. 4–6, Table S1 ). For trace element results (Fig.
7 ), we excluded those that were not reproduced by measurements on
reference materials (e.g., Nb and Ta for Isua ultramafic samples). The
trace element abundances of Isua ultramafic samples are mostly
0.1– 10 times those in the modelled primitive mantle (McDonough
and Sun, 1995; same below; Fig. 7 ). These samples show a
fractionation trend from light to medium rare earth elements with
(La/Sm)PM values
ranging from ~1.5– 3.8 (i.e., the ratio of
concentrations that have been normalized to those of the primitive
mantle) (Fig. 7a, 7c ). The heavy rare earth elements of Isua
ultramafic samples indicate a flat trend or variably fractionated trends
with (Dy/Yb)PM of ~0.3– 1.2
(Fig. 7a ). The Th concentrations and Dy/Yb ratios (proxies for
alterations; Fig. 7b ; Deschamps et al., 2013) of Isua
ultramafic samples range from 0.04– 0.13 ppm and
0.5– 1.9, respectively.