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 ). The trace element abundances of Isua
ultramafic samples are mostly 0.1– 10 times to those inferred
for the primitive mantle
(McDonough and Sun, 1995; same
below; Fig. 7 ). These samples show an unfractionated to mildly
fractionated trend from light to medium rare earth elements (LREE to
MREE) with
(La/Sm)PM values
ranging from ~1.1– 3.8 (Fig. 7a, 7c ).
The heavy rare earth elements (HREE) of Isua ultramafic samples indicate
a flat trend or variably fractionated trends with
(Gd/Yb)PM of ~0.3– 1.6
(Fig. 7a ). The Th concentrations and Gd/Yb ratios (proxies for
alterations; Fig. 7b ; Deschamps et al., 2013) of Isua
ultramafic samples range from 0.04– 1.13 ppm and
0.4– 2.1, respectively. Some Isua ultramafic samples show
negative Nb anomalies. Positive or negative Eu anomalies can both be
identified in Isua ultramafic samples (Fig. 7c ).