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 ).