Figure 4. Major element and loss-on-ignition (LOI) geochemical
characteristics of Pilbara and Isua ultramafic samples in comparison
with common primary and alteration minerals in ultramafic rocks. All
major element concentrations are anhydrous values (i.e., normalized to
zero LOI and 100 wt.% total). Panel a shows a ternary plot of
SiO2, LOI, and SumOxides (MgO +
Al2O3 + CaO + Na2O +
K2O + TiO2 +
Fe2O3T + MnO +
P2O5) (modified from
Deschamps et al., 2013). Panelb shows
MgO/SiO2–Al2O3/SiO2space with Pilbara and Isua ultramafic samples, common primary and
alteration minerals in ultramafic rocks, the terrestrial array of mantle
peridotites (fitted by abyssal peridotites, AP), and MgO-loss or
SiO2-gain alteration curves. The data in this figure
show that the major element systematics of our Isua and Pilbara samples
reflect various degrees of serpentinization without strong talc
alteration, consistent with thin-section petrography (Figs.
2–3). Two samples (AW17725-2B, AW17806-1) which were collected from
outcrops near the meta-tonalite have significantly elevated
Al2O3. These high Al concentrations
cannot be attributed to serpentinization and talc alteration. Panel b is
modified from Malvoisin et al.
(2015) which itself is a modified version of
Jagoutz (1979). PM: primitive
mantle; DM: depleted mantle. All mantle values are from McDonough and
Sun (1995).
Pilbara ultramafic rocks have
whole-rock SiO2 of ~43– 46
wt.%, MgO of ~41– 45 wt.%, CaO of
0.02– 0.12 wt.%, Al2O3 of
~1.6– 2.5 wt.%, FeOt of
~6.1– 12.8 wt.%, Mg# of 85– 93, and
LOI of 12.3– 12.9 wt.% (Figs. 4–6, Table
S1 ). The trace element abundances in these samples are also
0.1– 10 times to those of primitive mantle. Pilbara samples
show fractionated LREE to HREE, with (La/Sm)PM ranging
from 1.9 to 2.4 (Fig. 7 ). These samples have weak negative Nb
anomalies, negative Eu anomalies, and generally flat HREE trends [with
(Gd/Yb)PM of 0.8– 1.1].
The Th concentrations and Gd/Yb
ratios of Pilbara ultramafic samples range from 0.10 to 0.19 ppm and
1.2– 1.7, respectively (Fig. 7b ). The primitive
mantle-normalized (Becker et al.,
2006) HSE patterns of the Pilbara samples exhibit similar fractionated
patterns characterized by strong Ru enrichment over Os– Ir
[(Ru/Ir)PM = 2.0– 3.5] and Pt depletion
over Os-Ir [(Pt/Ir)PM =0.3– 0.6], whereas
Pd and Re contents are highly variable (Fig. 8 ). One Pilbara
sample (AW52514-4A) shows significantly higher Pd abundance (close to
the primitive mantle value) than the rest of the samples. The
present-day 187Os/188Os values range
between ~0.1094 and 0.1166. Rhenium contents are high in
two samples (0.13 ppb in AL52614 and 0.35 ppb in AW52514), resulting in
superchondritic 187Re/188Os (0.53
and 0.86, respectively) and together with consequently unrealistic low
initial 187Os/188Os values
(<0.078) calculated at ~3.4 Ga.