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). Panel b 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 those in the modelled primitive mantle. Pilbara
samples show fractionated light to medium rare earth elements, with
(La/Sm)PM ranging from 1.9 to 2.4 (Fig. 7 ).
These samples also have weak negative Nb anomalies, and generally flat
heavy rare earth element trends [with (Dy/Yb)PM of
0.8– 1.1]. The Th
concentrations and Dy/Yb ratios of Pilbara ultramafic samples range from
0.10 to 0.19 ppm and 1.2– 1.7, respectively (Fig. 7b ).
The PM-normalized (Becker et al., 2006) HSE patterns of the Pilbara
samples exhibit similar fractionated patterns characterized by
Os– Ir depletion over Ru [(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-day187Os/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 unrealistic low initial187Os/188Os values
(<0.078) calculated at ~3.4 Ga.