Earth’s earliest phaneritic ultramafic rocks 1: plate tectonic mantle
slices or crustal cumulates?
Abstract
How and when plate tectonics initiated remain uncertain. In part, this
is because many signals that have been interpreted as diagnostic of
plate tectonics can be alternatively explained via hot stagnant-lid
tectonics. One such signal involves early Archean phaneritic ultramafic
rocks. In the Eoarchean Isua supracrustal belt of southwestern
Greenland, some ultramafic rocks have been interpreted as
tectonically-exhumed mantle during Eoarchean subduction. To explore
whether all Archean phaneritic ultramafic rocks originated as cumulate
and/or komatiite – i.e., without requiring plate tectonics – we
examined the petrology and geochemistry of such rocks in the Isua
supracrustal belt and the Paleoarchean East Pilbara Terrane of
northwestern Australia, with Pilbara ultramafic rocks interpreted as
representative of rocks from non-plate tectonic settings. We found that
Pilbara ultramafic samples have relict cumulate textures, relative
enrichment of whole-rock Os, Ir, and Ru versus Pt and Pd, and spinel
with variable TiO2, relatively consistent Cr#, and variable and low
Mg#. Similar geochemical characteristics also occur in variably altered
Isua ultramafic rocks. We show that Isua and Pilbara ultramafic rocks
should have interacted with low Pt and Pd melts generated by
sequestration of Pd and Pt into sulphide and/or alloy during magma
generation or crystallization. Such melts cannot have interacted with a
mantle wedge. Furthermore, altered mantle rocks and altered cumulates
could have similar rock textures and whole-rock geochemistry such that
they may not distinguish mantle from cumulate. Our findings suggest that
depleted mantle interpretations are not consistent with geochemistry
and/or rock textures obtained from Isua and Pilbara ultramafic rocks.
Instead, cumulate textures of Pilbara samples, whole-rock Pt and Pd
concentrations, and spinel geochemistry of Isua and Pilbara ultramafic
rocks support cumulate origins and metasomatism involving co-genetic
melts that formed in hot stagnant-lid settings. Collectively, these
findings permit ≤ 3.2 Ga initiation of plate tectonics on Earth.