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 mantle rocks
tectonically exhumed 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 serpentinized Pilbara
ultramafic samples have relict cumulate textures, weakly fractionated to
unfractionated trace element patterns that are close to primitive mantle
values, relative enrichment of whole-rock Os, Ir, and Ru versus Pt, 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 mantle
rocks and cumulates may have similar rock textures and whole-rock
geochemistry, especially when their geochemistry is controlled by
similar alteration processes (e.g., serpentinization, melt-assisted
alterations). Therefore, depleted mantle interpretations are not
required to explain the data obtained from Isua and Pilbara ultramafic
rocks. Instead, cumulate textures of Pilbara samples and spinel
geochemistry of Isua and Pilbara ultramafic rocks imply cumulate
origins, which are consistent with their geochemistry indicating
interactions between cumulus olivine and co-genetic melts that formed in
hot stagnant-lid settings. Collectively, these findings permit ≤ 3.2 Ga
initiation of plate tectonics on Earth.