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 being representative of rocks from
non-plate tectonic settings. We found that Pilbara ultramafic samples
have cumulate textures and relative enrichment of whole-rock Os, Ir, and
Ru versus Pt. In comparison, polygonal textures and variable whole-rock
Os, Ir, Ru and Pt patterns are identified in Isua ultramafic samples.
Isua and Pilbara ultramafic samples have (1) mineral assemblages that
can form at crustal conditions; (2) broadly similar whole-rock major
element patterns; (3) weakly fractionated to unfractionated trace
element patterns that are close to primitive mantle values; and (4)
spinel with variable TiO2, relatively consistent Cr#,
and variable and low Mg#. Many features of Isua or Pilbara ultramafic
rocks are similar to depleted mantle rocks, except for spinel chemistry
and cumulate textures. However, all features are consistent with
cumulates. Collectively, these data permit ≤ 3.2 Ga initiation of plate
tectonics on Earth.