Plain Language Summary:
Earth’s rigid outer shell is broken into pieces that move relative to
each other. These motions are generally understood according to the
theory of plate tectonics. However, the origins of plate tectonics are
not well understood. This contribution focuses on an aspect of this
problem, namely, the lack of consensus concerning when plate tectonics
started. We examine some of the most ancient evidence which has been
speculated to record plate tectonic processes: ultramafic rocks from the
≥3.7 billion-years-old Isua supracrustal belt of southwestern Greenland.
A leading hypothesis suggests that these are mantle (deep) rocks
emplaced by plate tectonic deformation. We test the viability of an
alternative hypothesis: that these rocks may have crystallized from
magmas at crustal (shallow) levels, a history that would not require
plate tectonics. Specifically, we compare new and published mineral and
chemical features of the Isua ultramafic rocks with similar rocks from
known crustal and mantle settings, including new data from a
northwestern Australia crustal site which is similar, yet non-plate
tectonic. Results show that each feature of the Isua ultramafic rocks is
consistent with crustal crystallization. Therefore, these rocks do not
constrain early plate tectonics, which could have developed later.