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.