Paleoarchean ocean crust and mantle excavated by meteor impact: Insight into early crustal processes and tectonics

Abstract

Two major features of Earth’s geochemical evolution include (1) the extraction of continental crust from the mantle to produce a depleted upper mantle, and (2) the onset of plate tectonics and shallow melting of the upper mantle to produce oceanic crust (mid-ocean-ridge basalt, MORB). Prior studies have suggested that continental crust extraction and/or plate tectonics began prior to 4 Ga or were not initiated until as late as the Neoproterozoic. Earth’s geological record is very incomplete prior to ca. 3 Ga, and the limited terrains available for study have left the nature of the earliest tectonics and crustal compositions unresolved. Here we show that a meteor impact at 3.24 Ga excavated basaltic crust and depleted mantle material. We use high field strength elements and other immobile elements to model the composition of the target rock and meteor and compare it to analyses of spherules condensed from the vaporized meteor and impact target rock. This provides insight into otherwise unpreserved Archean crust. A simple mixture of 15% CV group carbonaceous chondrite, 65% ocean basalt, and 20% depleted MORB mantle (DMM) fully replicates the field results. A significant proportion of this basalt is MORB like, distinctly different from the tholeiitic rocks that dominate in Archean greenstone belts, and consistent with seafloor spreading. The presence of DMM implies that a significant volume of continental crust was extracted before 3.24 Ga. However, no granite or tonalite-trondhjemite-granodiorite can be incorporated into the model and replicate field results, consistent with the lack of any observed shocked zircons in any of the Paleoarchean impact layers to date.