LARGE-SCALE IMPACTS AND THE EVOLUTION OF THE EARTH'S CRUST: THE EARLY YEARS

Abstract

Modeling the effects of differential scaling of impact melt and transient cavity volumes indicates that impact melt volumes exceed transient cavity volumes at transient cavity diameters greater than ̃500 km on the Earth. This condition is not realized on the Moon until transient cavity diameters are greater than ̃3000 km. Areasoned case is made that because of this "differential scaling," the large impact "basins" comparable in size to such lunar basins as Orientale, which must have been formed on the Hadean Earth, did not have an Orientale-like form. While their exact form is unknown, they were likely shallow structures, and they would have been characterized by voluminous central impact pools. These melt pools with their closed-system environment would likely differentiate, leading to the crystallization of more felsic rocks. This reprocessing of the Hadean crust by large-scale impacts provides a mechanism to produce pre-3.9 Ga zircons, without calling for plate-tectonic-related or other mechanisms of crustal recycling to produce felsic rocks. While the impact melting at a single one of such Hadean impact "basins" would be impressive, the cumulative effects would be potentially staggering, particularly if impact velocities during Hadean time were, as believed, lower than current velocities. Based on the number of large multi-ring basins on the Moon scaled to terrestrial conditions, cumulative melt production on the Hadean Earth by such basins alone would reach ̃10 11-1012 km3. With the assumption that the impact melt pools were basaltic in composition, modeling with the crystal-melt fractionation software MELTS suggests the cumulative volume of felsic rocks potentially produced through the evolution of such impact melt pools could be significant. © 2006 Geological Society of America.