Geochemical signatures and magmatic stability of terrestrial impact produced zircon

Abstract

Understanding the role of impacts on early Earth has major implications to near surface conditions, but the apparent lack of preserved terrestrial craters >2Ga does not allow a direct sampling of such events. Ion microprobe U–Pb ages, REE abundances and Ti-in-zircon thermometry for impact produced zircon are reported here. These results from terrestrial impactites, ranging in age from ~35Ma to ~2Ga, are compared with the detrital Hadean zircon population from Western Australia. Such comparisons may provide the only terrestrial constraints on the role of impacts during the Hadean and early Archean, a time predicted to have a high bolide flux. Ti-in-zircon thermometry indicates an average of 773°C for impact-produced zircon, ~100°C higher than the average for Hadean zircon crystals. The agreement between whole-rock based zircon saturation temperatures for impactites and Ti-in-zircon thermometry (at aTiO2=1) implies that Ti-in-zircon thermometry record actual crystallization temperatures for impact melts. Zircon saturation modeling of Archean crustal rock compositions undergoing thermal excursions associated with the Late Heavy Bombardment predicts equally high zircon crystallization temperatures. The lack of such thermal signatures in the Hadean zircon record implies that impacts were not a dominant mechanism of producing the preserved Hadean detrital zircon record.