Search for petrographic and geochemical evidence for the late heavy bombardment on earth in early archean rocks from Isua, Greenland

Abstract

The Moon was subjected to intense post-accretionary bombardment between about 4.5 and 3.9 billion years ago, and there is evidence for a short and intense late heavy bombardment period, around 3.85 ± 0.05 Ga. If a late heavy bombardment occurred on the Moon, the Earth must have been subjected to an impact flux at least as intense. The consequences for the Earth must have been devastating. In an attempt to investigate if any record of such a late heavy bombardment period on the Earth has been preserved, we performed a petrographic and geochemical study of some of the oldest rocks on Earth, from Isua in Greenland. We attempted to identify any remnant evidence of shock metamorphism in these rocks by petrographic studies, and used geochemical methods to detect the possible presence of an extraterrestrial component in these rocks. For the shock metamorphic study, we studied zircon, a highly refractive mineral that is resistant to alteration and metamorphism. Zircon crystals from old and eroded impact structures were found earlier to contain a range of shock-induced features at the optical and electron microscope level. Many of the studied zircon grains from Isua are strongly fractured, and single planar fractures do occur, but never as part of sets; none of the crystals studied shows any evidence of optically visible shock deformation. Several samples of Isua rocks were analyzed for their chemical composition, including the platinum group element (PGE) abundances, by neutron activation analysis and ICP-MS. Three samples showed somewhat elevated Ir contents (up to 0.2 ppb) compared to the detection limit, which is similar to the present-day crustal background content (≤0.03 ppb), but the chondrite-normalized siderophile element abundance patterns are non-chondritic, which could be a sign of either a small extraterrestrial component (if an indigenous component is subtracted), or terrestrial (re)mobilization mechanisms. In absence of any evidence for shock metamorphism, and with ambiguous geochemical signals, no unequivocal conclusions regarding the presence of extraterrestrial matter (as a result of possible late heavy bombardment) in these Isua rocks can be reached.