Ni-rich spinels and platinum group element nuggets condensed from a Late Archaean impact vapour cloud

摘要

Deciphering Earth's impact history before ~2 Ga relies heavily on the lunar record and terrestrial spherule layers, which are distal ejecta from large impacts. This study focuses on the Paraburdoo and Reivilo spherule layers in Western Australia and South Africa respectively, that were probably formed by one impact around 2.57 Ga. Both layers contain an aggregate thickness of ~2 cm of spherules, known as microkrystites. These spherules are up to ~0.6 mm in diameter and crystallized during flight, but were diagenetically replaced by K-feldspar and phlogopite with remarkable textural retention. Unlike any other Archaean layer, except for the 3.2 Ga S3 layer in the Barberton greenstone belt, the Paraburdoo and Reivilo spherules contain Ni-rich spinel crystals and high concentrations of meteoritic material (up to 357 ng g~(-1) Ir for bulk samples of several gram). These exceptional characteristics shed new light on the distribution of the meteoritic component carrier phases (metallic alloys dispersed in the pristine glass) and the processes involved in impact spherule formation and secondary alteration.