We know little about the Hadean Earth but most agree that the entire inner Solar System was pummeled by impacting bolides. None of the terrestrial Hadean impact basins are preserved but by analogy with our neighbours, the Earth must have featured basins of up to thousands of kilometers across. Is there any hope of inferring processes that would have operated from studying the few much younger remaining large impact structures on Earth?
Of these, the Chicxulub basin is the best preserved but covered by a km of carbonate. Geophysical data define size and architecture of the basin but in terms of access to rocks, the situation is disappointing with only very few drill cores available. In the context of the Hadean, a further issue is that the asteroid struck a carbonate platform, unlikely to have been a major rock type in the Hadean. In this presentation, I will instead present recent work on the 1.85 Ga Sudbury basin. Only a remnant of the originally >150 km basin is preserved but fortuitously, this it is folded into a synform giving complete access across the entire stratigraphy. The base of the meltsheet is also mineralised with Ni-Cu sulphides and the drill core archive eclipses what is available for all other impact structures taken together.
The impact structure comprises the shock-metamorphosed basement, the crystallised melt sheet and the lithologically complex crater fill. The average composition of the ca. 2.5 km thick meltsheet is a quartz-diorite, probably too evolved to serve as a direct analogue for the Hadean. However, the key interest is the unexpectedly thick (1.8 km) crater fill. The lower part of this is composed of very complex breccias and tuffs that formed when seawater flowed onto the super-heated meltsheet. The thicker upper part contains volcanic rocks which imply sustained magmatism . I will postulate that the insulation of the meltsheet with a breccia roof was key to drive the internal differentiation. Thus, if the Hadean Earth sported a liquid hydrosphere, differentiated meltsheets could be the source of Hadean zircons .
The crater fill breccias and tuffs define interesting chemostratigraphic trends . The high field strength element evolution clearly indicates that the crater rim remained intact during the deposition of the entire formation. Several volatile metals (e.g., Pb, Sb, Zn) are depleted by > 95% in the lowermost fill, suggesting that the impact resulted in a net loss of volatile species, supporting the idea of “impact erosion”. I will discuss new Zn-isotope data (collaboration with Prof. R. Schoenberg), supporting volatile element loss from the impact plasma plume.
In the upper crater fill, reduced C contents reach 0.5-1 wt%, where δ13C becomes constant at -31‰, indicating a biogenic origin. Elevated Y/Ho and U/Th require that the ash interacted with seawater. Redox-sensitive trace metal chemostratigraphies (e.g. V and Mo) suggest that the crater basin was anoxic and possibly euxinic and became inhabited by plankton, whose rain-down led to a reservoir effect in certain elements. Importantly, hydrothermal systems were active in the crater, producing volcanogenic massive sulphides. These hydrothermal systems evidently did not require mid-ocean ridges and implicitly, the operation of plate tectonics. Thus, on the early Earth, seawater-filled impact ring basins were probably nutrient-rich “ponds” in which chemical experiments could proceed in isolation from the wider ocean.