Very large bolide impacts:insight from melt products and element behaviour in the crater fill

Abstract

Large (>100 km) impact events have shaped the surfaces of terrestrial bodies in the inner Solar System. They have fundamentally altered the Earth?s geochemistry and may even have contributed to making the Earth habitable. Early collision, during accretion, resulted in proto-planets evolving from a bulk chondritic composition to the modern non-chondritic compositions of planets. The considerable post-accretion influx of large to giant impactors on Earth, culminating in the late heavy bombardment (LHB), destroyed almost all of the proto-crust through vaporisation, mixing and resurfacing. With a sufficiently hot geothermal gradient, the largest impact events (> 200 km craters) also likely generated magmatism through decompression melting, which could have led to the emplacement of large igneous provinces (LIPs). This thesis presents in situ geochemical investigations of impactites from one impact ejecta layer, the 1.1 Ga Stac Fada Member (Scotland), and two impact craters, the 1.85 Ga Sudbury basin (Canada) and the 65 Ma Chicxulub structure (Mexico). Analysis of juvenile green vitric products from the Stac Fada Member has succeeded in isolating the chondritic siderophile element cargo. Detailed Pb isotope examination of the Sudbury impact basin melt sheet and lower basin infill identified considerable (>95%) Pb depletion. Additionally, magma-seawater interaction is shown to have resulted in a more pronounced, volatile metal loss. In situ rare earth element investigations reveal a deep lithosphere origin of melt fragments found in both Sudbury and Chicxulub impactites. New Nd isotope data on selected vitric components indicate that basalt from decompression melting assimilated Palaeo- or Mesoarchean lower crust en-route to the surface. This work provides empirical evidence for several new insights: i) Extra-terrestrial siderophile elements from meteorite impacts were transferred to magnesian glass fragments in impactites. Weathering broke down the material and aided siderophile element migration into the mantle; ii) Collisional erosion has resulted in significant loss of moderately volatile metals. The moon-forming impact and the influx of Large (>100 km) impacts during the LHB contributed to the depletion of approximately 90% of the Earth?s original Pb budget; iii) New evidence of post-impact magmatism has been discovered in large, Sudbury-sized impact structures; and iv) Lithospheric decompression melting associated with vertical movement of rocks during and immediately after impacts may have led to the emplacement of LIP into impact basins.