Identification of Earth's late accretion by large impactors through mass independent Cr isotopes

The late addition of extra-terrestrial material may represent an important source of Earth's volatiles. The composition of impactors can be reconstructed using Cr-54 abundances in impact related rocks preserved in the terrestrial record. The average epsilon Cr-53 and epsilon Cr-54 of Earth's mantle determined from mantle rocks of 3.8 Ga to present are 0.03 +/- 0.02 and 0.08 +/- 0.04, respectively. Impact melt rocks and spherule beds linked to impact structures larger than 50 km that formed between 3.4 Ga and 66 Ma have epsilon Cr-53 ranging from -0.04 to 0.17, and epsilon Cr-54 ranging from -0.64 to 1.41. A carbonaceous chondrite-like impactor contribution dominated the Meso- to Paleoarchean spherule layers (> 3.0 Ga), whereas a mixed chondrite flux composed of carbonaceous and non-carbonaceous chondrites, with a possible contribution of differentiated meteorites is observed in the younger spherule layers (2.5 Ga to 66 Ma). This likely reflects the break-up of distinct asteroid families through time. Although available impact materials are limited, the Cr isotope signatures of materials related to large impacts suggest a change in the composition of crater-forming impactors on Earth, from predominantly carbonaceous chondrite-like more oxidized material during the Archean to predominantly non-carbonaceous -like more reduced and volatile poor material in recent times. Chromium isotopes suggest that not more than 0.01 wt% of CC-like material added to the Earth's mantle after the Archean. Thus, it is inferred that the mass accreted after 3.0 Ga contributed insignificantly to the water and other volatile element budget of the Earth.