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

31/05/2024 15:38 · by karmaka · in carbonaceous chondrite, Earth, water

Yogita Kadlag, Aryavart Anand, Mario Fischer-Gödde, Klaus Mezger , Kristoffer Szilas , Steven Goderis, Ingo Leya

Icarus
Available online 31 May 2024, 116143

“Highlights

The average ε53Cr and ε54Cr of Earth’s mantle is 0.03 ± 0.02 and 0.08 ± 0.04, respectively.
CC-like material added to the Earth’s mantle after Archean is limited to 0.01 wt%.
A carbonaceous chondrite-like impactors dominated the Meso- to Paleoarchean spherule layers (> 3.0 Ga), whereas a mixed impactor flux dominated after 3.0 Ga.”

“The late addition of extra-terrestrial material may represent an important source of Earth’s volatiles. The composition of impactors can be reconstructed using 54Cr abundances in impact related rocks preserved in the terrestrial record. The average ε53Cr and ε54Cr 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 ε53Cr ranging from −0.04 to 0.17, and ε54Cr 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 >0.01 wt% of CC-like material added to the Earth’s mantle after 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.”