Abstract

The differentiation of Earth into a metallic core and silicate mantle has left its signature on the chemical and isotopic composition of the bulk silicate Earth (BSE). This is seen in the depletion of siderophile (metal-loving) relative to lithophile (rock-loving) elements in Earth’s mantle as well as, for instance, the silicon (Si) isotope offset between primitive meteorites (a good proxy for the composition of bulk Earth) and BSE , which further implies that Si is present in Earth’s core. Another putative light element in Earth’s core is sulfur; however, estimates of core S abundance vary significantly and, due to its volatile nature, no unequivocal S isotopic signature for core fractionation has thus far been detected. I present new high precision isotopic data for Cu, a siderophile and chalcophile (sulfur-loving) element, which shows that Earth’s mantle is isotopically fractionated relative to bulk Earth. Critically, high-pressure metal-sulfide-silicate equilibration experiments suggest that the sense of Cu isotopic fractionation between BSE and bulk Earth requires that a sulfide-rich liquid segregated from Earth’s mantle during differentiation, which most likely entered the core.

References: 1) Palme, H and O’Neill H. St.C. (2014) Cosmochemical estimates of mantle composition, Treatise on Geochemistry (Second Edition), Volume 3, pp.1-39 (a brilliant review of the composition of the Earth’s mantle, and how we think it got that way)

2) Badro, J., Cote, A.S. & Brodholt, J.P. (2014) A seismologically consistent compositional model of Earth’s core. Proc. Nat. Acad. Sci., 111, pp.7542-7545 (some interesting ideas of how to constrain the light element budget in the core – also, well referenced if you want to read further)

3) Wood, B. J. and Halliday A. N. (2010) The lead isotopic age of the Earth can be explained by core formation alone. Nature. 465, pp.767-770 (info about the Pb isotope composition of the mantle – and how core formation could explain it)

4) O’Neill H. St.C. (1991) The origin of the moon and the early history of the earth-A chemical model. Part 2: The earth. Geochim. Cosmochim. Acta. 55, pp.1159-1172. (optional – an oldie but a goodie – my inspiration for the sulphide matte model)