The differentiation of Earth into a metallic core and silicate mantle 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 the silicon isotope offset between primitive meteorites (i.e. bulk Earth) and BSE, which is generally interpreted as a proof that Si is present in Earth’s core. Another putative light element in Earth’s core is sulphur; 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. Here we present new high precision isotopic data for Cu, a chalcophile (sulphur-loving) element, which shows that Earth’s mantle is isotopically fractionated relative to bulk Earth. Results from high pressure equilibration experiments suggest that the sense of Cu isotopic fractionation between BSE and bulk Earth requires that a sulphide-rich liquid segregated from Earth’s mantle during differentiation, which likely entered the core. Such an early-stage removal of a sulphide-rich phase from the mantle presents a possible solution to the long-standing 1st terrestrial lead paradox. Letter Geochemical Perspectives Letters © 2015 European Association of Geochemistry Geochem. Persp. Let . (2015) 1, 53-64 | doi: 10.7185/geochemlet.1506 Geochem. Persp. Let . (2015) 1, 53-64 | doi: 10.7185/geochemlet.150653 54 Letter The budget of light elements in Earth’s core is a long-standing geochemical problem (Poirier, 1994), as constraining such elements and their abundances can tell us much about the physiochemical conditions of Earth’s differentiation. Sulphur is often cited as one such element: cosmochemical estimates suggest that the core contains ~2 wt. Further complications stem from the fact that late addition of extra-terrestrial S to the mantle, post-core formation, should overwhelm any pre-existing S (isotope) signature (the “late veneer”; Holzheid et al., 2000; Wang et al., 2013). In an effort to investigate the role of S during Earth’s differentiation, we have investigated the Cu isotope compositions of bulk Earth and BSE; this is because Cu is siderophile and strongly chalcophile (~2/3 of Earth’s Cu is thought to be in the core; Palme and O’Neill, 2014, McDonough, 2003) but is Shofner1, J. Siebert 1, J. Badro1, 5, I.S. Puchtel 6 Abstract doi: 10.7185/geochemlet.1506 The differentiation of Earth into a metallic core and silicate mantle 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 the silicon isotope offset between primitive meteorites ( i.e. bulk Earth) and BSE, which is generally interpreted as a proof that Si is present in Earth’s core. Another putative light element in Earth’s core is sulphur; 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. Here we present new high precision isotopic data for Cu, a chalcophile (sulphur-loving) element, which shows that Earth’s mantle is isotopically fractionated relative to bulk Earth. Results from high pressure equilibration experiments suggest that the sense of Cu isotopic fractionation between BSE and bulk Earth requires that a sulphide-rich liquid segregated from Earth’s mantle during differentiation, which likely entered the core. Therefore, in terms of Cu isotopes, BSE is enriched in the heavy Cu isotope compared to bulk Earth, with a minimum offset (taking bulk Earth to be –0.19 ‰) of +0.26 ± 0.14 ‰ (2 s.d., Fig. 1). This suggests that some process related to planetary differentiation and accretion has affected the Cu isotope composition of Earth’s mantle; we now consider the two most likely culprits: volatile loss of Cu, and core formation. Preferential removal of the lighter Cu isotope during volatile loss could lead to enrichment in isotopically heavy Cu in Earth’s mantle. This, however, can be discounted by considering the Zn isotope system. Geochemical Perspectives Letters Letter Letter Geochemical Perspectives Letters Geochem. Persp. Let . (2015) 1, 53-64 | doi: 10.7185/geochemlet.1506 Geochem. Persp. Let . (2015) 1, 53-64 | doi: 10.7185/geochemlet.150657 58 Planetary differentiation is therefore the most likely explanation for the Cu isotope difference between BSE and bulk Earth. To further investigate the behaviour of Cu isotopes during core formation, i.e. To conclude, the Cu isotope composition of BSE seems to require that large scale sulphide-silicate equilibration occurred sometime in Earth’s history; here, we have modelled it as the formation of a discreet Fe-O-S reservoir, a “Hadean Matte”, which ponded to the base of the mantle during the final stages of Earth’s differentiation. Such a feature likely admixed into Earth’s core; however, if any of this material remains, such material could account for recently detected non- chondritic S isotope compositions in Earth’s mantle (Labidi et al., 2013). Finally, the Martian core is thought to have up to 14 wt. (2013) Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth. Earth and Planetary Science Letters 369-370, 34-42. dAuPhAS, n., Chen, J.h., ZhAnG, J., PAPAnAStASSiou, d.A., dAviS, A.M., trAvAGlio, C. (2014) Calcium-48 isotopic anomalies in bulk chondrites and achondrites: Evidence for a uniform isotopic reservoir in the inner protoplanetary disk. Earth and Planetary Science Letters 407, 96-108. dreibu S, G., PAlMe, h. (1996) Cosmochemical constraints on the sulphur content in the Earth’s core. Geochimica et Cosmochimica Acta 60, 1125-1130. Fitou SSi, C., bourdon , b. (2012) Silicon isotope evidence against an enstatite chondrite earth.