Ratios of S, Se and Te in the silicate Earth require a volatile-rich late veneer

Abstract

The excess of highly siderophile (iron-loving) elements (HSEs) and the chondritic ratios of most HSEs in the bulk silicate Earth (BSE) may reflect the accretion of a chondritic 'late veneer' of about 0.5 per cent of Earth's mass after core formation. The amount of volatiles contained in the late veneer is a key constraint on the budget and the origin of the volatiles in Earth. At high pressures and temperatures, the moderately volatile chalcogen elements sulphur (S), selenium (Se) and tellurium (Te) are moderately to highly siderophile; thus, if depleted by core formation their mantle abundances should reflect the volatile composition of the late veneer. Here we report ratios and abundances of S, Se and Te in the mantle determined from new isotope dilution data for post-Archaean mantle peridotites. The mean S/Se and Se/Te ratios of mantle lherzolites overlap with CI (Ivuna-type) carbonaceous chondrite values. The Se/Te ratios of ordinary and enstatite chondrites are significantly different. The chalcogen/HSE ratio of the BSE is similar to that of CM (Mighei-type) carbonaceous chondrites, consistent with the view that the HSE signature of the BSE reflects a predominance of slightly volatile-depleted, carbonaceous-chondrite-like material, possibly with a minor proportion of non-chondritic material. Depending on the estimates for the abundances of water and carbon in the BSE, the late veneer may have supplied 20 to 100 per cent of the budget of hydrogen and carbon in the BSE.

Massive impact-induced release of carbon and sulfur gases in the early Earth's atmosphere

“…Gyr ago, compatible with geochemical data (Labidi et al, 2013;Wang and Becker, 2013). Here we demonstrate that impact melt pool degassing redistributes mantle carbon and (chiefly impactorderived) sulfur towards surface environments, drastically increasing the amount of sulfur available for Archean biological processes.…”

“…Some of the earliest known organisms (e.g., Shen et al, 2001) relied on sulfur for key metabolic functions. However, because the vast majority of Earth's primordial sulfur likely partitioned into the core during core formation (e.g., Labidi et al, 2013;Wang and Becker, 2013) its availability could have been (Bell et al, 2014). Time bins of 10 Myr.…”

The molecular composition of impact-generated atmospheres on terrestrial planets during the post-accretion stage

“…A recent geochemical measurement shows that the ratios of chalcogen elements (S, Se, Te), which become moderately to highly siderophile elements at high pressure conditions (e.g., Rose-Weston et al, 2009), in Earth's mantle are very similar to those of CI chondrites (Wang and Becker, 2013). This indicates that a volatile-rich carbonaceous impactor was likely to be a major volatile contributor after core formation.…”

Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation

“…However, recent molecular dynamics estimates suggest S may not be present at all in the core (Badro et al, 2014); also, it is unclear as to whether S entered the core as an iron alloy, or as a discrete sulphide phase (O'Neill, 1991). 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; O'Neill, 2014, McDonough, 2003) but is less volatile than S, so is abundant enough in Earth's mantle to have been largely unaffected by a late veneer.…”