Selenium isotopes as tracers of a late volatile contribution to Earth from the outer Solar System

Abstract

The origin of Earth’s volatiles has been attributed to a late addition of meteoritic material after core-mantle differentiation. The nature and consequences of this 'late veneer' are debated, but may be traced by isotopes of the highly siderophile, or iron-loving, and volatile element selenium. Here we present high-precision selenium isotope data for mantle peridotites, from double spike and hydride generation multi-collector inductively coupled plasma mass spectrometry. These data indicate that the selenium isotopic composition of peridotites is unaffected by petrological processes, such as melt depletion and melt-rock reaction, and thus a narrow range is preserved that is representative of the silicate Earth. We show that selenium isotopes record a signature of late accretion after core formation and that this signature overlaps only with that of the CI-type carbonaceous chondrites. We conclude that these isotopic constraints indicate the late veneer originated from the outer Solar System and was of lower mass than previously estimated. Thus, we suggest a late and highly concentrated delivery of volatiles enabled Earth to become habitable.

Isotope Fractionation during Condensation and Evaporation during Planet Formation Processes

“…However, the first sets of observations of K isotopes in meteorites, CAI, chondrules and in the Moon revealed a rather different pattern with little or no fractionation with increasing K depletion 24 . More recent work has shown that, except for CAI 25 , the magnitude of isotope fractionations reported in planetary materials are smaller than expected for a purely kinetic Rayleigh-type loss process (Rb, Zn, K, Mg, Si, Fe, Cu and Se) 15,[26][27][28][29][30][31][32][33][34][35] . Thus, at all scales of planetary accretion, some processes acted to limit the large kinetic isotope fractionation expected for evaporation or condensation.…”

“…(0, ) = ,0 > 0 (27) If the species of interest is in trace amount then the diffusion coefficient can be considered to be independent of the concentration. Under these conditions, the partial differential equation can be solved analytically to yield 81 the time-dependent solution of equation 24can be written as…”

Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks

“…Mass-independent ruthenium isotopic variations among meteorites and the Earth have provided evidence that the late veneer was derived from reduced and volatile-poor inner solar system materials most similar to enstatite chondrites 5 , 11 , 12 , 19 . This is in contrast to constraints from relative abundances of volatile elements such as selenium (Se) – tellurium (Te) – sulphur (S) and the Se isotope composition in the silicate Earth that were used to argue for a CM or CI carbonaceous chondrite-like late veneer composition 2 , 9 , 10 . Owing to its distinct Ru isotope composition, volatile-rich carbonaceous chondrite-like material from the outer solar system was excluded as possible late veneer source material 5 , 11 and thus, the late veneer appeared unlikely to be the primary source of water and volatiles on Earth 5 , 11 .…”

“…However, the timing of this accretion remains controversial 5 – 8 . It was proposed that volatile elements were added to Earth by late accretion of a late veneer consisting of carbonaceous chondrite-like material after core formation had ceased 6 , 9 , 10 . This view, however, could not be reconciled with the distinct ruthenium (Ru) isotope composition of carbonaceous chondrites 5 , 11 compared to the modern mantle 12 , and in fact also not with any known meteorite group 5 .…”

“…Approximately chondritic relative abundances of HSEs in the mantle suggest that the late veneer must have consisted of primitive meteoritic material 17 , 18 , amounting to ~0.5% of the Earth’s mass 18 . The chemical composition of the late veneer and its origin are a longstanding matter of debate, especially in the context of how and when Earth accreted its water and volatiles 3 , 6 , 9 , 10 . In this regard, previous studies debated whether significant amounts of volatile-rich carbonaceous chondrite-like material were added by the late veneer during the final stages of Earth’s accretion 6 , 9 , 10 or had already been incorporated during earlier stages of Earth’s growth 3 , 5 , 7 , 8 , 11 .…”