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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.

The redox‐sensitive, chalcophile, and volatile Se stable isotope system offers new perspectives to investigate the origin and evolution of terrestrial volatiles and the roles of magmatic and recycling processes in the development of the redox contrast between Earth's reservoirs. Selenium isotope systematics become more robust in a well‐constrained petrogenetic context as can be inferred from Se‐Te elemental signatures of sulfides and igneous rocks. In this study, we present a high‐yield chemical sample processing method that allows the determination of Se‐Te concentrations and Se isotope composition from the same sample digest of silicate rocks by hydride generation isotope dilution (ID) quadrupole inductively coupled plasma mass spectrometry (ICP‐MS) and double spike (DS) multicollector (MC)‐ICP‐MS, respectively. Our procedure yields ∼80% Se‐Te recoveries with quantitative separation of relevant interfering elements such as Ge and HG‐buffering metals. Replicate analyses of selected international reference materials yield uncertainties better than 0.11‰ (2 s.d.) on δ82/76Se and 3% (r.s.d.) on Se concentration for DS MC‐ICP‐MS determinations for as low as ∼10 ng sample Se. The precision of Se‐Te concentration measurements by ID ICP‐MS is better than 3% and 5% (r.s.d.) for total amounts of ∼0.5–1 ng Se and ∼0.2–0.5 ng Te, respectively. The basaltic reference materials have variable Se‐Te contents, but their δ82/76Se values are rather uniform (on average 0.23 ± 0.14‰; 2 s.d.) and different from the chondritic value. This altogether provides the methodology and potential to extend the limited data set of coupled Se isotope and Se‐Te elemental systematics of samples relevant to study the terrestrial igneous inventory.

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