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Selenium isotopes as tracers of a late volatile contribution to Earth from the outer Solar System
Selenium isotopes in mantle rocks compared to those of a range of meteorites allowed to identify the type of meteoritic material that contributed a significant amount of volatiles to the Earth at the very end of planetary accretion.
María I. Varas-Reus
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What makes our planet Earth unique in the Solar System is its ability to support life. This critical feature depends greatly on life-essential volatile elements such as hydrogen, carbon, and nitrogen, which make up a large proportion of our atmosphere. Thus, understanding the origin of Earth's volatiles and their timing of delivery is fundamental to understand the evolution of a habitable planet.
It has been known for many years now that the abundance of highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au), or also called iron-loving elements, in the Earth's mantle are higher than predicted by experimentally determined metal-silicate partition coefficients and core-mantle differentiation models. The HSE prefer to partition into metal compared to silicate and so they were expected to be preferentially removed from the mantle and sequestered into the planet's iron core during planetary differentiation. This adds to the fact that the relative abundances of HSE in the mantle are generally chondritic even though their metal-silicate partition coefficients are significantly different. The over-abundance and near-chondritic proportion of these elements in the mantle have been best explained by the 'late veneer' model, which sustains that HSE were replenished in the mantle by a late addition of chondritic material after core formation had ceased. According to studies based on the refractory HSE Re, Os and Ru, the late veneer would come from the inner Solar System, where volatile substances are rare, and as such it could not have brought significant amounts of volatiles and water to the Earth.
The late veneer model refers to the late accretion of meteoritic material to terrestrial planets after core formation. Image credit: Johan Swanepoel.
Interestingly, a study based on the ratios of S, Se, and Te —elements that are primarily volatile and chalcophile but at core-forming conditions behave as siderophile— shows an over-abundance of these elements in the Earth's mantle and near-chondritic ratios. The latter are similar to those of carbonaceous chondrites, which originated in the outer solar system and are characterized by having the highest abundances of volatile elements of any meteorite group. For these reasons, a volatile-rich late veneer was proposed as the most possible source of these elements in the Earth's mantle. The main problem with this study is that its findings depend on elemental ratios that do not really represent a pristine feature of the mantle and instead, reflect the action of secondary mantle processes.
The contradiction between the information provided by refractory HSE and volatile siderophile elements regarding the volatile nature of the late veneer led us to think about how we could shed light on this ongoing debate from a new perspective. With this in mind, we decided to study for the first time the Se isotope signature of mantle rocks. Although Se isotopes have been used for some years now as tracers of redox processes in surface environments, it has been only until now that an analytical method was specially developed to perform high-precision Se isotope measurements in rocks that are very depleted in Se, such as mantle rocks. This technique was developed at the University of Tübingen (Germany) as part of the ERC Project O2rigin led by Dr. Stephan König, co-author of this study. To obtain precise and accurate Se isotope data it is necessary to follow a rigorous sample preparation in a clean lab, where special care needs to be taken regarding temperature conditions for sample processing, as Se is a volatile element that starts to evaporate above ca. 85°C.
Clean lab facilities of the Isotope Geochemistry Laboratory, University of Tübingen (from left to right): Dr. María Isabel Varas-Reus, Dr. Stephan König and Ph.D. candidate Aierken Yierpan. Image credit: Daniel Fuente Friend.
Samples analyzed in this study are peridotites, which are mantle rocks that have been brought to the surface by plate tectonic processes. Although evidence shows that these rocks have suffered complex melt-rock interaction processes in the mantle that might have affected their Se budget, they had remained unchanged with regard to its Se isotope composition. In a previous study, also carried out by the O2rigin group, Se isotope variations were reported for different classes of chondrites. Strikingly, the Se isotope signature of mantle peridotites is identical only to that of CI carbonaceous chondrites. This implies that Se, which behaves as a HSE and must have been stripped from the mantle during core formation, was most likely delivered to Earth by a late veneer composed of CI chondrite-like material. We also quantified additional components that were – apart from Se– delivered by these meteorites to the early Earth. This revealed that large amounts of water and other life-essential elements such as carbon and nitrogen were also brought by this late veneer. These new results provide one of the strongest evidence so far for an outer Solar System origin of the late veneer, directly challenging the most widely accepted paradigm that the late veneer was "dry" and had an inner Solar System origin.
Although these findings clearly indicate that carbonaceous chondrites dominated the volatile late-accreted material, the calculated mass of a CI chondrite-like late veneer is not sufficient to account for the over-abundance of refractory HSE unless their metal-silicate partition coefficients were actually lower than experimentally predicted. This seems to be the case at least of Ru as ongoing studies suggest that not all Ru was removed from the mantle prior to the late veneer. However, firmer constraints and further experimental studies at high pressure and high temperature are still needed. Future research should now be directed to explain why Se isotopes indicate a carbonaceous chondrite-like late veneer, whereas Os isotopes support and enstatite and/or ordinary chondrite-like late veneer. Hence we may have made an important step towards constraining the origin of terrestrial volatiles, but many aspects of our Planet's early history remain to be discovered.
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