The origins and concentrations of water, carbon, nitrogen and noble gases on Earth (1405.6336v1)

Abstract: The isotopic compositions of terrestrial hydrogen and nitrogen are clearly different from those of the nebular gas from which the solar system formed, and also differ from most of cometary values. Terrestrial N and H isotopic compositions are in the range of values characterizing primitive meteorites, which suggests that water, nitrogen, and other volatile elements on Earth originated from a cosmochemical reservoir that also sourced the parent bodies of primitive meteorites. Remnants of the proto-solar nebula (PSN) are still present in the mantle, presumably signing the sequestration of PSN gas at an early stage of planetary growth. The contribution of cometary volatiles appears limited to a few percents at most of the total volatile inventory of the Earth. The isotope signatures of H, N, Ne and Ar can be explained by mixing between two end-members of solar and chondritic compositions, respectively, and do not require isotopic fractionation during hydrodynamic escape of an early atmosphere. The terrestrial inventory of 40Ar (produced by the decay of 40K throughout the Earth's history) suggests that a significant fraction of radiogenic argon may be still trapped in the silicate Earth. By normalizing other volatile element abundances to this isotope, it is proposed that the Earth is not as volatile-poor as previously thought. Our planet may indeed contain up to ~3000 ppm water (preferred range : 1000-3000 ppm), and up to ~500 ppm C, both largely sequestrated in the solid Earth. This volatile content is equivalent to a ~2 (+/-1) % contribution of carbonaceous chondrite (CI-CM) material to a dry proto-Earth, which is higher than the contribution of chondritic material advocated to account for the platinum group element budget of the mantle. Such a (relatively) high contribution of volatile-rich matter is consistent with the accretion of a few wet planetesimals during Earth accretion.

• The paper demonstrates that Earth's volatiles primarily originate from chondritic sources rather than comets, based on detailed isotopic comparisons.
• The paper uses mixing models of chondritic and solar-like components to explain the observed isotopic signatures in water, nitrogen, and noble gases.
• The paper reveals a higher-than-expected volatile content in the Earth’s mantle, influencing theories on planetary formation and atmospheric evolution.

The Origins and Concentrations of Water, Carbon, Nitrogen, and Noble Gases on Earth

The paper presents an examination of the origins and concentrations of volatile elements such as water, carbon, nitrogen, and noble gases on Earth, integrating isotopic analyses and cosmochemical datasets. It advances a perspective that Earth’s volatiles did not primarily originate from cometary material, despite previous suggestions, but rather from a chondritic source with some contributions from proto-solar nebular gases.

Key Findings and Methodology

1. Isotopic Compositions and Sources: The isotopic compositions of terrestrial hydrogen and nitrogen are revealed to notably differ from those found in nebular gases and most cometary bodies but align closely with primitive meteorites. This suggests that the primary source of Earth's volatile elements is similar to that which contributed to the primitive meteorites, as opposed to comets.
2. Limited Cometary Contributions: The paper infers that cometary contributions to Earth's volatile inventory are minimal. Isotopic data, specifically the differences in the N/N and D/H ratios, indicate that terrestrial water and nitrogen must have been derived largely from a non-cometary source.
3. Chondritic and Solar-like Components: An important conclusion is that the isotopic signatures of H, N, Ne, and Ar can be modeled by a mixing scenario between chondritic and solar-like end-members rather than by isotopic fractionation processes during hydrodynamic escape. Notably, there is a remnant solar-like component in the Earth's mantle, suggesting ancient trapping of proto-solar nebular gases during Earth's formation.
4. Terrestrial Volatile Inventory: The terrestrial volatile inventory, especially concerning argon, suggests a higher volatile content in the solid Earth than traditionally believed. The Earth may harbour up to 3000 ppm of water and 500 ppm of carbon, correlating to a contribution of ~2% carbonaceous chondrite material to a dry proto-Earth. This contribution is posited to be higher than what is required to account for the platinum group elements in the Earth's mantle.

Implications and Theoretical Considerations

1. Evolving Understanding of Volatile Delivery: The findings propose a revision of the conventional understanding of Earth's volatile delivery, emphasizing accretion of chondritic material rather than post-accretionary cometary delivery. This has implications for understanding planetary formation and differentiation processes, especially for terrestrial bodies.
2. Noble Gas and Volatile Element Interactions: The hypothetical models for noble gases and volatiles suggest an intricate early Earth story with respect to atmospheric retention and recycling of volatiles, essential for understanding Earth's long-term geothermal and atmospheric evolution.
3. The Xenon Paradox: An exceptional case within the composition of volatiles is xenon, which is isotopically light in the atmosphere compared to chondritic data. This xenon paradox suggests prolonged interaction with high-energy solar radiation possibly altered isotopic ratios over geological time.

Future Directions

Further exploration is indicated in the manipulation of isotopic analyses from both primitive and differentiated cosmochemical materials, alongside potential collection of pristine samples from cometary bodies and other solar system objects. Such studies will enhance our understanding of Earth’s early atmosphere and volatile budgets, delineating the detailed processes that governed the initial capture and evolution of planetary volatiles.

In summary, this paper puts forward a robust narrative characteristic of terrestrial volatiles largely accruing from chondritic bodies with minor solar component contributions, challenging the previously overstated role of comets. It provides a foundation for further research into planetary formation, atmospheric evolution, and the inner workings of primordial Earth conditions.