Annuaire

PhD. subject: Origin of terrestrial neon: constrained by the current composition of neon in the Earth's mantle
Supervisor: Manuel Moreira; Co-supervisor: Bruno Scaillet

The origin of volatile elements on Earth is one of the major questions in Earth sciences and planetology. Several scenarios are generally proposed for this origin, with two main ones emerging.

The first proposes a "dry" accretion, followed by a late input of volatile elements (H2O, C, N) transported by carbonaceous chondrite-type meteorites and/or comets, after the end of Earth's accretion.

The second scenario, on the other hand, suggests that volatile elements were present in the Earth's parent bodies. The notion of a "dry" Earth is based on the idea that the environment in which Earth's parent bodies were formed – close to the Sun – did not allow condensation, or favored evaporation, of volatile and highly volatile elements, including chemical elements such as K, Rb, Zn, Pb. However, this cosmochemical notion of volatility in no way precludes enrichment in volatile elements, such as helium, neon, hydrogen, elements with very low condensation temperatures – if a physical phenomenon independent of condensation or evaporation exists. Such a process may be irradiation by the solar wind, which can re-enrich "dry" materials with certain elements.

Neon is a geochemical tracer that can be used to test this model. Neon is a rare gas with a condensation temperature of just a few Kelvin, so it is present in minute quantities in the solids of the forming solar system. On the other hand, it is one of the major elements of the sun (the 5th), and therefore of the solar wind. Irradiation by the solar wind is easily traceable by neon, and in particular by its isotopic signature, which makes it possible to distinguish between solar wind and solar nebula. Neon and its isotopes can be measured by vacuum grinding of terrestrial rocks coupled with noble gas mass spectrometry. The results of neon analysis in ridge or hotspot basalts suggest that neon may indeed have been incorporated into the Earth's parent bodies by solar wind irradiation on the first dust formed in the solar system. However, these analyses on centimetric samples often reveal the presence of an atmospheric component that contaminates the magmatic gas, making interpretation difficult, as all scenarios for the origin of neon remain possible.


The aim of this PhD. is to develop an experimental protocol for the analysis of neon in individual gas bubbles in oceanic basalts. This method is free from atmospheric contamination. The isotopic composition of neon in these bubbles will be determined by mass spectrometry coupled to laser ablation, using state-of-the-art equipment currently being acquired as part of Manuel Moreira's APATE ERC.

The implications for the origin of hydrogen and water in the deep Earth are then significant, for if neon is a major element of the solar wind, hydrogen is even more so. The cosmochemical implications of the thesis will focus on the pressure, temperature and irradiation conditions in the young Earth's formative environment, and the existence of a solar primordial atmosphere captured around this young Earth will also be discussed.

This project is funded by the ERC APATE directed by Manuel Moreira.

Keywords: origin of volatile elements on Earth – noble gases – neon – isotopic geochemistry – mass spectrometry