Solar wind settling on early dust, a source of volatile elements for the Earth

As the lower mantle is a reservoir of primitive rare gases, several studies have estimated its composition, particularly in neon and argon, in order to understand the origin of the volatile elements. Two models have been proposed: the implantation of the solar wind on dust at the beginning of the formation of the solar system and the dissolution of gas from the solar nebula in an ocean of magma on a primitive Earth. However, the analyses of all these studies are subject to atmospheric contamination of the samples, making it impossible to interpret the results clearly. In fact, the samples are very often contaminated by air at the level of micro-fractures: by analysing the samples as a whole (by crushing them, for example), the mantle signal contained in the bubbles mixes with the air contained in the micro-fractures.

In this study, a new protocol was introduced, laser ablation. This consists of analysing a sample bubble by bubble in order to avoid atmospheric contamination, and we succeeded in reducing the analytical blanks as much as possible. Samples from the Galapagos hotspot were selected to analyse helium, neon and argon. In order to pierce the bubbles, they were first identified using X-ray microtomography, a non-destructive three-dimensional imaging technique. The images acquired make it possible to identify fractures, intact bubbles and bubbles connected to fractures, and to calculate their volumes. In this way, microtomography is proving to be a valuable tool for the analysis of rare gases by laser ablation.