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

The origin of volatile elements such as water, carbon and nitrogen on telluric planets is still widely debated. Due to their inert nature, noble gases (He, Ne, Ar, Kr and Xe) are unique tracers of possible sources of volatile elements (such as comets, solar wind or meteorites). Determining the isotopic and elemental composition of the Earth's noble gases can therefore provide a better understanding of their origins.

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

Publication date: 17/03/2017

Research

Related themes : Origins

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.

3D image (top) of a sample of volcanic glass from Fernandina (Galápagos) obtained by X-ray microtomography. The bottom image is a cross-section of the sample where the bubbles and micro-fractures appear in white, the matrix in light grey and some crystals in different shades of grey.

Remarkable precision was obtained on the isotopic ratios measured, in particular for the 20Ne/22Ne and 38Ar/36Ar ratios. These new results support the solar wind implantation model to explain the origin of light rare gases on Earth. This scenario implies that some of the volatile elements, such as water, were implanted directly into grains at the start of the formation of the solar system. The implantation of the solar wind is a surface phenomenon that affects the first few hundred nanometres of grains in the solar nebula located relatively close to the Sun. Certain volatile elements would therefore have been present in the Earth’s parent bodies.

Ref : S. Péron, M. Moreira, B. Putlitz, M.D. Kurz – Solar wind implantation supplied light volatiles during the first stage of Earth accretion: www.geochemicalperspectivesletters.org/article1718

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