Air like 38Ar/36Ar in the mantle: implications for the nature of the parent bodies of the Earth
Résumé
The `'stable'' 20Ne/22Ne ratios in mantle-derived samples clearly differ from the atmospheric ratio. 20Ne/22Ne in oceanic basalts show obviously a solar origin (>12). Nevertheless, the mantle ratio (e.g. not contaminated by air) is not precisely determined. No mantle-derived samples have precise 20Ne/22Ne ratios above 13.0±0.2 (1σ). Two models are generally discussed to explain this solar origin. One is to dissolve solar wind gases in a previously degassed magma ocean. The other is that the mantle value is similar to the so-called Neon B (12.5 ± 0.2; (1σ)), which corresponds to mixing between the `'normal'' solar wind and solar energetic particles (SEP). This ratio is notably observed in gas rich meteorites, which are breccias irradiated in an early phase of the planetary accretion. The knowledge of the isotopic composition of rare gases in the mantle has important implications for the model of volatile acquisition on Earth. Some mantle-derived samples show 38Ar/36Ar ratios different from the atmosphere ratio and trend to the solar-wind isotopic composition. These results were challenged and an air like argon composition in the mantle has been suggested. As 40Ar signal is up to one million times higher than the 38Ar, we can suspect an influence of the 40Ar signal on the 38Ar base line (tail effect) during mass spectrometer analyses, giving low 38Ar/36Ar. Moreover, step heating appears to fractionate argon isotopes. We have performed a complete study of He, Ne and Ar in MORB and OIB in order to constrain the argon isotopic composition of the mantle. Using samples that have suffered only little air contamination and degassing, analyzed by crushing with a new analytical procedure, we show that the mantle 38Ar/36Ar is not different from the atmospheric ratio even for 20Ne/22Ne higher than 12.5. Starting from isotopic compositions close to the solar wind for both Ne and Ar, one may suppose than atmospheric argon was recycled within the mantle without neon during subduction. However, the 129Xe anomalies observed in mantle-derived sample suggest that the subduction of argon is probably not the source of the atmospheric 38Ar/36Ar in the mantle. The simplest explanation is that the parent bodies of the Earth were irradiated by a solar wind during the planetary accretion (T Tauri ?), giving these specific neon and argon isotopic compositions to the Earth (gas rich meteorite show air-like argon associated to neon B).