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Abstract

Studying the origin and evolution of cosmo- and geochemical reservoirs particularly requires knowledge about the composition and occurrence of the inert noble gases (He, Ne, Ar, Kr, Xe). Earth's atmosphere is characterized by a "planetary" noble gas signature, i.e., depleted from solar element abundances more intensively in lighter than in heavier gases, whereas Earth's interior hosts light noble gases (He and Ne) with a distinct "solar" composition. In particular, Ne isotopic ratios of both the convecting and more primitive mantle, the latter sampled by oceanic island basalts (OIBs), resemble the solar wind (SW) implanted Ne-B component in meteorites with 20Ne/22NeNe-B ~12.7. The atmosphere, instead, displays a lower 20Ne/22Ne ratio of 9.80. The reservoir of the primitive noble gas signatures, traditionally assumed to be isolated in the deep mantle, is not precisely located and some models speculate about Earth’s core as possible source. High resolution release experiments on interior samples of the iron meteorite Washington County (WC) were carried out in this study to identify volume correlated trapped noble gases and to investigate the possibility of noble gas partitioning into metal upon core segregation. Consisting of a mixture of predominantly cosmogenic and solar components, with only minor atmospheric additions, gases are released from schreibersite ((Fe,Ni)3P) at ~1100 °C and kamacite-taenite (Fe,Ni) at ≳1400 °C. The solar signatures are distinct in Ne and He/Ne isotopic ratios with clear 4He excess. Ar, Kr and Xe isotopic ratios are either dominated by spallation or are overprinted by air contamination. Measured 20Ne concentrations of ~4*10-8 cm³STP/g imply that solar wind-implantation into terrestrial precursors and incorporation of <1% core material that resembled Washington County metal would have been sufficient to provide solar type Ne in the core that satisfies observed mantle fluxes. This would be consistent with the core as potential source region. The actual acquisition of the light solar noble gases on Earth can be either explained by solar nebula gas dissolution into a magma ocean or accretion of solar wind irradiated material. The solar wind implantation model is assessed by applying constraints for the present terrestrial influx of particles ranging from 10-16–1025 g, and the size-specific Ne inventory of extraterrestrial matter. Present-day Ne contributions to Earth’s surface peak at interplanetary dust particle sizes of ~9 µm which contain a mean 20Ne/22Ne ratio of 12.61±0.41. This value represents Ne-B in unablated solar wind saturated particle surfaces and dominates the inventory of irradiated, though volatile-poor, matter that accreted to form Earth in the inner Solar system. This is opposed to volatile-rich objects from the outer Solar system containing planetary Ne-A with 20Ne/22Ne ~8.20. The data compilations allow determining the mass and size dependent upper atmosphere Ne flux and infer the contribution during early Earth formation of a) surface correlated Ne-B, dominated by ~75 µm particles with high surface/volume ratio and b) volume correlated Ne-A, dominated by larger bodies. The Ne-acquisition scenario considers delivery of solar wind implanted Ne-B shortly after dissipation of disk gas and Ne incorporation into Earth with 20Ne/22Ne: 12.61±0.41 by dissolution into a magma ocean before the Moon-forming impact. The late veneer contribution of Ne-A to degassed mantle Ne-B establishes the atmospheric inventory with 20Ne/22Ne: 9.80. The model calculations show that, because dominated by implanted components in cosmic dust, only a fraction of a few % of irradiated precursor material is sufficient to account for the solar Ne budget of Earth, thus, demonstrating the significance of dust accretion for the origin of volatiles.