The origin of the neon isotopes in chondrites and on Earth

- The neon isotope signature on Earth reflects solar wind irradiation on dust during the first stage of the solar system formation.
- Irradiation of grains during a few kyr is able to explain the Neon B composition.
- A new value for the Neon B component is estimated at ∼12.7 instead of the ancient estimate of 12.5.
- Chondrites reflect the mixture between these irradiated grains and pre-solar grain having the neon A signature.
- There is no need for a dense primordial solar atmosphere to explain the neon composition of the Earth's mantle.

We discuss the origin of the neon isotopic signatures in chondrites and in the terrestrial mantle. There are two primary possible origins for neon in the Earth's mantle. One origin is the dissolution of a dense primordial atmosphere with a solar composition of 20Ne/22Ne >13.4 into the mantle in a possible magma ocean stage during Earth's accretion. The second origin, developed in this study, is that mantle neon was already in Earth's parent bodies because of refractory grain irradiation by solar wind. We propose that solar wind implantation occurred early on dust within the accretion disk to allow such irradiation. Because solar wind implantation fractionates neon isotopes, the heavier isotopes are implanted deeper than the lighter ones because of different kinetic energies, and the process of implantation, if coupled with sputtering, leads to a steady state neon isotopic ratio (20Ne/22Ne ∼12.7) that is similar to what is observed in mantle-derived rocks (12.5-12.9), lunar soil grains (∼12.9) and certain gas-rich chondrites from all classes (enstatite, ordinary, rumuruti). Using a dust transport model in a turbulent and irradiated solar nebula, we estimated the equivalent irradiation age of a population of dust particles at three different distances from the sun (0.8, 1, 1.2 AU) and converted these ages into neon concentrations and isotopic ratios. The dust subsequently coagulated to form Earth's parent bodies, which have the mean neon isotopic composition of the irradiated dust (non-irradiated dust is assumed to be free of neon). If this scenario of solar wind implantation coupled with sputtering in the precursors of Earth's parent bodies is correct, it offers a simple alternative to the model of solar nebula gas incorporation by dissolution in a magma ocean.