  The isotopic compositions of terrestrial hydrogen and nitrogen are clearly
different from those of the nebular gas from which the solar system formed, and
also differ from most of cometary values. Terrestrial N and H isotopic
compositions are in the range of values characterizing primitive meteorites,
which suggests that water, nitrogen, and other volatile elements on Earth
originated from a cosmochemical reservoir that also sourced the parent bodies
of primitive meteorites. Remnants of the proto-solar nebula (PSN) are still
present in the mantle, presumably signing the sequestration of PSN gas at an
early stage of planetary growth. The contribution of cometary volatiles appears
limited to a few percents at most of the total volatile inventory of the Earth.
The isotope signatures of H, N, Ne and Ar can be explained by mixing between
two end-members of solar and chondritic compositions, respectively, and do not
require isotopic fractionation during hydrodynamic escape of an early
atmosphere. The terrestrial inventory of 40Ar (produced by the decay of 40K
throughout the Earth's history) suggests that a significant fraction of
radiogenic argon may be still trapped in the silicate Earth. By normalizing
other volatile element abundances to this isotope, it is proposed that the
Earth is not as volatile-poor as previously thought. Our planet may indeed
contain up to ~3000 ppm water (preferred range : 1000-3000 ppm), and up to ~500
ppm C, both largely sequestrated in the solid Earth. This volatile content is
equivalent to a ~2 (+/-1) % contribution of carbonaceous chondrite (CI-CM)
material to a dry proto-Earth, which is higher than the contribution of
chondritic material advocated to account for the platinum group element budget
of the mantle. Such a (relatively) high contribution of volatile-rich matter is
consistent with the accretion of a few wet planetesimals during Earth
accretion.
