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survived in a homogenous state. Some workers
argue that it is more reasonable that the Earth
accreted from material with major-element com-
positions that were distinct from any meteorite
type than to accept chemical stratification and
petrological differentiation of the mantle.
processing by accretional or petrological pro-
cesses. This is not unexpected since they cannot
avoid processing by accretional or petrological
processes. Both the bulk of the Earth, and the
late veneer probably have multiple sources and
it is too much to expect that all the chemical
characteristics of the Earth must be found in
one kind of extraterrestrial object or its frag-
ments which themselves have a history of pro-
cessing. The alternatives are that all the distinctly
Earth-forming material is either contained in
the Earth and Moon or was ejected from the
inner Solar System, during giant impacts, mak-
ing the Earth--Moon system unique in the Solar
System.
There are two views about the source of
volatiles on Earth. One view holds that temper-
atures were too high at 1 AU for hydrous and
volatile-rich phases to exist in the accretion disk,
so that the bulk of the Earth accreted 'dry.' Water,
and noble gases, were delivered from sources
such as comets or meteorites after the bulk of the
Earth had formed. An alternative view holds that
the Earth accreted 'wet' and gas-rich, with anhy-
drous and hydrous silicate phases among the
material accreted to the growing planet through-
out the accretionary history. In this view, Earth's
water, and other volatiles such as the noble gases,
have an indigenous origin. This implies a cold ori-
gin and early history and reaction of water with
Fe. The late veneer of the Earth may have come
from more than one source. The noble metals,
noble gases and water may have been brought in
by different components and at different times.
The Os-isotopic compositions of the crust and
upper mantle of the Earth, are inconsistent with
water-bearing carbonaceous chondrites, but are
consistent with anhydrous ordinary chondrites.
Various components have also been recognized in
the noble gas inventory of the Earth. These may
have been in large accreting objects, or in dust
particles affected by Solar radiation. They may, in
part, be due to cosmic radiation at the surface of
early Earth.
Late veneer
Most of the volatiles in the Earth may have been
brought in as a late veneer, or late veneers. The
late veneer material is mixed into the upper man-
tle, and the term does not imply a thin surface
coating. The heterogenous accretion hypothesis
makes dynamical sense in that the feeding zone
of the Earth must have extended further out
from the Sun as the growth of planets increased
the relative velocities and the eccentricities of
the accreting material. Earth probably developed
magma oceans throughout the accretion pro-
cess, including late in its accretion, effectively
processing and fractionating incoming material.
Magma oceans and extensive melting are some-
times viewed as homogenizers rather than frac-
tionators or filters.
Free water in the Earth and the trace
siderophiles in the upper mantle, and possibly
the lighter rare gases, may all have been deliv-
ered to the Earth's surface long after the bulk
of the Earth had formed and the core was in
place, and the bulk of the mantle had cooled,
differentiated and solidified. Comets may have
delivered some fraction of the water. Solar wind
and IDP have helium and neon isotopic charac-
teristics suitable to be an end member in some
basalts. The similarity of Os-isotope ratios in the
mantle and ordinary chondrites and oxygen iso-
topic composition with the enstatite meteorites
is evidence for the existence of Earth-building
materials sharing some properties with various
meteorites.
No meteorite or other ET material has all of
the properties of Earth material, at least without
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