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et al
., 2002; Albar ede, 2009). Unfortunately,
there is a large variation in the contents of water,
carbon and other volatiles between the different
kinds of chondritic meteorites and the Earth,
likely formed by accretion of a mixture of these
different materials, the precise fractions being
poorly constrained. Moreover, during accretion,
massive loss of volatiles to space likely occurred
caused by impacts. This volatile loss has to
be
values, while they are much lower than those
observed in comets. This limits the cometary
contribution to the terrestrial water and nitro-
gen budget to a few percent at most (Marty &
Yokochi, 2006).
Recent models of the Earth's formation (e.g.
Rubie
et al
., 2011) suggest that during accretion,
initially very volatile depleted chondritic mate-
rial accreted, which possibly became more water
and volatile-rich towards the end of accretion,
but still before core formation. Such models are
consistent with the observed depletion of mod-
erately volatile elements (e.g. Na, K, Zn) on the
Earth relative to CI chondrites; these elements
may have failed to condense in chondritic ma-
terial that formed close to the sun. Numerical
models of early solar system evolution suggest
that at later stages of accretion, stronger radial
mixing in the solar system occurred, so that
water and volatile-rich material from the cold
outer part of the solar system entered the growing
planet (Morbidelli
et al
., 2002). Taking all of the
available evidence together, it is plausible that
the Earth after complete accretion contained 1-5
ocean masses of water (Jambon & Zimmermann,
1990; Hirschmann, 2006). A major depletion of
hydrogen and other light elements by loss to
space during later Earth history can be ruled out,
because the expected depletions of light isotopes
resulting from such a distillation process are not
observed on the Earth.
Evidence on the present-day volatile content of
the Earth's mantle comes from direct studies of
mantle samples, particularly xenoliths, from mea-
surements of water contents in basalts, which are
partial melts formed in the shallow part of the up-
per mantle and from remote sensing by seismic
methods and magnetotelluric studies of electrical
conductivity. While the first two methods may
provide constraints on all volatiles, remote sens-
ing techniques are primarily sensitive to water
(Karato 2006).
Pyroxenes in mantle xenoliths that were
rapidly transported to the surface contain from
<
100 to about 1000 ppm of water (Skogby, 2006);
olivines may be nearly anhydrous but some-
times contain up to 300 ppm of water (Beran &
accounted
for,
which
introduces
another,
considerable uncertainty.
Estimating the volatile content of the bulk
mantle or of the bulk silicate Earth (crust
+
mantle) from cosmochemical arguments is even
more difficult, since the iron-nickel alloy of
the Earth's core very likely sequestered at least
some fraction of the available volatiles. Evidence
for this comes from the occurrence of sulfides
(troilite, FeS), carbides (cohenite, Fe
3
C) and ni-
trides (osbornite, TiN) as minerals in iron mete-
orites and from various experimental studies that
show that under appropriate conditions, carbon,
sulfur, nitrogen and hydrogen are quite soluble in
molten iron (Fukai, 1984; Wood, 1993; Okuchi,
1997; Adler & Williams, 2005; Terasaki
et al
.,
2011). Another line of evidence is the density
deficit of the Earth's outer core (Birch, 1952),
which requires the presence of some light ele-
ments in the iron nickel melt. While most present
models suggest that Si and/or O account for most
of the density deficit, a significant contribution
from other volatiles is possible. The recent model
by Rubie
et al
. (2011) yields 8 wt % Si, 2 wt % S
and 0.5 wt % O as light elements in the core. The
low oxygen content appears to be consistent with
shock wave data on melts in the Fe-S-O system
(Huang
et al
., 2011).
The timing of volatile acquisition on the Earth
is another poorly constrained variable. One type
of models assumes that volatiles were acquired
during the main phase of accretion, while another
view holds that volatiles, in particular water were
delivered to the Earth very late (Albar ede, 2009),
possibly during the formation of a ''late veneer'' of
chondritic materials or perhaps by comets. How-
ever, both the D/H and
15
N/
14
N isotope ratios of
terrestrial reservoirs are close to the chondritic
