Geoscience Reference
In-Depth Information
two-step redox process, and a very late accretion
of 0.8% of CI type material. Formation of the core
and mantle from fully oxidized chondritic mate-
rial is even more complex since little free iron is
present.
The accessible and sampled parts of Earth
are clearly deficient in elements more volatile
than Li and Si, and this includes Rb, Na and K.
However, in geochemical models based on noble
gases, these missing elements are assumed to be
sequestered in the lower mantle or core rather
than being deficient in the Earth as a whole.
Although there is no inconsistency -- from
a major element and refractory element point
of view -- in building the planets from known
meteorite classes, or from estimates of cosmic
abundances, part of the Earth may have accreted
from hydrous materials that are not represented
in meteorite collections; that is, they are no
longer extant in the inner Solar System, at least
as meteorite-size objects. This is not surprising.
Comet dust and interplanetary dust particles
(IDP) may be important for certain elements or
isotopes. The isotopic compositions of at least the
lighter rare gases are similar to those in IDPs
falling on the Earth today.
Table 3.7
Compositions of possible compo-
nents of the terrestrial planets (% or ppm)
Species
Cl
EC
HTC
SiO
2
30.9
39.1
20.2
TiO
2
0.11
0.06
1.9
Al
2
O
3
2.4
1.9
36.5
Cr
2
O
3
0.38
0.35
—
MgO
20.8
21.3
7.1
FeO
32.5
1.7
MnO
0.25
0.14
—
CaO
2.0
1.6
34.1
Na
2
O
1.0
1.0
K (ppm)
800
920
—
U (ppm)
0.013
0.009
0.19
Th (ppm)
0.059
0.034
0.90
Fe
0
26.7
—
N
1.3
1.7
—
S
8.3
4.5
—
Cl: Average Cl carbonaceous chondrite, on
a C-, H
2
O-free basis (Wood, 1962).
EC: Average enstatite chondrite (Wood, 1962).
HTC: High-temperature condensate (Gross-
man, 1972).
Minerals in mantles
6.0
38.5
EARTH
Several cosmochemical estimates of mineralogy
of Earth's mantle are given in Table 3.8. These
tend to be less rich in olivine than estimates of
the composition of the upper mantle (column 4)
and some estimates of the whole mantle, which
in turn are based on the assumption of a homoge-
nous mantle. The mineralogy changes with depth
because of solid--solid phase and compositional
changes.
Although Al
2
O
3
, CaO and Na
2
O are minor con-
stituents of the average mantle, their presence
changes the mineralogy, and this in turn affects
the physical properties. The effect on density can
result in chemical stratification of the mantle
and concentration of these, and related, elements
into certain layers. They also influence the melt-
ing point and tend to be concentrated in melts.
Olivine is an essential component in most
groups of meteorites except the irons. Pallasites
5.5
MERCURY
VENUS
5.0
M
=
28.3
27.0
25.1
4.5
4.0
22.4
MARS
3.5
MOON
0
−
0.5
−
1.0
−
1.5
−
2.0
Mass relative to Earth (log (M
/
M
o
))
Fig. 3.4
Mean density versus mass, relative to Earth, of
planets having the same structure
as
the Earth and various
metal/silicate ratios, expressed as M, mean atomic weight.
Earth and Venus have similar bulk chemistries while Mars and
Moon are clearly deficient in iron. Mercury is enriched in
iron. Earth and Venus might be considered
average terrestrial
planets
.
