Geoscience Reference
In-Depth Information
Table 3.5
Solar and cosmic abundances in
atoms/1000 Si atoms
Table 3.6
Short table of cosmic abundances
(Atoms/Si)
Corona Photosphere
'Cosmic'
Cameron
Anders and Ebihara
Z Element
(1)
(1)
(2)
Element
(1982)
(1982)
6
C
2350
6490
12 100
O
18.4
20.1
7
N
700
2775
2480
Na
0.06
0.057
8
O
5680
22 900
20 100
Mg
1.06
1.07
9
F
0.28
1.1
0.843
Al
0.085
0.0849
10
Ne
783
3140
3760
Si
1.00
1.00
11
Na
67.0
67.0
57.0
K
0.0035
0.003 77
12
Mg
1089
1089
1075
Ca
0.0625
0.0611
13
Al
83.7
83.7
84.9
Ti
0.0024
0.0024
14
Si
1000
1000
1000
Fe
0.90
0.90
15
P
4.89
9.24
10.4
Ni
0.0478
0.0493
16
S
242
460
515
17
Cl
2.38
9.6
5.24
18
Ar
24.1
102
104
estimated from lead isotope data. The amount
of argon-40 in the atmosphere provides a lower
bound on the amount of potassium in the crust
and mantle. Most of these are very weak con-
straints, but they do allow rough estimates to be
made of the refractory, siderophile, volatile and
other contents of the Earth and terrestrial plan-
ets. The elements that are correlated in magmatic
processes have very similar patterns of geochem-
ical behavior, even though they may be strongly
fractionated during nebular condensation. Thus,
some abundance patterns established during con-
densation tend not to be disturbed by subsequent
planetary melting and igneous fractionation. On
the other hand, some elements are so strongly
fractionated from one another by magmatic
and core formation processes that discovering a
'cosmic' or 'chondritic' pattern can constrain the
nature of these processes.
The outer planets and satellites are much
more volatile-rich than the inner planets. Mete-
orites also vary substantially in composition and
volatile content. The above considerations sug-
gest that there may be an element of inhomo-
geneity in the accretion of the planets, perhaps
caused by temperature and pressure gradients
in the early solar nebula. Early forming plan-
etesimals would have been refractory- and iron-
rich and the later forming planetesimals more
volatile-rich. If planetary accretion was occurring
simultaneously with cooling and condensation,
19
K
3.9
3.9
3.77
20
Ca
82
82
61.1
21
Sc
0.31
0.31
0.034
22
Ti
4.9
4.9
2.4
23
V
0.48
0.48
0.295
24
Cr
18.3
18.3
13.4
25
Mn
6.8
6.8
9.51
26
Fe
1270
1270
900
27
Co
18.1
18.1
2.25
<
<
28
Ni
46.5
46.5
49.3
29
Cu
0.57
0.57
0.514
30
Zn
1.61
1.61
1.26
(1) Breneman and Stone (1985).
(2) Anders and Ebihara (1982).
typical compositions of possible components of
the terrestrial planets are listed in Table 3.7.
There are some constraints on the amounts or
ratios of a number of key elements in a planet.
For example, the mean density of a planet, or
the size of the core, constrains the iron con-
tent. Using cosmic ratios of elements of simi-
lar geochemical properties (say Co, Ni, refractory
siderophiles), a whole group of elements can be
constrained. The uranium and thorium content
are constrained by the heat flow and thermal his-
tory calculations. The K/U ratio, roughly constant
in terrestrial magmas, is a common constraint
in this kind of modeling. The Pb/U ratio can be
