Chapter
3
The
building
blocks
of
planets
For
as
the
sun
draws
into
himself
the
parts
of
which
he
has
been
composed,
so
earth
receives
the
stone
as
belonging
to
her,
and
draws
it
toward
herself
...
Plutarch
TI1e
Earth
is
part
of
the
solar
system
and
the
com-
position
of
the
Sun,
meteorites,
comets,
inter-
planetary
dust
particles
and
other
planets
pro-
yjde
information
that
may
be useful
in
deducing
the
overall
composition
of
our
planet,
most
of
which
is
inaccessible
to
direct
observation.
C
ar
-
b o n
ac
eou
s
chondrite
s
(CI)
appear
to
be
the
most
primitive
and
low
-
temperature
extraterres-
trial
(ET)
objects
available
to
us.
Even
if
these
were
the
only
building
blocks
of
planets
the
final
planet
would
differ
in
composition
from
them
because
of
vaporization
during
accretion,
and
loss
of
low-molecular-weight
material.
But
the
ratios
of
refractory
elements
in
ET
materials
may
proyjde
a
useful
constraint.
Since
most
of
the
vol-
ume
of
a
terrestrial
planet
is
oxygen.
the
oxygen
isotopes
of
candidate
materials
play
a key
role
in
deciding
how
to
assemble
a
planet.
Oxygen
iso-
topes
require
that
the
Earth
either
be
made
of
enstatite
met
eorites
or
a
mixture
of
meteorites
that
bracket
the
isotopic
composition
of
the
Earth
or
enstatite
meteorites
.
The
bulk
oxygen-isotopic
composition
of
the
Earth
precludes
more
than
a few
percent
of
carbonaceous
chondritic
mate-
rial
accreting
to
the
Earth
. Mars
has
a
different
oxygen-isotopic
composition
from
the
Earth,
sug-
gesting
that
distinct
oxygen
reservoirs
were
avail-
able
in
the
early
solar
system
over
relatively
small
annuli
of
heliocentric
distance.
Most
workers
assume
that
the
Earth
accreted
from
some
sort
of
primitive
material
delivered
to
the
Earth
as
meteorites
and
probably
originat-
ing
in
the
asteroid
belt.
But
it
is
understood
that
when
the
mantle
is
referred
to
as
hayjng
the
com-
position
of
CI
c h o
ndrit
es
,
or
chondritic,
it
is
usually
only
the
refractory
parts
that
are
meant.
In
the
cosmological
context
refractory
and
volatil
e
refer
to
the
condensation
temperature
in
a cool-
ing
nebula
of
sol
ar
composition.
The
Earth
is
clearly
deficient
in
elements
more
volatile
than
about
Si,
and
this
includes
Na
and
K.
In
some
models
constructed
by
noble
gas
geochemists,
however.
the
lower
mantle
is
actually
assumed
to
be
primordial
or
undegassed
and
to
approach
CI
in
overall
composition.
The
actual
material
form-
ing
the
Earth
is
unlikely
to
be
represented
by
a
single
meteorite
class.
It
may
be
a
mixture
of
vari-
ous
kinds
of
meteorites
and
the
composition
may
have
changed
with
time.
The
oxidation
state
of
accreting
material
may
also
have
changed
with
time.
Me
t
eor
it
es
Usin
g
terrestrial
samples,
we
cannot
see
very
far
back
in
time
or
very
deep
into
a
planet's
interior.
Meteorites
offer
us
the
opportunity
to
extend
both
of
these
dimensions.
Some
meteorites.
the
chondrites,
are
chemically
primitiv
e,
hayjng
com-
positions
-
volatile
elements
excluded
-
very
Table
3. 1
I
Compositions
of
chondrites
(wt.%)
Ordinary
Enstatite
H
L
Si
16.47-20.48
17.08
18.67
Ti
O.o3-0.Q4
0.06
om
AI
0.77-1.06
1.22
1.27
Cr
0.24-0.23
0.29
0.31
Fe
33.15-22.17
27.81
21.64
Mn
0.19-0.12
0.26
0.27
Mg
I 0.40-1
3.84
14.10
15.01
Ca
1.19-0.96
1.26
1.36
Na
0.75-0.67
0.64
0.70
K
0.09-0.05
0.08
O.Q9
p
0.30-0.15
0.15
0.15
Ni
1.83-1.29
1.64
1.10
Co
0.08-0.09
O.Q9
0.06
s
5.78-3.19
1.91
2.19
H
0.13
c
0.43-0.84
Fe
0
/Fe
tot
0.70-0.75
0.60
0.29
Mason
(1962).
similar
to
that
of
the
sun.
The
volatile-rich
car-
bonaceous
chondrites
are
samples
of
slightly
altered,
ancient
planetesimal
material
that
con-
densed
at
moderate
to
low
temperatures
in
the
solar
nebula.
The
nonchondritic
meteorites
are
differentiated
materials
of
nonsolar
composition
that
have
undergone
chemical
processing
like
that
which
has
affected
all
known
terrestrial
and
lunar
rocks.
Meteorites
are
assigned
to
three
main
categor-
ies.
Irons
(or
siderites)
consist
primarily
of
metal;
stones
(or
aerolites)
consist
of
silicates
with
little
metal;
stony
irons
(or
siderolites)
contain
abun-
dant
metal
and
silicates.
Carbonaceous
chondrites
Carbonaceous
chondrites
contain
high
abun-
dances
of
volatile
components
such
as
water
and
organic
compounds,
have
low
densities,
and
con-
tain
the
heavier
elements
in
ne
ar
ly
solar
pro-
portions.
They
also
conta
in
carbon
and
mag-
netite.
These
characteristics
show
that
they
have
not
been
strongly
heated,
compressed
or
altered
METEORITES
23
Carbonaceous
Cl
CM
co
cv
10.40
12.96
15.75
15.46
O.Q4
0.06
0.10
O.Q9
0.84
1.17
1.41
1.44
0.23
0.29
0.36
0.35
18.67
21.56
25.82
24.28
0
.1
7
0.16
0.16
0.16
9.60
11.72
14.52
14.13
1.01
1.32
1.57
1.57
0.55
0.42
0.46
0.38
0.05
0.06
0.10
om
0.14
0.13
0.11
0.13
l.o3
1.25
1.41
1.33
0.05
0.06
0.08
0.08
5.92
3.38
2.0
I
2.14
2.08
1.42
O.Q9
0.38
3.61
2.30
0.
31
1.08
0.00
0.00
0.09
0.11
since
their
formation;
that
is,
they
have
not
been
buried
deep
inside
planetary
objects.
The
CI
or
Cl
meteorites
are
the
most
extreme
in
their
primordial
characteristics
and
are
us
ed
to
supplement
solar
values
in
the
estimation
of
cosmic
composition.
The
other
categories
of
car-
bonaceous
chondrites.
CII
(CM)
and
CIII
(CO
and
CV),
are
le
ss
volatile-rich.
Some
carbonaceo
us
chondrites
contain
calcium-aluminum-rich
inclusions
(CAl),
which
appear
to
be
high-temperature
condensates
from
the
solar
nebula.
Theoretical
calculations
show
that
compounds
rich
in
Ca,
AI
and
Ti
are
among
the
first
to
con
dens
e
in
a
cooling
solar
nebul
a.
Highly
refractory
elements
are
strongly
enriched
in
the
CAl
compared
to
Cl
meteorites,
but
they
occur
in
Cl,
or
cosmic,
ratios.
Cl
'chond
rites'
are
fine
grained,
do
not
con-
tain
c
hondrules
and
are
chemically
simi
lar,
to
the
true
chondrites
(see
Table
3.1).
As
the
name
suggests,
ordinary
chondrites
are
more
abundant,
at
l
east
in
Earth-crossing
orbits,
than
all
other
types
of
meteorites.
They
are
chem-
ically
similar
but
differ
in
their
contents
of
iron
24
THE
BUILDING
BLOCKS
OF
PLANETS
Table
3.2
I
Normative
mineralogy
of
ordi-
nary
chondrites
(Mason,
1962)
Species
Hi
gh
Iron
Low
Iro n
Olivine
36.2
47.0
Hypersthene
24.5
22.7
Diopside
4.0
4.6
Feldspar
1
0.0
1
0.7
Apatite
0.6
0.6
Chromite
0.6
0.6
Ilm
en
i
te
0.2
0.2
Troilite
5.3
6
.1
Ni-Fe
18.6
7.5
Mason
(1962).
and
other
siderophiles,
and
in
the
ratio
of
oxi-
dized
to
metallic
iron.
As
the
amount
of
oxi-
dized
iron
decreases,
the
amount
of
reduced
iron
increases.
Olivine
is
the
most
abundant
mineral
in
chondrites,
followed
by
hypersthene,
feldspar,
nickel-iron
,
troilite
and
diopside
with
minor
apatite,
chromite
and
ilmenite
(Table
3 .
2).
The
composition
of
the
olivine
varies
widely,
from
0
to
30
mole
%
Fe
2
Si0
4
(Fa).
Enstatite
chondrites
are
distin
g
uished
from
ordinary
chondrites
by
lower
Mg/Si
ratios
(Table
3.3),
givi
n g
rise
to
a
min-
eralogy
dominated
by
MgSi0
3
and
havin
g
little
or
no
olivine.
They
formed
in
a
uniquely
reduc
i
ng
environment
and
contain
silicon-bearing
metal
and
very
low
FeO
silicates
.
They
contain
several
m
inera
ls
not
found
elsewhere
(CaS,
TiN,
Si
2
N
2
0).
In
spite
of
these
unusual
properties,
e
nstatite
chondrites
are
within
20
%
of
solar
composition
for
most
elements.
They
are
extre
mely
old
and
have
not
been
involved
in
major
planetary
pro-
cess
ing
.
They
have
been
suggested
as
possible
con-
stituents
of
the
Earth
becaus
e
of
their
high
free-
iron
content
,
their
oxidation
state
and
oxygen
iso-
topic
ratios.
If
Earth
is
to
be
made
out
of
a
single
meterorite
class,
the
enstatite
chondrites
are
the
closest
match.
Achon
drit
es
The
ac
h
ondrites
are
meteorites
of
igneous
ori-
gin
that
are
thought
to
have
been
dislodged
by
impact
from
small
bodies
in
the
solar
system.
Some
of
these
may
have
come
from
the
asteroid
belt,
ot
h
ers
are
almost
certainly
from
the
Moon,
Table
3.3
I
Element
ratios
(by
weight)
in
four
s
ubtyp
es
of
chondritic
meteorites
Ratio
C l
H
L
E6
AI/Si
0.080
0.063
0.063
0.044
Mg
/Si
0.91
0.80
0.79
0.71
Cal
AI
1.10
1.11
1.08
1
.06
Cr/Mg
0.025
O.Q25
0.026
0.024
and
one
subclass
(the
SNC
group)
have
apparently
come
from
Mars
.
Many
of
the
achondrites
crystal-
lized
between
4.4
and
4.6
billion
years
ago.
They
range
from
almost
monomineralic
olivine
and
pyroxene
rocks
to
objects
that
resemble
lun
ar
and
terrestrial
basalts.
Two
important
subgroups
,
classified
as
basaltic
achondrites,
are
the
eucrites
and
the
shergottites.
Two
gro
ups
of
meteoriti
c
breccias,
the
howardites
and
the
m
eso
siderit
es,
also
contain
basaltic
material.
The
eucrites,
howardites,
mesosiderites
and
diogenites
appear
to
be
related
and
may
come
from
different
depth
s
of
a
common
parent
body.
They
comprise
the
eucritic
association.
The
shergottites
,
nakhlites
and
chassignites
form
another
assoc
i
ation
and
are
collectively
called
the
SNC
meteorites
.
Eucrites
are
plagioclase-pyroxene
rocks
simi-
l
ar
to
basalts
and
have
textures
similar
to
basalt
s.
However
,
terrestrial
basalts
have
higher
abundan-
ces
of
sodium,
potassium,
rubidium
and
other
volatile
elements
and
have
more
calcium-rich
pyroxenes
.
Eucrite
p l
agioclase
is
richer
in
cal-
c
ium
and
poorer
in
sodium
than
terrestrial
bas
al
-
tic
feldspar
.
The
presence
of
free
iron
in
eucrites
demonstrates
that
they
are
more
reduced
than
terrestrial
basalts.
Studies
of
ba
sa
lts
from
the
Moon
and
the
eucrite
parent
body
have
severa
l
import
an
t
impli-
cations
for
the
ear
ly
history
of
the
Earth
and
the
other
terrestrial
p l
anets
.
They
show
that
even
very
small
bodies
can
melt
and
differentiate.
The
energy
source
must
be
due
to
impact,
rapid
accre-
tion,
short-lived
radioactive
isotopes
or
formation
in
a
hot
nebula.
The
widespread
occurrence
of
chondrules
in
chondritic
meteorites
a lso
is
evi-
dence
for
high
temperatures
and
melting
in
the
early
solar
system.
The
depletion
of
volatiles
in
eucrites
and
lunar
material
suggests
that
small
planet
and
the
early
planetesimal
stage
of
planet
forma-
tion,
may
be
characterized
by
volatile
loss.
T11ese
extraterrestrial
basalts
also
contain
evidence
that
free
iron
was
removed
from
their
source
region.
Alternatively,
these
objects
are
fragments
of
giant
impacts
that
caused
melting
and
silicate/iron
sep-
aration.
Nevertheless,
the
process
of
core
forma-
tion
must
start
very
early
and
is
probably
con-
temporaneous
with
accretion.
Shergottites
are
remarkably
similar
to
ter-
restrial
basalts.
TI1ey
are
unusual
,
among
mete-
orites,
for
having
very
low
crystallization
ages,
about
10
9
years,
and,
among
basalts.
for
having
abundant
shocked
plagioclase
.
The
shergottites
are
so
similar
to
terrestrial
basalts
that
their
source
regions
must
be
similar
to
the
upper
mantle
of
the
Earth.
The
similarities
extend
to
the
trace
elements,
be
they
refractory,
volatile
or
siderophile,
suggesting
a
similar
evolution
for
both
bodies
.
The
young
crystallization
ages
imply
that
the
shergottites
are
from
a
large
body
,
one
that
could
maintain
igneous
processes
for
3
billion
years.
Cosmic-ray-exposure
ages
show
that
they
were
in
space
for
several
million
years
after
ejection
from
their
parent
body
.
Shergottites
are
slightly
richer
in
iron
and
manganese
than
terrestrial
basalts,
and,
in
this
respect,
they
are
similar
to
the
eucrites.
They
con-
tain
no
water
and
have
different
oxygen
isotopic
compositions
than
terrestrial
basalts.
T11e
major-
element
chemistry
is
similar
to
that
inferred
for
the
martian
soil.
The
rare-gas
contents
of
sher-
gottites
are
similar
to
the
martian
atmosphere,
giving
strong
circumstantial
support
to
the
idea
that
these
meteorites
may
have
come
from
the
surface
of
Mars.
In
any
case,
these
meteorites
provide
evidence
that
other
objects
in
the
solar
system
have
similar
chemistries
and
undergo
sim-
ilar
processes
as
the
Earth's
upper
mantle
.
The
growing
Earth
probably
always
had
basalt
at
the
surface
and,
consequently.
was
continu-
ously
zone-refining
the
incompatible
elements
toward
the
surface.
The
corollary
is
that
the
deep
interior
of
a
planet
is
refractory
and
depleted
in
volatile
and
incompatible
elements.
T11e
main
difference
between
the
Earth
and
the
other
ter-
restrial
planets,
including
any
meteorite
parent
body,
is
that
the
Earth
can
recycle
material
back
into
the
interior.
Present-day
basalts
on
Earth
may
be
recycled
basaltic
material
that
formed
COSMIC
ABUNDANCES
25
during
accretion
and
in
early
Earth
history
rather
than
initial
melts
from
a
previously
unprocessed
peridotitic
parent.
Indeed,
no
terrestrial
basalt
shows
evidence,
if
all
the
isotopic
and
geochem-
ical
properties
are
taken
into
account,
of
being
from
a
primitive,
undifferentiated
reservoir.
Cosmic
abundances
The
Sun
and
planets
probably
formed
more
or
less
contemporaneously
from
a
common
mass
of
interstellar
dust
and
gas.
There
is
a
close
sim-
ilarity
in
the
relative
abundances
of
the
con-
densable
elements
in
the
atmosphere
of
the
Sun,
in
chondritic
meteorites
and
in
the
Earth.
To
a first
approximation
one
can
assume
that
the
planets
incorporated
the
condensable
elements
in
the
proportions
observed
in
the
Sun
and
the
chondrites.
On
the
other
hand,
the
differences
in
the
mean
densities
of
the
planets,
corrected
for
differences
in
pressure,
show
that
they
can-
not
all
be
composed
of
materials
having
exactly
the
same
composition.
Variations
in
iron
con-
tent
and
oxidation
state
of
iron
can
cause
large
density
variations
among
the
terrestrial
plan-
ets.
TI1e
giant,
or
Jovian
planets.
must
contain
much
larger
proportions
of
low-atomic-weight
e l
ements
than
Mercury,
Venus.
Earth,
Moon
and
Mars.
With
the
exception
of
a
few
elements
such
as
Li,
Be
and
B,
the
composition
of
the
solar
atmosphere
is
essentially
equal
to
the
composi-
tion
of
the
material
out
of
which
the
solar
system
formed.
T11e
planets
are
assumed
to
accrete
from
material
that
condensed
from
a
cooling
prim-
itive
solar
nebula
. Various
attempts
have
been
made
to
compile
tables
of
'co
smic'
abundances.
The
Sun
contains
most
of
the
mass
of
the
solar
system;
therefore
,
when
we
speak
of
the
elemen-
tal
abundances
in
the
solar
system,
we
really
refer
to
those
in
the
Sun
.
The
spectroscopic
anal-
yses
of
elemental
abundances
in
the
solar
photo-
sphere
do
not
have
as
great
an
accuracy
as
chem-
ical
analyses
of
solid
materials
.
Carbonaceous
chondrite
meteorites.
which
appear
to
be
the
most
representative
samples
of
the
relatively
nonvolatile
constituents
of
the
solar
system,
are
used
for
compilations
of
the
abundances
of
most
of
the
elements
(Tables
3.4
to
3.6).
For
the
very
26
THE
BUILDING
BLOCKS
OF
PLANETS
Table
3.4
\
Cosmic
abundances
of
the
elements
(Atoms/10
6
Si)
I
H
2.72
X
I 0
10
24
Cr
1.34
X
I 0
4
48
Cd
1.69
72
Hf
0.176
2
He
2.
18
X
I 0
9
25
Mn
95
10
49
In
0.184
73
Ta
0.0226
3
Li
59.7
26
Fe
9.00
X
10
5
50
Sn
3.82
74
w
0.
137
4
Be
0.78
27
Co
2250
51
Sb
0.352
75
Re
0.
0507
5
B
24
28
N i
4
.93
X
10
4
52
Te
4.91
76
O s
0.717
6
c
1.
2 1
X
10
7
29
Cu
514
7
N
2.48
X
I 0
6
30
Zn
1
260
8
0
2.01
X
10
7
31
G a
37.8
9
F
843
32
Ge
11
8
10
Ne
3.76
X
10
6
33
A
s
6.79
II
Na
5.70
X
I 0
4
34
Se
62
.1
12
Mg
1.075
X
10
6
35
Br
11
.8
13
AI
8.49
X
I 0
4
36
Kr
45.3
14
Si
I.
QQ
X
10
6
37
Rb
7.09
IS
p
1.
04
X
10
4
38
Sr
23.8
16
s
5.15
X
10
5
39
y
4.64
17
Cl
5240
40
Zr
10
.7
18
Ar
1.04
X
10
5
41
Nb
0.7
1
19
K
3770
42
Mo
2.52
20
Ca
6
.11
X
10
4
44
Ru
1.86
2 1
Sc
33.8
45
Rh
0.344
22
Ti
2400
46
Pd
1.39
23
v
295
47
AG
0.529
And
e rs
and
Ebihara
(1982)
.
abundant
volatile
elements,
solar
abunda
nce
values
are
used.
The
very
light
and
volatile
elements
(H,
He,
C,
N)
are
extremely
depleted
in
the
Earth
rel-
at
ive
to
the
Sun
or
carbonaceous
chondrites
.
Moderately
volatile
elements
(such
as
K,
Na,
Rb,
Cs
and
S)
are
moderately
depleted
in
the
Earth
.
Refractory
elements
(such
as
Ca,
AI
,
Sr,
Ti,
Ba,
U
and
Th)
are
generally
assumed
to
be
retained
by
the
planets
in
their
cosmic
ratios
.
It
is
also
likely
that
magnesium
and
silicon
occur
in
a
planet
in
chondritic
or
cosmic
ratios
with
the
more
refractory
elements
.
The
Mg/Si
ratio
, how-
ever,
varies
somewhat
among
meteorite
classes.
Sometimes
it
is
assumed
that
magnesium
,
iron
and
silicon
may
be
fractionated
by
accretional
or
pre-accretional
processes,
but
these
effects,
if
they
exist,
are
slight.
The
upper
mantle
of
the
Earth
is
olivine-rich
and
has
a
high
Mg/Si
ratio
compared
with
the
cosmic
ratio
(Figures
3.1
to
3.3).
If
the
Earth
is
53
I
0.90
77
lr
0.660
54
Xe
4.35
78
Pt
1.37
55
Cs
0.372
79
Au
0.186
56
Ba
4.36
80
HG
0.52
57
La
0.448
8 1
Tl
0.184
58
Ce
1.16
82
Pb
3.15
59
Pr
0.174
83
Bi
0.144
60
Nd
0.836
90
Th
0.0335
62
Sm
0.26
1
92
u
0.0090
63
Eu
0.
0972
64
Gd
0.33
1
65
Tb
0.0589
66
Dy
0.398
67
Ho
0.0875
68
Er
0.253
69
Tm
0.
0386
70
Yb
0.243
71
Lu
0.0369
chondritic
in
major-element
chemistry
,
then
the
deeper
mantle
must
be
rich
in
pyroxene
and
garnet
and
their
hi
gh-pressure
phases
.
Figure
3.3
is
a
schematic
illustration
of
how
the
original
accreting
silicate
material
of
a
planet
(primitive
mantl
e)
may
fractionate
into
a
melt
(magma
ocean)
and
dense
refractory
crystals
.
Crystallization
of
the
magma
ocean
creates
the
m
ateria
ls
that
we
sample
from
the
upper
mantle
.
In
a l
arge
planet
,
the
original
differentiation
may
be
irreversible
because
of
the
effects
of
pressure
on
material
properties,
such
as
the
thermal
expansion
coefficient.
Composition
of
the
terrestrial
planets
The
mean
uncompressed
densities
of
the
ter-
restrial
planets
decreases
in
the
order
Mercury,
Earth,
Venus,
Mars,
Moon
(Figure
3.4).
Some