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generally in excess of 200 km, and mass-balance
calculations require that most or all of the Moon
has experienced partial melting and melt extrac-
tion. Evidence in support of the magma ocean
concept, or at least widespread and extensive
melting, include: (1) the complementary high-
land and mare basalt trace-element patterns, par-
ticularly the europium anomaly; (2) the enrich-
ment of incompatible elements in the crust and
KREEP; (3) the isotopic uniformity of KREEP; and
(4) the isotopic evidence for early differentiation
of the mare basalt source region, which was com-
plete by about 4.4 Ga.
The separation of crustal material and frac-
tionation of trace elements is so extreme that the
concept of a deep magma ocean plays a central
role in theories of lunar evolution. The cool-
ing and crystallization of such an ocean permits
efficient separation of various density crystals
and magmas and the trace elements that accom-
pany these products of cooling. This concept
does not require a continuous globally connected
ocean that extends to the surface nor one that
is even completely molten. Part of the evidence
for a magma ocean on the Moon is the thick
anorthositic highland crust and the widespread
occurrence of KREEP, an incompatible-element-
rich material best interpreted as the final liquid
dregs of a Moon-wide melt zone. The absence of
an extensive early terrestrial anorthositic crust
and the presumed absence of a counterpart to
KREEP have kept the magma ocean concept from
being adopted as a central principle in theories
of the early evolution of the Earth. However, a
magma ocean is also quite likely for the Earth
and probably the other terrestrial planets as well.
Tables 2.1 and 2.2 gives comparisons between
the crusts of the Moon and the Earth. In spite of
the differences in size, the bulk composition and
magmatic history of these two bodies, the prod-
ucts of differentiation are remarkably similar.
The lunar crust is less silicon rich and poorer in
volatiles, probably reflecting the overall depletion
of the Moon in volatiles. The lunar-highland crust
and the mare basalts are both more similar to
the terrestrial oceanic crust than to the continen-
tal crust. Depletion of the Moon in siderophiles
and similarity of the Earth and Moon in oxygen
isotopes are consistent with the Moon forming
Table 2.1
Crustal compositions in the Moon
and Earth
Lunar
Highland
Crust
Lunar
Mare
Basalt
Terrestrial
Continental
Crust
Terrestrial
Oceanic
Crust
Major Elements (percent)
Mg
4.10
3.91
2.11
4.64
Al
13.0
5.7
9.5
8.47
Si
21.0
21.6
27.1
23.1
Ca
11.3
8.4
5.36
8.08
Na
0.33
0.21
2.60
2.08
Fe
5.1
15.5
5.83
8.16
Ti
0.34
2.39
0.48
0.90
Refractory Elements (ppm)
Sr
120
135
400
130
Y
13.4
41
22
32
Zr
63
115
100
80
Nb
4.5
7
11
2.2
Ba
66
70
350
25
La
5.3
6.8
19
3.7
Yb
1.4
4.6
2.2
5.1
Hf
1.4
3.9
3.0
2.5
Th
0.9
0.8
4.8
0.22
U
0.24
0.22
1.25
0.10
Volatile Elements (ppm)
K
600
580
12,500
1250
Rb
1.7
1.1
42
2.2
Cs
0.07
0.04
1.7
0.03
Taylor (1982), Taylor and McLennan (1985).
from the Earth's mantle, after separation of the
core.
The origin of the Moon
Prior to the Apollo landings in 1969 there were
three different theories of lunar origin. The fis-
sion theory, proposed by G. H. Darwin, Charles
Darwin's son, supposed that the moon was spun
out of Earth's mantle during an early era of rapid
Earth rotation. The capture theory supposed that
the moon formed somewhere else in the solar
system and was later captured in orbit about the
Earth. The co-accretion or 'double planet' theory
supposed that the Earth and moon grew together
out of a primordial swarm of small 'planetes-
imals.' All three theories made predictions at
