Encyclopedia of the Solar System 2nd ed

The Moon 243

TABLE 2 Elemental Abundancesa

Oxide (weight Cl Earth Mantle Bulk Highlands Low-Ti High-Ti

percent) +Crust Moon Basalt Basalt

SiO 2 34.2 49.9 47 45.0 43.6 37.8

TiO 2 0.11 0.16 0.3 0.56 2.60 13.0

Al 2 O 3 2.44 3.64 6.0 24.6 7.87 8.85

FeO 35.8 8.0 13.0 6.6 21.7 19.7

MgO 23.7 35.1 29 6.8 14.9 8.44

CaO 1.89 2.89 4.5 15.8 8.26 10.7

Na 2 O 0.98 0.34 0.09 0.45 0.23 0.36

K∑ 2 O 0.10 0.02 0.01 0.03 0.05 0.05

99.2 100.1 99.9 100 100.4 99.5

Volatile elements

K (ppm) 854 180 83 200 420 500

Rb (ppm) 3.45 0.55 0.28 0.7 1.0 1.2

Cs (ppb) 279 18 12 20 40 30

Moderately volatile element

Mn (ppm) 2940 1000 1200 570 2150 2080

Moderately refractory element

Cr (ppm) 3975 3000 4200 800 5260 3030

Refractory elements

Sr (ppm) 11.9 17.8 30 130 101 121

U (ppb) 12.2 18 30 80 220 130

La (ppm) 0.367 0.55 0.90 2.0 6.0 5.22

Eu (ppm) 0.087 0.13 0.21 1.0 0.84 1.37

V (ppm) 85 128 150 30 175 50

Siderophile elements

Ni (ppm) 16500 2000 400 100 64 2

Ir (ppb) 710 3.2 0.01 — 0.02 0.04

Mo (ppb) 1380 59 1.4 5 50 50

Ge (ppm) 48.3 1.2 0.0035 0.02 0.003 0.003

aElemental abundances in Cl chondrites (volatile=primitive solar nebula). Earth mantle+crust=primitive Earth

mantle; bulk Moon; average lunar highland crust; low-Ti mare basalt (12002) and high-Ti mare basalt (70215). Both of these

later samples are probably primary basaltic magmas. Data sources from Taylor (1982) and Hartmann et al. (1986).

ages of 4440±20 million years and 4460±40 million
years have been obtained, and their average of 4450 mil-
lion years is taken to represent the crystallization age of
ferroan anorthosites from the lunar magma ocean and the
flotation of the feldspathic highland crust as “rockbergs.”
Alternatively, this date may represent the “isotopic closure
age” during cooling of the crust.

8.2 Mg Suite

The Mg suite comprises norites, troctolites, dunites, spinel
troctolites, and gabbroic anorthosites. They are character-
ized by higher, and so more primitive, Mg/ (Mg+Fe^2 +)
ratios compared to the ferroan anorthosites. They range in

age from about 4.44 billion years down to about 4.2 billion

years, but typical ages are 100–200 million years younger

than those of the ferroan anorthosites. The Mg suite is pet-

rographically distinct from the older ferroan anorthosites

and does not appear to be related directly to the crystal-

lization from the magma ocean. It makes up a minor but

significant proportion (perhaps 10%) of the highland crust

and has two distinct and contradictory components in terms

of conventional petrology. It is Mg-rich, and so primitive

in terms of igneous differentiation, but also contains high

concentrations of incompatible elements, typical or highly

evolved or differentiated igneous systems. These charac-

teristics point to an origin by mixing of these two distinct

components.