242 Encyclopedia of the Solar System
breccias range in age from 3.9 to 4.3 billion years. The oldest
basalt from a visible maria isApollosample number 10003, a
low-K basalt from Mare Tranquilitatis with an age of 3.86±
0.03 billion years. This gives a younger limit for the age of the
Imbrium collision because the lavas of Mare Tranquilitatis
overlie the Imbrium ejecta blanket.
The youngest dated sample is number 12022, an ilmenite
basalt with an age of 3.08±0.05 billion years, although
some doubtful younger ages are in the literature. Low-Ti
basalts are generally younger than high-Ti basalts. Strati-
graphically younger flows, some of which appear to embay
young ray craters, may be as young as 1 billion years but
are of very limited extent. The most voluminous period of
eruption of lavas appears to have been between about 3.8
and 3.1 billion years ago. Isotopic measurements show that
the mare basalt source regions formed at about 4.4 billion
years, and this age must represent the solidification of much
of the magma ocean.
7.2 Composition of the Mare Basalts
The basic classification is chemical, with finer subdivisions
based on mineral composition. The basalts are divided into
low-Ti, high-Ti, and high-Al basalts. The low-Ti basalts in-
clude VLT (very-low-Ti), olivine, pigeonite, and ilmenite
basalts. The high-Ti basalts include high-K, low-K, and
VHT (very-high-Ti) basalts. TheClementinedata suggest
that there is a continuous variation in Ti contents. The ma-
jor minerals are pyroxene, olivine (Mg-rich), plagioclase
(Ca-rich), and opaques, mainly ilmenite. The basalts are
highly reduced, with oxygen fugacities of 10−^14 at 1100◦C
or about a factor of 10^6 lower than those of terrestrial basalts
at any given temperature. Ferric iron is effectively absent,
and 90% of Cr and 70% of Eu in the Moon is divalent.
An alloy of FeNi metal is a common late-stage crystallizing
phase.
In comparison with terrestrial basalts, the silica contents
of mare basalts are low (37–45%), and the lavas are iron-
rich (18–22% FeO). The lunar basalts are notably high in
Ti, Cr, and Fe/Mg ratios and low in Ni, Al, Ca, Na, and
K compared with terrestrial counterparts (Table 2). They
are depleted in volatile (e.g., K, Rb, Pb, Bi) and siderophile
(e.g., Ni, Co, Ir, Au) elements. The ratio of volatile (e.g., K)
to refractory elements (e.g., U) is low. Thus, lunar K/U ratios
average about 2500 compared to terrestrial values of about
12,000. The rare earth elements (REEs) display a charac-
teristic depletion in divalent Eu or europium anomaly (Fig.
19). This is one of the several pieces of evidence that the
mare basalts come from a previously differentiated interior,
rather than being melted from a primitive undifferentiated
lunar composition. Even the lunar glasses that may come
from deeper show the tell-tale evidence of depletion in Eu,
indicating that they too come from a differentiated interior.
The differences in composition of the mare basalts are
mostly due to source region heterogeneity, with only mi-
nor evidence for near-surface fractionation. Variations in
the amount of partial melting from a uniform source, sub-
sequent fractional crystallization, or assimilation cannot ac-
count for the observed diversity. Some mare basalts are
vesicular, evidence for a now-vanished gas phase, usually
thought to be CO.7.3 Origin of the Mare Basalts
Mare basalts originate by partial melting, at temperatures of
about 1200◦C, deep in the lunar interior (see Fig. 8), prob-
ably at depths between 200 and 400 km. The lunar volcanic
glasses appear to come from greater depths, but still from
a differentiated source. The basalts are derived from the
zones and piles of cumulate minerals developed, at various
depths, during crystallization of the magma ocean. The iso-
topic systematics of the mare basalts indicate that the source
region had crystallized by 4.4 billion years. Partial melting
occurred in these diverse mineral zones some hundreds of
millions of years later due to the slow buildup of heat from
the presence of the radioactive elements K, U, and Th. The
melting was not extensive. Over 25 distinct types of mare
basalt were erupted over an interval of more than 1 billion
years, but the total amount of melt so generated amounted
to only about 0.1% of the volume of the Moon. This forms
a stark contrast to the state of the Moon at accretion, when
it may have been entirely molten.8. Lunar Highland Crust
Most of the rocks returned from the highlands are polymict
breccias, pulverized by the massive bombardment. How-
ever, some monomict breccias have low siderophile ele-
ment contents. These are considered to be “pristine” rocks
that represent the original igneous components making up
the highland crust. Three pristine constituents make up the
lunar highland crust, namely, ferroan anorthosites that are
the dominant component, the Mg suite, and KREEP.8.1 Ferroan Anorthosite
Ferroan anorthosite is the single most common pristine
highland rock type, making up probably 80% of the high-
land crust. The pristine clasts in lunar meteorites are
mostly ferroan anorthosites. The major component (95%)
is highly calcium-rich plagioclase, typically An 95 – 97 with
a pronounced enrichment in Eu (Eu/Eu∗=50). Low-
Mg pyroxene is the next most abundant mineral, but the
mafic minerals are only minor constitutents in this nearly
monomineralic feldspathic rock. The anorthosites are typ-
ically coarsely crystalline with cumulate textures. Reliable