244 Encyclopedia of the Solar System
FIGURE 19 The abundances of rare earth elements in the
source regions of the mare basalts, the highland crust, and
KREEP, relative to bulk moon concentrations. These patterns
result from the preferential entry of divalent europium (similar
radius to strontium) into plagioclase feldspar. This mineral floats
to form the highland crust, and so depletes the interior of Eu.
Mare basalts that subsequently erupted from this region deep
within the Moon bear the signature of this early depletion.
KREEP is the final residue of the crystallization of the magma
ocean. It is strongly depleted in Eu owing to prior crystallization
of plagioclase and is enriched in the other rare earth elements
(e.g., K, U, Th, Ba, Rb, Cs, Zr, P) that are excluded from olivine,
pyroxene, and ilmenite during the crystallization of the major
mineral phases of the magma ocean.
The source of the highly evolved component is clearly
KREEP. The source of the “primitive” Mg-rich component
is less obvious. If the primitive component came from deep
cumulates, the concentrations of Ni in olivine of the Mg
suite are low, not high as predicted. Conventional theories
propose that the Mg suite arose as separate plutons that
intruded the crust as separate igneous intrusions. However,
all Mg suite rocks have parallel REE patterns, a character-
istic compatible with mixing, but not expected to occur in
separate igneous intrusions. This is a major constraint on the
concept that the lunar highland crust formed through “serial
magmatism.” Furthermore, it is of interest that the Mg suite
contains Mg-rich orthopyroxene, a mineral that is lacking
in mare basalts. Clearly the Mg suite originates in a location
distinct from the source region of the mare basalts. During
crystallization of the magma ocean, Mg-rich minerals (e.g.,
olivine and orthopyroxene) are among the first to crystallize
and accumulate on the bottom of the magma chamber, in
this case at depths exceeding 400 km. It is sometimes sug-
gested that massive overturning has occurred to bring these
within reach of the surface. However, the magma ocean had
completed crystallization by 4400 million years with only the
KREEP component remaining liquid until about 4360 mil-
lion years; it was solid at the time of the formation of the Mg
suite. There is no obvious source of energy for remelting
early refractory Mg-rich cumulates. Such material may have
been derived from a late infall ofplanetesimalsthat might
provide both the primitive component and the energy for
melting. Subsequent melting to produce mare basalts took
place in more differentiated cumulates and produced lavas
with a different mineralogy (e.g., lacking orthopyroxene),
without the primitive characteristics of the Mg suite.8.3 Alkali Suite
A rare component of the highlands crust is the Alkali suite.
The largest sample is 1.6 gm and they seem to have un-
dergone severe thermal metamorphism but their origin is
not well understood. They are commonly 85% plagioclase
feldspar, the remainder being mostly pyroxene. Their signif-
icant feature is an enrichment in the alkali elements so that
they contain Na-rich rather than Ca-rich feldspar. They are
probably related to KREEP as the trace element patterns
are similar.8.4 Kreep
KREEP is enriched in potassium, rare earth elements, and
phosphorus, hence the name. It is commonly applied as
an adjective to refer to highland rocks with an enhanced
and characteristic trace element signature. KREEP origi-
nated as the final 2% or so melt phase from the crystalliza-
tion of the magma ocean and is strongly enriched in those
“incompatible” trace elements excluded from the major
mineral phases (olivine, orthopyroxene, clinopyroxene, pla-
gioclase, ilmenite) during crystallization of the bulk of the
magma ocean. This residual phase was the last to crystal-
lize, at about 4.36 billion years, and apparently pervaded
the crust, with which it was intimately mixed by cratering.
Its presence tends to dominate the trace element chem-
istry of the highland crust. Extreme REE enrichment up
to 1000 times the chondritic (or solar nebula abundances)
are known (see Fig. 19). This extreme concentration of trace
elements amounts to a significant part of the total lunar bud-
get and so provides strong evidence for the magma ocean
hypothesis and for large-scale lunar melting.