Geoscience Reference
In-Depth Information
Pyroxene and olivine
Plagioclase
Anorthositic crust
Magma ocean
Lower mantle
High-Ti basalts
KREEP basalts
Gabbros and anorthosites
Pyroxene-rich cumulates
Figure 12.15
The magma ocean from which the lunar mantle and crust originated. Heat from the impact that
separated the Earth and the Moon left the outer part of the Moon molten. Plagioclase feldspar
floated to create the anorthositic crust (lunar highlands), while olivine and pyroxene settled out
to form the mantle. The KREEP basalts derived from the last residual liquid accumulated beneath
the crust. Basalts from lunar maria derive from the subsequent remelting of mantle cumulates.
that the LHB was an isolated event which resulted from the destabilization of small plan-
etary bodies in the outer Solar System upon outward migration of the giant planets. The
LHB produced a consolidated rubble known as breccia, which is particularly abundant
in the samples brought back by the astronauts. Chemical maps produced by the satel-
lites Clementine and Lunar Prospector show that the ejecta forming the wall of the Mare
Imbrium are particularly rich in KREEP. The surface of the Moon is covered with regolith,
a thick soil created by incessant meteoritic bombardment (gardening) of a body with no
protective atmosphere.
The best documented and most popular model for the formation of lunar rocks is that
of the magma ocean (
Fig. 12.15
)
. Observations on the first samples of breccia, brought
back by Apollo 11, suggested that the relatively abundant anorthosite is an essential
component of the surface, while lunar mare basalt chemistry indicated a form of basalt-
anorthosite complementarity. It is thought that after the impact that formed the Moon some
