Geoscience Reference
In-Depth Information
15.0
Depleted mantle
12.5
Mare basalts
10.0
Apollo 17
7.5
Apollo 12
5.0
Apollo 11
2.5
Apollo 15
0.0
Intrusive
highland
rocks
KREEP
−
2.5
Enriched crust
−
5.0
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
Age (Ga)
Figure 12.17
Evolution of the Sm-Nd isotope system on the Moon. Anorthosite of the lunar maria and KREEP
basalts indicate geochemical “enrichment” (low Sm/Nd ratio and therefore unradiogenic Nd). By
contrast, Nd of the source of lunar maria basalts is radiogenic, indicating that the mantle source is
geochemically depleted, which is consistent with the hypothesis of pyroxene accumulation.
Compare this with the evolution of the Earth's mantle and crust (
Figs. 11.16
and
11.20
).
How did the basalts of the lunar maria form in this process? Their relatively young ages
exclude their being erupted out of the magma ocean system and their modest enrichment
in incompatible elements indicates that they do not represent residual liquids. Nonetheless,
these basalts exhibit a negative Eu anomaly indicating that their mantle source is a com-
plement to the lunar highland crust. The most common interpretation is that they formed
by remelting, at around 3.8-3.2 billion years, of pyroxene-rich cumulates present in great
quantities in the mantle and produced by crystallization of the magma ocean and accu-
mulated in the lowest points of the topography, the impact craters. The highly variable
titanium contents of these basalts show that another mineral, ilmenite (FeTiO
3
), was also
present in very variable quantities at the time of melting. The sources of these lavas in the
lunar mantle therefore represent cumulates of a magma ocean at relatively advanced stages
of crystallization.
The essential features of lunar mantle differentiation are the saturation in plagioclase of
the magma ocean and its lack of water. On Earth, basaltic magmas can precipitate plagio-
clase down to about 30 km. On the Moon, where gravity is six times less, the equivalent
pressure is only reached at about 180 km. It can be seen therefore that potential plagioclase
production by lunar magmas is much more substantial than on Earth and that the potential
production of anorthositic crust is much greater on the Moon.
