Geoscience Reference
In-Depth Information
10000
Plagioclase flotation
1000
KREEP basalts
100
High-Ti basalts
(maria)
10
1
0.1
Anorthosite (highlands)
0.01
Cs Rb Ba Th U Nb La Ce Pr Sr Nd Sm Zr Hf Eu Gd Tb Dy
Y Ho Er Tm Yb Lu
Figure 12.16
Trace elements in lunar rocks. Anorthosite and gabbro of the lunar highlands exhibit surplus Eu
and Sr from the accumulation of plagioclase by buoyancy (cf. the partition coefficient of
Fig. 2.5
).
The KREEP basalts exhibit negative anomalies complementary to those of anorthosite. Lunar
maria basalt, being older, displays similar negative anomalies inherited from the source in the
mantle and depleted in plagioclase.
4.56 billion years ago, its mantle was largely molten. As this enormous mass of magma
cooled, which may have taken several hundred million years, all of its upper part was
saturated in plagioclase, olivine, and pyroxene. Plagioclase, being lighter than the liquid,
floated to form the highland crust, while the heavier olivine and pyroxene sedimented out,
filling the magma ocean from the bottom upward. The last residual liquids, particularly
rich in incompatible elements (including K, P, the rare-earths, and Ti), subsisted for several
hundred million years beneath the anorthositic crust. What arguments are there to sup-
port this model? The presence of a strong positive europium anomaly in anorthosites and
other intrusive rocks, while KREEP and other basalts display very marked negative anoma-
lies (
Fig. 12.16
). Incorporation of Eu in plagioclase is very efficient in comparison with
other rare-earths (
Fig. 2.5
). Such a large anomaly cannot be generated by plagioclase frac-
tionation from the basalts themselves, and must already be present in the source of these
melting event (the magma ocean). Efficient segregation of plagioclase by buoyancy from a
magmatic liquid was at the origin of the intrusive rocks of the lunar highlands. The KREEP
basalts are formed out of a mantle impregnated with residual liquids: the very high content
of their mantle source in heat-producing elements (U, Th, K) may actually have kept this
particular layer partially molten for billions of years. This model is supported by the con-
trasting unradiogenic character of the Nd of the anorthosites at the time they formed and
the radiogenic Nd of the basalts (
Fig. 12.17
). The inference from this is of complemen-
tary Sm/Nd fractionation processes of the early lunar mantle, which is further evidence of
magmatic fractionation.
