Rare Earth Elements (REE) in Lunar Meteorites
Rocks from the Moon and Earth differ in their relative concentrations of the REE - rare earth elements (atomic numbers 57 to 71). These differences are usually depicted on a "rare earth plot" such as those below. The concentration of each element in the rock is divided by its concentration in chondritic meteorites and these normalized concentrations are plotted against atomic number of the REE. Thus, the REE pattern of most chondrites is a horizontal line at 1 on the Y axis. Most rocks from the Earth and Moon are "enriched" in REE compared to chondrites. Also, on both planets the "light REE" - those on the left side of the plots are relatively more enriched than the heavy REE - those of the right side.
A distinct characteristic of lunar rocks compared to many Earth rocks is behavior of the element Eu (europium). Eu can exist in both the +2 and +3 oxidation states whereas the other REE exist only in the +3 state. On Earth, where conditions are much more oxidizing than on the Moon, most Eu is in the +3 state, so Eu behaves like the other (trivalent) REE. On the Moon, where there is no free oxygen, most of the Eu is in the +2 (reduced) oxidation state. During geochemical processes like crystallization of a magma, Eu2+ preferentially goes into the plagioclase feldspar whereas the other REE "prefer" to stay in the liquid phase. Thus, feldspathic lunar rocks have a "positive Eu anomaly" and later crystallizing rocks have a "negative Eu anomaly." Some terrestrial rocks have Eu anomalies, but seldom as strong as those in lunar rocks.
This plot compares REE
concentrations in feldspathic lunar meteorites. All the meteorites are
breccias. Only a few of these meteorites have moderately high concentrations
of REE. The others provide the best estimates of
the composition of the surface of the feldspathic highlands crust distance
from the Procellarum KREEP Terrane (Korotev et al., 2003). They are all
characterized by a strong positive Eu anomaly. For comparison, also shown are
the concentrations in the average upper crust of the Earth (mean, from
compilation Lodders and Fegley, 1998). On average, the terrestrial crust has
distinctly different relative concentrations of REE than the lunar
This plot compares REE
concentrations in lunar meteorites of intermediate iron concentration. All
are breccias. The meteorites at the bottom are mafic anorthosites - rocks of
the feldspathic highlands. Many of the rocks in the middle of the
distribution are mixtures of mare and highlands material. Meteorites plotting
at the top contain a large component of KREEP and likely originate from in or
near the Procellarum KREEP Terrane. Many of the meteorites in the middle of
the distribution also contain a component of KREEP, thus the shape of the
patterns are the same.
All of the meteorites depicted on this plot are basalts or breccias composed mainly of basaltic material. Some lunar basalts are "depleted in light REE" compared to rocks from the highlands. Most lunar basalts have negative Eu anomalies
Korotev R. L., Jolliff B. L., Zeigler R. A., Gillis J. J., and Haskin L. A. (2003) Feldspathic lunar meteorites and their implications for compositional remote sensing of the lunar surface and the composition of the lunar crust. Geochimica et Cosmochimica Acta 67, 4895-4923.
Lodders K. and Fegley B, Jr. (1998) The Planetary Scientist's Companion. Oxford University Press, New York. 371 pp.