Rare earth elements (REE) in lunar meteorites

Relative concentrations of the REE (rare earth elements) in rocks from the moon usually differ substantially from those of rocks from the earth. These differences are typically demonstrated using a rare earth element diagram such as Fig. 1 where the absolute concentrations (usually in units of ppm – parts per million) are divided by the concentrations in some well characterized reference material, often chondritic meteorites (“chondrites”). On such plots, data for the trivalent REE (3+ oxidation state) usually form smooth curves. Data for Eu (2+, europium), which is divalent on the moon, usually plot above or below the smooth curve (Eu anomaly). Many terrestrial rocks have little of no Eu anomaly (but some do!).

Figure 1. Comparison of average concentrations of rare earth elements in Earth’s continental crust (Taylor and McClennan, 1985) to concentrations in the typical feldspathic crust of the moon (Korotev and Irving, 2021). The lunar data are averages for 26 feldspathic lunar meteorites (Fig. 3). For simplicity, in this and subsequent REE diagrams, the pattern for lunar meteorites is based only on the 7 elements represented by blue squares here; data for other elements are either interpolated (e.g. Er) or extrapolated (Gd).

On average, the feldspathic lunar crust of the moon has a distinct “positive europium (Eu) anomaly,” but as shown below, individual lunar rocks have both positive and negative Eu anomalies.

Figure 2. Lunar meteorites in Fe-Sm (iron-samarium) space. Feldspathic, Mafic, and Basalts refer to the next three figures. All of the green “mafic” points represent breccias. Many of the mafic points that lie between the feldspathic and basaltic points represent meteorites that are mainly mixtures of felspathic rocks from the highlands and basalts from the maria. Petrographically, however, several are not highlands-mare mixtures (they contain minimal basalt) but are predominantly nonmare anorthositic norites and norites that may derive from rocks of the lower crust or the South Pole-Aitken Basin. The high-Sm points represent meteorites that derive from or near the anomalous, REE-rich PKT (Procellarum KREEP Terrane) of the lunar nearside.

**Figure 3. Chondrite-normalized concentrations of REE in feldspathic lunar meteorites.**

All the feldspathic lunar meteorites represented in Fig. 3 are breccias of anorthosite, noritic anorthosite, or troctolitic anorthosite compositions, i.e., they are dominated (greater than about 75 volume %) by plagioclase feldspar. Those meteorites with the lowest REE concentrations (bottom of plot) contain virtually no KREEP component and are typical of the farside highlands. The “Moon” data of Fig. 1 were derived from 26 meteorites for which the chondrite-normalized La concentration was between 2.4 and 10.0 (mean: 6.2).

KREEP-poor feldspathic rock from the moon have strong positive Eu anomalies. Ninety-two percent of the meteorites of this plot have chondrite-normalized Eu concentrations in the narrow range of 11 to 21 (i.e., 0.6-1.2 ppm Eu; the range is 0.6-1.0 ppm Eu for those with <2.5 ppm Sm, Fig. 2). The nearly constant Eu concentration of ~0.8±0.2 ppm Eu is one of the most characteristic feature of material from the lunar highlands.

Most of the high-REE meteorites of Fig. 3 have compositions like that of mature Apollo 16 soils, which are composed of 20-25% KREEP-rich, noritic impact-melt breccias probably originating from the PKT mixed with feldspathic material. Some lunar breccias have no Eu anomaly. This characteristic has no petrogenetic significance. It is just a necessary consequence of binary mixing of anorthosite with a strong positive Eu anomaly and KREEP rocks with a strong negative Eu anomalies in just the right and fortuitous proportions.

**Figure 4. Lunar meteorites with intermediate concentration of iron and related elements.**

Meteorites of Fig. 4 are largely polymict breccias composed of varying proportions of (1) material of the feldspathic highlands, (2) mare basalt and volcanic glass, and (3) nonmare norites, gabbronorites, troctolites, and their more feldspathic equivalents (e.g., anorthositic norite). Several are rich in KREEP lithologies (Fig. 2) that, mineralogically, are typically norites. The highest-REE patten is that for the regolith breccia lithology of Sayh al Uhaymir (SaU) 169. The lowest-REE patten is for Kalahari 009, an unusual basaltic breccia.

Figure 5. REEs in lunar meteorites that are crystalline basalts or gabbros, except that NWA 11886 is a fragmental breccia with no compositional of petrographic evidence for a component of nonmare material.

Unfortunately, there are no data for many of the most recently classified basaltic lunar meteorites. When likely occurrences of terrestrial and launch pairings are considered, Fig. 5 represents as few as 7 locations on the moon: (1) NEA 003, (2) Dhofar 287, (3) NWA 4898, (4) NWA 11886, (5) the YAMM clan, (6) the meteorites in the asterisk (*) list of the figure plus NWA 8632, and (7) the NWA 773 clan, for which the figure contains data for four of the igneous and plutonic lithologies observed in the breccia (Valencia et al., 2019).

Fine Print

Chondrite data used in Figs. 1, 2, 3, and 4 are those Anders and Grevesse for CI chondrites multiplied by the factor 1.36 to convert to a volatile-free basis.

All lunar data are those from my lab (Korotev and Irving, 2021, and papers cited therein). Data for a few lunar meteorites for which REE concentration have been substantially altered by terrestrial processes (Korotev and Irving , 2021) are not included.

References

Anders E. and Grevesse N. (1989) Abundances of the elments: Meteoritic and solar.
Geochimica et Cosmochimica Acta 53, 197-214.

Korotev R. L. and Irving A. J. (2021) Lunar meteorites from northern Africa. Meteoritics & Planetary Science 56, 206-240.

Taylor S. R. and McClennan S. M. (1985) The Continental Crust: Its Origin and Evolution, Blackwell Sci. Publ., Oxford, pp. 312.

Valencia S. N., Jolliff B. L., Korotev R. L. (2019) Petrography, relationships, and petrogenesis of the gabbroic lithologies in Northwest Africa 773 clan members Northwest Africa 773, 2727, 3160, 3170, 7007, and 10656. Meteoritics & Planetary Science 54, 2083-2115.

Lunar meteorites

Lists of lunar meteorites

Lunar meteorites in composition space