Lunar Meteorites in Composition Space

Lunar meteorites span a wide range of compositions, a range that far exceeds that of meteorites from any other meteorite parent body. The charts below are useful for distinguishing different lunar meteorites from EACH OTHER. They are not particularly useful for distinguishing lunar meteorites from terrestrial rocks. If you are interested in whether your rock has a composition consistent with a meteorite, go here.

The chart above is one of several that can be used for classifying lunar meteorites by composition and distinguishing one lunar meteorite from another. This particular chart is useful because both FeO (total iron expressed as percent FeO) and Th (thorium expressed in µg/g or ppm [parts-per-million]) have been measured on the surface of the Moon by orbital spacecraft (Clementine and Lunar Prospector). Th is shown on a logarithmic scale because the range in Th concentrations is so great. “KREEPic” rocks contain high levels of incompatible elements like Th. Each point represents a named lunar meteorite stone (or an unnamed stone that I’ve analyzed but which does not yet have a name). Keep in mind that meteorites that plot together on this chart may plot apart in charts using other element pairs (below). All the data represented here are from my laboratory.

Comparison of compositions of lunar meteorites (blue squares) to surface and trench soils from the Apollo mission (colored fields) and core soils from the Russian Luna missions (diagonal pink squares). Non-basaltic Apollo samples tend to have higher concentrations of incompatible elements like Th than do the meteorites because all the Apollo missions landed on the nearside of the Moon in or near the Th-rich Procellarum KREEP Terrane. Half of the lunar meteorites originate from the farside of the Moon where Th concentrations are lower.

On Earth, SiO2 (silica) concentrations are used as a 1st-order chemical classification parameter. On the Moon, the three major minerals, plagioclase feldspar, pyroxene, and olivine, all have about the same SiO2 concentration, so SiO2 does not vary much among lunar rocks and isn’t particularly useful for classification. “KREEPic” rocks often contain minor amounts of silica minerals like quartz or cristobalite, so SiO2 is greater in the KREEPic meteorites.

Aluminum anticorrelates with iron + magnesium in lunar samples because nearly all the Al2O3 is in plagioclase feldspar (~0% FeO, 36% Al2O3) and all the FeO and MgO is in pyroxene, olivine, and ilmenite (high FeO+MgO, <5% Al2O3). KREEPic rocks contain small proportions of silica phases (0% FeO, 0% Al2O3), pulling them off the trend toward the origin. On the Moon, Al2O3 or FeO is a much better 1st-order chemical classification parameter than is SiO2.

Likewise, most of the CaO in lunar rocks is in plagioclase feldspar (CaO = 20%; CaO/Al2O3= 0.58) and most of the FeO is in pyroxene, olivine, and ilmenite (high FeO, <5% CaO). The CaO/Al2O3 ratio increases with FeO in lunar rocks because in basaltic rocks some CaO is carried by clinopyroxene, which contains some Ca. Meteorites from hot deserts are often contaminated with terrestrial calcite.

The MgO/FeO ratio varies greatly among feldspathic lunar rocks. This observation is perhaps one of the most important to be obtained from lunar meteorites. The observation argues that not all highlands rocks derive from “ferroan anorthosite” (typically, MgO/FeO ranging from 0.85-1.30). Basalts have lower MgO/FeO than rocks of the feldspathic highlands.

Because all the iron and manganese in lunar silicate and oxide minerals is Fe2+ and Mn2+, FeO and MnO are strongly correlated in lunar rocks. The mean and standard deviation of FeO/MnO in the data depicted here is 67± 9 (1 standard deviation). This ratio is greater for lunar meteorites than for any other type of meteorite. The FeO/MnO ratio is often used as “proof” that a meteorite is from the Moon.

The Sc-Sm (scandium-samarium) chart above is similar to the FeO-Th chart in that Sc, which is carried mainly by pyroxenes, increases from feldspathic meteorites (high plagioclase, low pyroxene) to basaltic meteorites (high pyroxene, low plagioclase). Sc does a better job of resolving the feldspathic lunar meteorites than does FeO.

Some lunar meteorites are distinct in Sm/Th. The three highest-Sm/Th meteorites with <30 mg/g Sc are enriched in Sm from terrestrial weathering processes.

The Cr/Sc ratio (chromium/scandium) is a proxy for the olivine/pyroxene ratio.

All iridium (Ir) in lunar meteorites derives from asteroidal meteorites (e.g., chondrites, iron meteorites) that strike the lunar surface. The crystalline mare basalts contain essentially zero Ir because they are not breccias. The most Ir-rich lunar meteorites are regolith and impact-melt breccias, which contain up to several percent asteroidal material. The highest-Ir meteorites each contain nuggets of FeNi metal, probably from iron meteorites.