Lunar meteorites in composition space

Lunar meteorites span a wide range of compositions, a range that far exceeds that of meteorites from any other planetary 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 any kind of meteorite, go here.

This chart 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. All meteorites that plot above the dotted line at 3.5 ppm Th likely originate from the Procellarum KREEP Terrane (PKT). Each point represents a named lunar meteorite stone (or an unnamed stone that I have 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 for igneous rocks. On the Moon only three minerals account for >95% of the crystalline material for nearly all rocks: plagioclase feldspar (mainly anorthite), pyroxene, and olivine. All three have about the same SiO2 concentration, so SiO2 does not vary much among lunar rocks and is not particularly useful for classification. “KREEPic” rocks often contain minor amounts of silica minerals like quartz or cristobalite, so SiO2 tends to be greater in the KREEPic meteorites. Of the 3 main minerals, olivine has the lowest SiO2 so the olivine-rich troctolitic rocks have the lowest SiO2. (Some basalts have concentrations of iron-titanium oxide minerals like ilmenite that may exceed 10% of the rock by volume.)
Al2O3 anticorrelates with FeO plus MgO in lunar samples because nearly all the Al2O3 is in plagioclase feldspar (~0% FeO+MgO, 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+MgO, 0% Al2O3), pulling them off the trend toward the origin.
CaO also correlates negatively with FeO because for low-FeO rocks most of the CaO is also in the plagioclase. Notice the change in the slope of the trend around 7% FeO. For more mafic rocks some of the CaO is also carried by pyroxenes, particularly clinopyroxenes in mare basalt. Many, but probably not all, of the breccias of anorthositic norite composition contain some clasts of mare basalt.
CaO/Al2O3 increases with FeO in lunar rocks because, again, in noritic and basaltic rocks a significant proportion of the CaO is carried by pyroxenes. Meteorites from hot deserts are often contaminated with terrestrial calcite. The horizontal blue line represents the average CaO/Al2O3 in the 12 feldspathic meteorites from Antarctica. These rocks are not contaminated by terrestrial calcite. Their CaO/Al2O3 is essentially that of pure anorthite (typically An96-97), 0.569.
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.
Basaltic lunar rocks typically contain titanium-rich FeTi-oxide minerals like ilmenite, armalcolite, and ulvöspinel. These minerals are sparse in feldspathic rocks. Breccias of anorthositic norite and mafic compositions with greater than about 1% TiO2 likely contain clastic basalt or volcanic glass. TiO2 concentrations in basalts from the Apollo and Luna missions range from 0.8% to 13%; the high-Ti basalts are from Apollos 11 and 17. No high Ti basalts have yet been found among the lunar meteorites.
Because (nearly) all of the iron and manganese in lunar silicate and oxide minerals is in the Fe2+ and Mn2+ oxidation states, 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 high FeO/MnO ratio is often used as “proof” that a meteorite is from the Moon. Many terrestrial rocks have FeO/MnO in the lunar range, however.
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. Meteorites in the blue ellipse are “typical feldspathic lunar meteorites” and are those from which the composition of the typical surface crust of the feldspathic highlands is derived.
The Cr/Sc ratio (chromium/scandium) varies considerably in lunar rocks and is a proxy for the olivine/pyroxene ratio thus troctolitic meteorites have the highest Cr/Sc.
All iridium (Ir, values in parts-per-billion = ng/g) in lunar meteorites derives from asteroidal meteorites (e.g., chondrites, iron meteorites) that strike the lunar surface. Most lunar meteorites are breccias formed from numerous impacts of asteroidal meteorite, thus all brecciated meteorites contain Ir from asteroidal sources. 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 high-Ir lunar meteorites each contain nuggets of FeNi metal, probably from iron meteorites.