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. For most meteorites, one point representing the mean composition of all stones of a pair group is plotted. Keep in mind that meteorites that plot together on this chart may plot apart in charts using other element pairs (below). All of the data represented here are from my laboratory.
Comparison of compositions of lunar meteorites (blue circles) to surface and trench soils from the Apollo mission (colored fields) and core soils from the 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 in or near the Th-rich Procellarum KREEP Terrane.
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 crystobalite, 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 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.
The MgO/FeO ratio varies greatly among feldspathic lunar rocks. This observation is perhaps one of the most important to be provided by the lunar meteorites, that is, non all highlands rock 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.
TiO2/FeO does not vary much among feldspathic rocks. Those with high TiO2 usually also have high concentrations of incompatible elements like Th and Sm. TiO2 concentrations are highly variable among mare basalts.
Most feldspathic lunar meteorites are dominated by plagioclase with 97±1 % anorthite component. These meteorites have 0.3-0.4% Na2O. Some meteorites are distinct in being richer in sodium (Na2O) because the plagioclase is more albitic. Compare with Sc-Eu, below. NWA 5744 and NWA 773 are low in sodium because they are rich in olivine, which contains very low sodium.
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 meteorites are distinct in Th/Sm. A number of lunar meteorites have Th/Sm substantially different from KREEP (defined by the range of Sm- and Th-rich impact-melt breccias from the Apollo missions).
Europium (Eu) is a trace element that reflects plagioclase abundance and composition. Some lunar meteorites are distinct on this chart.
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.