Rock Types and Terminology

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Rock Types and Terminology

Lunar meteorites have been discussed in three different groupings - basalts, anorthosites, KREEP-rich samples, and brecciated mixtures of these three end members. The latter type has also been referred to as "mingled" (Table 1).

There is some precedence for terminology from the extensive work on Apollo and Luna rocks returned from the Moon. The classification scheme of Le Bas (2001) is used for the eight unbrecciated basaltic lunar meteorites.

The rest of the lunar meteorites are breccias, and the terminology recommended by Stoffler et al. (1980) on highlands rocks is used here. Because this terminology is less than simple, it will be reviewed here for clarity.

There are three different subgroups of breccias - monomict, dimict, and polymict.

Monomict breccias exhibit intergranular in-situ brecciation of a single lithology; they can also be recrystallized.

Dimict breccias exhibit intrusive-like veined textures of fine grained melt breccias within coarse grained plutonic or metamorphic rock types.

And polymict breccias can come in five different forms. Regolith breccias contain clastic regolith constituents including glass spherules and brown vesiculated matrix glass. Fragmental breccias contain rocks clasts in a porous clastic matrix of fine grained rock debris. Crystalline melt or impact melt breccias contain rock and mineral clasts in an igneous textured matrix (can be granular, ophitic, sub-ophitic, porphyritic, poikilitic, dendritic, fibrous, sheaf-like, etc.). Impact glass or glassy melt breccias contain rock and mineral clasts in a coherent glassy or partially devitrified matrix. And finally, granulitic breccias contain rock and mineral clasts in a granoblastic to poikiloblastic matrix.

Wherever possible, these terms will be applied to the lunar meteorite breccias in this compendium, but it should be emphasized that all brecciated lunar meteorites are polymict breccias (Table 1).

Regolith breccias from the Apollo collections have been studied extensively, and three characteristics allow their maturity (length of exposure near the lunar surface) to be estimated. Implantation of noble gases by solar wind leads to higher levels in more mature regolith (e.g., Eugster, 1989). Siderophile elements (e.g., Ni, Ir, Co) become higher and more uniform in mature regolith, approaching levels of that of the impacting materials such as chondrites (e.g., Korotev, 1994). Finally, the ferromagnetic resonance 5 analysis (FMR) maturity index, or Is/FeO, is correlated with other indicators of maturity and has been measured on many lunar soils and regolith samples (e.g., Morris, 1978; McKay et al., 1986).

All three of these parameters have been used to characterize maturity of breccias in lunar feldspathic meteorites and will be part of the discussions for individual meteorites.

Finally, bulk compositional data are being used to make initial classifications of many lunar meteorites, and often times before careful petrography has been done. As a result many lunar meteorites have been classified based on their trace element content by inference from previously studied and classified samples. Although the first few decades of lunar meteorite research led to an understanding of compositional variation that can be explained in terms of three end member components - anorthosite/FHT - KREEP/PKT - mare basalts (Fig. 4), the many additional meteorite samples that have become available have led to recognition of a fourth component, which is mafic anorthositic and noritic lithologies (Korotev et al., 2009b).

The compositional variation required by this fourth component is clear, and has fundamental implications for interpreting geological and petrologic data for lunar samples (Fig. 5). As with KREEP component, it is not clear if this fourth component can be recognized in thin section or petrographically.

Three oxygen isotope diagram for lunar meteorites

Figure 4: Three oxygen isotope diagram for lunar meteorites (data summarized in Table 2), and other achondrites (data from Clayton and Mayeda, 1996).

Bulk Composition of Lunar Meteorites

Figure 5: Comparison of lunar meteorite and Apollo sample bulk compositions, from the INAA data and laboratory studies of Haskin/Korotev (from Korotev et al., 2009)

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