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A Survey of Lunar Rock Types and Comparison of the Crusts of Earth and Moon

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A Survey of Lunar Rock Types and Comparison of the Crusts of Earth and Moon A Survey of Lunar Rock Types and Comparison of the Crusts of Earth and Moon John A. Wood Center for Astrophysics Harvard College Observatory and Smithsonian Astrophysical Observatory Cambridge, Massachusetts The principal known types of lunar rocks are briefly reviewed, and their chemical relation- ships discussed. In the suite of low-KREEP highland rocks, Fe/(Fe + Mg) in the normative mafic minerals increases and the albite content of normative plagioclase decreases as the total amount of normative plagioclase increases, the opposite of the trend predicted by the Bowen reaction principle. Lunar highland samples analyzed are uniformly distributed in this sequence, in which normative plagioclase contents range from ~ 40 percent to ~ 100 percent. The distri- bution of compositions of rocks from terrestrial layered mafic intrusives is substantially different: here the analyses fall in several discrete clusters (anorthositic rocks, norites, granophyres and ferrogabbros, ultramafics), and the chemical trends noted above are not reproduced. It is suggested that the observed trends in lunar highland rocks could be produced by crystal fractionation in a deep global surface magma system if (1) plagioclase tended to float, upon crystallization, and (2) the magma was kept agitated and well mixed (probably by thermal convection) until crystallization was far advanced and relatively little residual liquid was left. When such a system was finally immobilized, the Fe-, Na-rich residual liquid would produce Fe-rich mafic minerals in the upper levels of the system, but could not much alter the composition of abundant calcic plagioclase. Conversely, the same liquid would produce sodic plagioclase deep in the sequence, but could not much alter the composition of abundant magnesian mafic minerals. After the crustal system solidified, but before extensive cooling had developed a thick, strong lithosphere, mantle convection was able to draw portions of the lunar anorthositic crust down into the mantle in a manner analogous to the present-day behavior of the terrestrial mantle and crust. At depth, the crustal material was heated; KREEP-rich norite was extracted by partial melting and erupted at the surface as a lava, analogous to terres- trial andesite eruptions. Five years have passed since Eagle, the lections returned from two or more sites. The Lunar Module of the Apollo 11 mission, tendency for certain rock types to occur at landed at Tranquillity Base. In the time since, several widely separated points on the Moon samples of lunar material returned by six is sufficiently great to make us confident that Apollo and two Luna missions to the Moon we have obtained a fairly good sampling of have been studied intensively, and thousands the surface layers of the lunar nearside. of pages of descriptive material have been In reading the literature, it is not easy to published. The samples collected at the eight gain an impression of the total range of com- sites are not totally different from one an- positions of lunar materials encountered, or other: in many cases, essentially the same . their frequency of occurrence in the samples rock type has been observed among the col- returned. Papers descriptive of lunar samples 35 36 COSMOCHEMISTRY OF THE MOON AND PLANETS are typically specialized and detailed, and highlands can one exclude, on petrographic present only the results from a single labora- grounds, the possibility of derivation from tory, employing a single technique of anal- an impact melt, in which the parent material ysis. Sometimes data from the samples of melted was a complex mixture of earlier rock other missions, or data obtained by other types. workers, are shown for comparison, but the There are three major categories of lunar body of data included for comparison tends samples, the analyses of which might be ex- to be small and highly selective. The need pected to inform us of the major lunar rock exists for a broad comparison of the proper- types. The first of these is lunar rocks of hand ties of all lunar materials studied to date. specimen size, which are usually analyzed The chemical composition of its major ele- by traditional wet-chemical or X-ray fluores- ment is the most fundamental property of a cence techniques. The second is smaller lithic lunar rock, and the one most useful for sep- fragments from soil samples and discrete arating categories of lunar rocks and under- clasts from complex breccias. These can be standing the relationship of the various analyzed by a variety of techniques, including categories to one another. Petrographic tex- neutron activation analysis and electron- tural information is, in my opinion, of sec- microprobe analysis of fused beads; but the ondary importance. In the case of highland great majority of lithic fragment and clast rocks, textures record a history of relatively analyses reported in the literature were ob- superficial processes (brecciation, mixing, tained by using an electron microprobe with thermal recrystallization) that are less im- a broad (defocused) beam to analyze a large portant than the large-scale internal geo- number of spots on a polished section surface. chemical processes that largely determined The third category consists of fragments or the chemical compositions of the lunar rocks. globules of glass from the lunar soil samples, It might be argued that textural information which are analyzed individually by the tra- is essential in order to distinguish between ditional electron-microprobe method. pristine igneous rocks and polymict breccias Each of these three categories of material, in which several rock types with different including its method of analysis, has its own chemical compositions have been mixed; but, strengths and weaknesses, which are sum- unfortunately, major impacts are capable of marized in table 1. Here the comment that remelting complex breccias on the Moon and studies of lithic fragments from soils (and giving them igneous textures. For few, if clasts) are least accurate alludes to the fact any, crystalline igneous rocks from the lunar that most such analyses reported in the liter- Table 1.—Sources of Information About Lunar Rock Compositions No. of Advantages Disadvantages Analyses in SAO Library Large Rocks Most accurate Small number of samples 156 Representative (?) analyzed Petrography, other properties Integrates polymict breccias known Lithic Fragments Petrography known Least accurate 530 From Soils Large number analyzed Poorly representative (?) Glass Particles Accurate Petrography not known; 2389 From Soils Probably very representative parent may be polymict (well-mixed) Large number analyzed SURVEY OF LUNAR ROCK TYPES 37 ature were made by the defocused-beam tech- DBA technique has an advantage that should nique. The traditional procedure for reducing be weighed against the disadvantage of the and correcting electron-microprobe analyses small volume sampled, i.e., the fact that lithic assumes a homogeneous target volume and fragments can be assessed petrographically, is not strictly applicable when the micro- in thin section, at the time the analysis is probe beam is enlarged to embrace many performed. Thus, materials that are obvi- mineral grains of differing compositions si- ously polymict in character can be avoided, multaneously. For this reason, it is and analyses can be limited to fragments or recognized that defocused-beam analyses clasts of uniform lithology. (DBA) provide no more than an approxima- During the past year, my group has assem- tion of the true composition of the material bled from the literature a library of chemical analyzed, although the approximation can be analyses of lunar materials from all three a very good one, if the data reduction is car- categories. The current number of entries is ried out thoughtfully. shown in table 1. These come from many In table 1, the question of which type of different sources, too many for each to be sample is most representative of its parent referenced explicitly. The library includes rock is not so straightforward as it might all analyses reported in the Proceedings of seem. Clearly, a large rock can be sampled the first through fourth Lunar Science Con- and analyzed in a much more representative ferences, in Lunar Science I through IV, and way than a 1-mm lithic fragment, especially in the Apollo 15, 16, and 17 Preliminary when only a section surface through the Science Reports. The largest contributors to lithic fragment is accessible for DBA sam- the library were the group headed by K. Keil pling. However, this assumes that the large (DBA's of lithic fragments and point micro- rock, or a substantial fraction of it, was ho- probe analyses of glass particles) and the mogenized and properly sampled to obtain Apollo Soil Survey (A. M. Reid and cowork- an aliquot for chemical analysis. In fact, with ers; point analyses of glass particles from a handful of exceptions, this procedure was soil samples). No analyses of bulk soil sam- not carried out for lunar hand specimens be- ples were included in the library. cause of the wish to consume as little of them My objective was to plot large bodies of as possible, retaining the great bulk of each lunar data in a number of types of chemical specimen intact and uncontaminated for and mineralogical variation diagrams, and study by future generations of scientists. to seek broad trends and clusterings of com- Typically, rock in the amount of 0.5 g was positions. A conscious effort was made to allocated to an investigator for major ele- escape the detailed relationships between ment chemical analysis, and this was de- individual analyses that have dominated livered in the form of a single chip, rather lunar science, and to try to see "the forest than as a powder representative of a much instead of the trees." A computer plotting larger mass of material. If the rock was routine was developed that samples the li- coarse grained or otherwise heterogeneous brary and enters the appropriate analyses in on a large scale, the sample analyzed may a three-variable orthogonal reference frame, have been a poor representative of the whole.
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