Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...61s&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 61
Mathematics
Logic
Craters, Igneous Rocks, Lunar Composition, Lunar Crust, Lunar Geology, Lunar Maria, Lunar Rocks, Lunar Soil, Lunar Surface, Moon, Clementine Spacecraft, Iron Oxides, Lunar Maps, Meteorites, Petrology, Regolith, Lunar Prospector
Scientific paper
Since the first return of lunar samples indicated that global differentiation of the Moon had occurred, numerous models of crustal structure have been proposed. With the completion of the first global reconnaissance mapping by Clementine and Lunar Prospector, we are now in position to re-evaluate crustal structure and composition at a global scale. Although this is a difficult and complex task, and one requiring significant study, some first-order results are apparent now and are quite telling. We here summarize our current view of crustal structure and identify some required knowledge to better understand the origin and evolution of the lunar crust. Wood et al. attempted to estimate the amount of plagioclase in the crust, based on the average elevation difference between mare and highlands and some simple assumptions about anorthosite and basalt as responsible for the principal lunar rock types. Later, more complex models emerged, involving layered crusts of feldspathic material over more basaltic material or a laterally variable crust, with Mg-suite plutons intruding a grossly anorthositic crust. Later models attempted to reconcile these contrasting styles by incorporating both features. In part, crustal structure was inferred by the envisioned mode of crustal formation. A decade-long debate on the reality of the lunar "magma ocean," stimulated by the provocative notion of Walker that the Moon never had a magma ocean, and the recognition that the anorthosites and Mg suite probably recorded different and unrelated magmatic events. Such a scenario leaves much about crustal structure an open question, but allows for both lateral and vertical heterogeneity, thus accommodating both principal crustal models. Global maps of Fe , Ti, and Th both confirm old ideas and create new problems. It is clear that vast areas of the lunar highlands are extremely low in Fe, consistent with a significant amount of anorthosite. Such a distribution supports the magma ocean. However, the average lunar highlands composition is, as long suspected, that of "anorthositic norite", a mixed rock type, somewhat similar to many of the lunar meteorites (e.g,., ALHA 81005 and more mafic than pure ferroan anorthosite. Anorthosite proper does occur on the Moon; it is found almost exclusively within the inner rings of multiring basins. These basins span a range of ages and distributions. Mafic provinces occur in the central Procellarum region of the front side and on the floor of the South Pole Aitken Basin. In these areas, the lunar surface is "highland basaltic" composition (FeO about 9-10 wt%). Additional highland basaltic areas occur in the vicinity of nearside basins, such as Serenitatis. The major lunar "hot spot" of high Th concentration (about 10 ppm) occurs within a broad, oval depression approximately coincident with Oceanus Procellarum. Slightly less elevated amounts (about ppm) are associated with the basaltic floor of SPA Basin on the farside. Aside from this, Th highs are isolated and minor. On the basis of the new global data, as well as from our continuing study of the composition of basin ejecta to probe the deep crust, we have modified slightly our existing crustal model to accommodate the new findings. We propose a three-layer model of crustal configuration. The uppermost zone, down to depths of about 15-20 km, consists of mega breccia of mostly anorthositic norite composition (FeO about 4-6 wt%; Al2O3 about 26 wt%). This zone is neither laterally or vertically uniform, displaying anomalous compositional zones at scales of tens to hundreds of kilometers, but is remarkably homogeneous at planetwide scales. In bulk composition, it resembles the "ferroan anorthositic norite" suite of mixed rocks described by Lindstorm et al. and many of the highlands regolith breccias found as lunar meteorites. It is also similar to the average crustal composition inferred by Taylor , on the basis of Apollo granulitic breccias and limited orbital chemical data. Although some areas on the northern farside appear very anorthositic, most areas of the upper highlands different from this composition are more mafic, not more feldspathic, showing affinities to highland basalt, with or without KREEP. The next zone of the crust is found at depths between 15 and about 35 km. It appears to consist largely of nearly pure, ferroan anorthosite (FeO < 2 wt%; Al2O3 > 33 wt%). Outcrops of pure anorthosite principally occur on the Moon in the inner rings of multiring basins, which are structurally uplifted blocks from midcrustal levels, or rarely, as central peaks in some selected craters (Alphonsus, Aristarchus). The anorthosite is apparently confined to middle levels in the crust; moreover, it appears to be at least partly of global extent, as basin rings of anorthosite are found in basins spanning the globe, from Orientale to Humboldtianum. Because anorthosite is most likely to represent the primordial crust , we interpret this global "layer" of anorthosite as the remnant of the original crust of the Moon. The petrological nature of the roughly half of the crust below the anorthosite zone (depths of 35-65 km) remains obscure, but several observations may be made about the likely nature of rocks tobe found there. First, where most of the upper crust has been removed by large, basin-forming impact (such as the floors of the SPA and Imbrium Basins), the crustal composition appears to be that of highland basalt (FeO 9-12 wt%; Al2O3 18-20 wt%). Second, a plot of the total Fe content of basin ejecta (determined from orbital measurements;) against basin size shows that larger basins excavate more mafic (Fe-rich) material. This relation suggests that the lower levels of the crust are more "basaltic" than middle or upper levels. Additional information contained in original.
Bussey Ben J. D.
Hawke Bernard Ray
Spudis Paul D.
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