Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p34a..08p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P34A-08
Other
[5410] Planetary Sciences: Solid Surface Planets / Composition, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The lunar crust is thought to have formed as a result of global melting of the outer parts of the Moon in its earliest history, a lunar magma ocean (LMO). Crystallization of this magma ocean set the stage for the ensuing history of the planet. Models for the formation of the lunar crust and the evolution of the LMO were derived from individual Apollo samples that could not be placed directly in the context of crustal bedrock with remote sensing data that was available. Data from modern sensors, such as the Moon Mineralogy Mapper (M3) on Chandrayaan-1, now allow such bedrock issues to be addressed. The ~930 km diameter Orientale multi-ringed impact basin in the western highlands provides an opportunity to evaluate the mineralogy of the in situ crust of the Moon in the search for LMO mineralogy and structure. Orientale is the youngest large basin on the Moon, and the basin deposits and ring structures expose progressively deeper bedrock layering that can be used to determine lunar crustal structure and test the LMO model. With its high spatial and spectral resolution, M3 data show that the ejecta of the basin is composed of mixed assemblages of processed feldspathic breccias with small amounts of low-Ca pyroxene comprising the upper kilometers-thick mega-regolith layer of the crust. Exposures in the outermost (Cordillera) ring reveal less processed examples of this material. The M3 data show that the next interior ring (Outer Rook), representing deeper material, is characterized by distinctly more crystalline blocks of impact-shocked anorthosite and noritic anorthosite. Most importantly, M3 data reveal that the mountains of the closest ring toward the basin interior (Inner Rook) consist of pure anorthosite, including outcrops of the unshocked crystalline form. This massive exposure of anorthosite across the entire mountain range provides validation for the LMO hypothesis. These mountains are believed to have originated in the upper crust below the impact fragmented regolith and were formed and uplifted during the basin event. Such extensive exposures of this rock type, consisting of almost pure plagioclase, could not have formed in any other way than plagioclase flotation in the crystallizing lunar magma ocean. Figure 1. Schematic cross section of the Orientale basin illustrating the relation of the Inner Rook Ring to the basin deposits of the exterior (after Head et al., 1993, JGR, 98, 17149).
Boardman Joseph W.
Burratti B.
Cheek L.
Clark Roger Nelson
Combe J.
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