Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p34a..05p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P34A-05
Other
[5410] Planetary Sciences: Solid Surface Planets / Composition, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
A new and unexpected rock-type has been detected on the farside of the Moon by the Moon Mineralogy Mapper (M3) on Chandrayaan-1. The rock-type is dominated by Mg-rich spinel with <5% pyroxene or olivine present. It occurs as one of several discrete areas along the western inner ring of Moscoviense Basin that exhibit very unusual compositions relative to their surroundings. The non-mare components comprising the basin and its rings are highly feldspathic and contain very few mafic minerals. However, small unconnected and diffuse areas, often less than 1 km in size, exhibit three distinct lithologies scattered and separated by several kilometers along the inner ring (210 km in diameter): one dominated by a very Mg-rich pyroxene, another dominated by olivine (Isaacson et al., this session), sometimes in association with the pyroxene, and the third dominated by Mg-rich spinel (Taylor et al., this session). Although no crystalline plagioclase has been detected within the region containing these materials, the entire region is highly anorthositic and all three unusual rock-types probably contain plagioclase. None of these exposures of olivine-orthopyroxene-spinel (OOS) vary significantly from their surroundings in albedo and none are associated with craters or unusually steep slopes (which commonly expose relatively unweathered underlying material). We have considered both exogenic and endogenic origins for the OOS lithologies. The olivine-orthopyroxene combination is reminiscent of ordinary chondrites and the only other spinel-rich surfaces detected with remote sensors are a few main-belt asteroids (e.g. Sunshine et al., 2008). This leads to the hypothesis that these exposures represent the breakup of a multi-component (rubblepile) near-Earth asteroid as it swung by the Earth-Moon system. Although the OOS exposures are relatively linearly aligned, special circumstances would be required to allow the impactor to survive. Alternatively, if these materials are lunar (endogenic), the OOS suite represents a fundamental and previously undiscovered crustal component of the Moon, uplifted from depth during the basin-forming event. Since the OOS suite is embedded and dispersed within a highly feldspathic basin ring, a simple lunar magma ocean crystallization (LMO) sequence is inadequate. Separation of relatively dense spinel and concentration into the observed new spinel rock-type requires processes such as fractional crystallization during secondary magmatic processes early in crustal evolution. Additional processes are required to embed the OOS products within the LMO feldspathic crust. All these events had to have occurred prior to the basin-forming impact.
Boardman Joseph W.
Burratti B.
Clark Roger Nelson
Combe J.
Green Robert O.
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