Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1701g&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1701
Other
[1026] Geochemistry / Composition Of The Moon, [3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The composition of the lunar crust (and its pristine rocks) provides clues about the processes that formed it, and hence provides constraints on the origin and evolution of the Moon [e.g., 1,2]. The lunar crust is inferred to contain abundant ferroan anorthosite, which formed by the crystallization and flotation of plagioclase from a global Lunar Magma Ocean (LMO) early in the Moon's history [3,4]. The global LMO hypothesis is based on analyses of returned Apollo samples, which show that ferroan anorthositic rock (Mg# of mafic silicates [molar Mg/(Mg + Fe)] = 50 - 70) is common among Apollo non-mare material. Remote sensing data show that the Moon's highland crust is anorthositic [5-7], suggesting that Apollo ferroan anorthosites are characteristic of the whole lunar crust. However, the Apollo non-mare materials come from only seven sites, and their source areas are now known to include the continuous ejecta blanket of the Imbrium [8,9]. Thus, it is not clear that Apollo ferroan anorthosites are representative of the whole Moon. Lunar feldspathic meteorites, which come from random sites across the lunar highlands [10], provide tests of the global distribution of LMO products. Most of the feldspathic meteorites are breccias; interestingly, most of them do not contain ferroan anorthosite. Here we present new data on lunar feldspathic meteorite ALHA81005 which, combined with literature data from other feldspathic meteorites and the Luna returned samples, suggest that anorthosite with Mg# = 50 - 70 (typical ferroan anorthosite) is only one of several anorthosite assemblages on the lunar highlands. True ferroan anorthosite seems to be abundant only in the continuous Imbrium ejecta and may not be distributed globally. Anorthosites in ALHA81005 and Luna 20 represent a second group, with a continuous range of Mg#s from 90 - 10. Other anorthosite assemblages in meteorites include: Mg-anorthosites that concentrate at Mg# = 65-85; and anorthosites that range from Mg# = 50 - 90 (like a mixture of Mg- and ferroan anorthosites). These distinct assemblages of anorthosites are unexpected, and may not be consistent with predictions of the LMO model (i.e., that lunar anorthosites are consistent across the 'ocean'); the LMO model may need revision. In addition, the grouping of anorthosites compared to each other and to returned samples seems to be a promising tool, in combination with orbital remote sensing, to place constraints on possible source regions of meteorites.
Gross John
Treiman Allan H.
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