Other
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009psrd.repte.138t&link_type=abstract
Planetary Science Research Discoveries
Other
Moon, Lunar, Nwa 032, Northwest Africa 032, Nwa032, Meteorite, Mare Basalt, Lunar Basalt, Lunar Magma Ocean
Scientific paper
Lars Borg (Lawrence Livermore National Laboratory, LLNL) and colleagues at LLNL, the University of New Mexico, the University of California, Berkeley, and Arizona State University have made detailed measurements of trace elements and isotopes in Northwest Africa (NWA) 032, a lunar basaltic meteorite. Previous studies had already shown that the rock is a low-titanium mare basalt. Among other things, Borg and coworkers determined the isotopic compositions of strontium (Sr), rubidium (Rb), samarium (Sm), and neodymium (Nd) present in the rock when it crystallized. The abundances of these elements and their initial isotopic ratios reflect the chemical characteristics of the region of the lunar interior where the NWA 032 magma formed. Borg and his colleagues found that this place in the lunar mantle (called "mantle source area") is quite different from the places where other lunar mare basalts formed. The Rb-Sr data indicate that the mantle source has higher concentrations of incompatible trace elements (they concentrate in magma, not in the major minerals making up planetary interiors) than do the source regions of other low-Ti mare basalts. On the other hand, the Sm-Nd data suggest that the concentrations of incompatible trace elements are lower than in other mare basalts. This mysterious conflicting evidence sets NWA 032 aside from other mare basalts. Geochemical modeling leads Borg and colleagues to conclude that three factors led to the unique chemical characteristics of the NWA 032 mantle source area. One is that it does not contain any "urKREEP," the last dregs of crystallization of the lunar magma ocean, the vast globe-encircling magma body that existed when the Moon formed. The second is that it does not seem to contain any plagioclase, whereas other mare basalt source regions did contain this mineral. The third factor is that the NWA 032 magma probably formed by much smaller amounts of partial melting than did other basalts, 2% versus 5-10% for other lunar basalts. The more samples we get, the more we learn about the Moon.
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