Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.448t&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 448
Other
Kreep, Lunar Crust, Magma Ocean, Magnesium Suite
Scientific paper
Norites, troctolites, dunites, spinel troctolites and gabbronorites form the Mg suite, which constitutes perhaps 20% of the highland crust. Their ages range from about 4.43 b.y. down to about 4.17 b.y. The Mg suite does not appear to be related to crystallization from the magma ocean. These rock types commonly have Mg# > 90 and so are "primitive," but also contain high concentrations of incompatible elements, typical of highly "evolved" igneous rocks. An origin by mixing of these two distinct components, one "primitive" to account for the major elements (particularly the high Mg#), and the other "evolved" to account for the high trace-element abundances, is suggested by these contradictory petrochemical characteristics. The source of the highly evolved trace-element component is clearly KREEP. The source of the "primitive" Mg-rich component is less clear. Many theories propose that the Mg-suite rocks are derived from different plutons that intruded the crust as separate igneous intrusions. However, all Mg-suite rocks have REE patterns parallel to those of KREEP and the ferroan anorthosites. This characteristic is compatible with mixing, but should not be expected to be duplicated in many separate igneous intrusions. The Mg suite also contains Mg-rich orthopyroxene, a mineral lacking in most mare basalts, so that the source regions of the mare basalts were distinct from those of the Mg suite. During crystallization of the lunar magma ocean, Mg-rich minerals accumulate on the bottom of the magma chamber at depths probably exceeding 400 km. It has been suggested that massive overturning of the crystal pile has occurred to bring these Mg-rich minerals close to the surface. The source regions of the mare basalts were solid by 4400 m.y. with only the minor KREEP component remaining liquid until about 4360 m.y. Thus the lunar interior was effectively solid, although still hot, at the time of the formation of the Mg suite. It thus seems difficult to envisage a massive overturning of the upper 400 km of the solid lunar interior after 4400 m.y. Near-surface remelting of such early refractory Mg-rich cumulates would be difficult in the absence of an obvious internal source of energy. Subsequent melting in the lunar interior that produced the mare basalts took place much later within differentiated cumulates. These lavas do not have the primitive and evolved characteristics nor the mineralogy of the Mg-suite. The total amount of mare basalt melt was very small, about 0.1% of lunar volume. This was produced over 1000 m.y. in more than 20 separate locations and was essentially trivial on a Moon-wide scale. The Mg suite with a volume about 20 times that of the mare basalts, was produced in 100-200 m.y. The formation of the Mg suite thus requires a major source of energy and a large volume of a primitive component. Large-scale overturning of the lunar mantle seems unlikely to produce the compositionally distinct and voluminous Mg-suite magmas. If the primitive compositions cannot be derived from the interior, then they may have come from above. The giant impact hypothesis for lunar origin spins out the silicate mantle of the impactor into Earth orbit. It thus provides a ring of debris of primitive lunar composition from which the Moon accreted. If accretion of this material into the Moon is not 100% efficient, sweep-up of some left-over bodies at relatively low velocities could occur following the formation of the lunar crust. The impact of such bodies could result in mixing of whole Moon, hence "primitive" compositions with some remelted ferroan anorthosite and with the residual KREEP liquid. The magmas so formed could then pond beneath the ferroan anorthositic crust and subsequently intrude the crust. Such a model can account for the mineralogy of the Mg suite, for the old ages, and provide both the primitive "whole Moon" component and an adequate energy source.
Esat Tezer
Norman Douglas M.
Taylor Stephen R.
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