On the Scale of Lunar Mantle Overturn following Magma Ocean Fractional Solidification: the Role for Multiple Scales of Convective Motion,

Introduction: In understanding the evolution of the Moon, a number of its fundamental magmatic characteristics must be explained in the context of other geophysical and geological observations. Volcanic activity subsequent to the formation of anorthositic crust was dominated by the eruption of mare basalt. (1) The main phase of mare volcanism began ~500 Myr after the crystallization of the anorthositic crust and continued for ~1 Gyr. (2) The picritic glasses, considered to be representative of primitive mare basalt liquid , were generated by melting, at 400-600 km depth [1,2], of a source containing components that, on the basis of the magma ocean hypothesis, should have crystallized at much shallower depth during fractiona-tion of the anorthositic crust. (3) Mare basalts occur primarily in one region of the Moon. Recent topog-raphic data [3] demonstrate that the earlier idea that mare basalt flooded all areas of sufficiently low elevation is not correct. Large areas of very low elevation do not contain mare basalt. The hemispheric asymmetry of mare basalt distribution on the lunar surface must be explained in some other way [4]. (4) A region of the surface roughly correlating with that containing mare basalts also is thought to contain high subsurface concentrations of KREEP which was excavated during the formation of large impact basins. This so-called Procellarum KREEP Terrane (PKT) [5] is responsible for the Imbrium basin centered thorium anomaly mapped by Lunar Prospector [6]. KREEP, as used here, refers to late stage ilmenite-bearing cumulates of the hypothetical magma ocean that would have crystallized near the base of the anorthositic crust. Interestingly , the region of mare basalt eruptions is near the equator of the Moon's rotational axis, as would be expected if this region is underlain by dense mantle. MO fractional solidification and overturn: The Moon should have been significantly melted if it formed by a large impact with the Earth [7]. Since the solidus and liquidus temperatures of mantle mineral assemblages increase with pressure more rapidly than temperature along an adiabat, solidification of a thermally well-mixed magma ocean (MO) is expected to occur from the bottom up. Ideal fractional solidifica-tion would result in an unstable stratigraphy primarily due to increasing Fe/Mg of residual liquid as solidifi-cation proceeds. Highly incompatible elements, including heat producing U, Th, and K, would be progressively enriched in the residual liquid. The unstable stratigraphy resulting from fractional solidification would overturn on relatively short time scales resulting …