Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29s.441b&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 441-442
Mathematics
Logic
Chronology, Crystallization, Hafnium Isotopes, Ilmenite, Lunar Mantle, Basalt, Kreep, Lunar Composition, Lunar Evolution, Magma, Mineralogy, Titanium
Scientific paper
The prevailing view of lunar mantle evolution is based on crystallization of the Moon's magmasphere. The accumulation of crystals from this magma ocean resulted in a compositionally stratified mantle that was olivine rich in the lower portions, clinopyroxene bearing in the upper portions, and ilmenite bearing in the uppermost mantle. Recent rheological models of the mantle suggest that this ilmenite-bearing upper mantle is likely to have become gravitationally unstable and sunk to its density-compensation depth in the lower mantle. However, it is unclear as to the effects and extent convective overturn had in homogenizing the stratified lunar mantle. We suggest that the amount of intermingling between the ilmenite layer and other layers during sinking of the dense ilmenite layer was probably minor. Data is given and discussed to support this observation. As originally pointed out by Unruh et al., accumulation of ilmenite during crystallization of the lunar magmasphere will produce a mantle layer with a relatively low Lu/Hf ratio without greatly modifying the Sm/Nd ratio of the cumulate pile. We note that the problem of achieving a mantle layer with a Lu/Hf fractionation 4x greater than the Sm/Nd fractionation is magnified if ilmenite was present in the source of these low-Ti basalts, including the ilmenite basalts from Apollo 12, with up to 5 wt% TiO2. The relationship between Ti content of mare basalts and their epsilonHf values can be taken as strong evidence that the source regions of high-Ti and low-Ti basalts have remained mineralogically distinct since their time of formation, and destruction of these mineralogical layers by convective overturn, is highly unlikely.
Beard Brian L.
Mitchell Nick J.
Snyder Glenn A.
Taylor Lawrence A.
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