Solidifying the lunar magma ocean: Model results and geochronology (Invited)

Details Solidifying the lunar magma ocean: Model results and geochronology (Invited) Solidifying the lunar magma ocean: Model results and geochronology (Invited)

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p33d..03e&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P33D-03

Physics

[5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

The Moon is posited to have formed by reconsolidation of materials produced during a giant impact with the Earth early in solar system evolution. The young Moon appears to have experienced a magma ocean of some depth, which resulted in the formation of an anorthosite flotation crust. There is no simple way to reconcile W-Hf results for the age of Moon formation, U-Pb and Sm-Nd ages of lunar crustal crystallization, and modeling results for magma ocean solidification. At the beginning of magma ocean solidification the dense iron- and magnesium-rich phases crystallizing from the cooling magma are believed to have sunk to the bottom of the magma ocean. When approximately 80% of the lunar magma ocean solidified, anorthite began to crystallize and float upward through the more dense magma ocean liquid; anorthite will continue to be added to this flotation crust until the last dregs of the magma ocean solidify. The crystallization times of the anorthite in the flotation crust, therefore, could span the range from about 80% solidification to what has been interpreted as the lunar magma ocean solidification age. Models including convection in the remaining magma ocean, conduction through the growing anorthosite lid, and radiation into space indicate that the magma ocean may freeze to the point of anorthosite formation in less than 104 years, and perhaps as little as 103 years. After this brief free-surface cooling period the growth of the anorthosite lid radically slows heat loss, and complete solidification of the magma ocean will require additional tens of millions of years. Young anorthosite crustal ages, far younger than models would predict possible, may be explained by further investigations into the evolution of the lunar orbit. Tidal heating of the anorthosite crust as the young Moon experiences a period of high eccentricity may delay closure of minerals with radiogenic phases; these late-closing minerals will then yield young ages, though they originally formed far closer to the lunar origin age. We will present magma ocean and orbital models, and compare with geochronology.

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