Lunar ferroan anorthosites and mare basalt sources: The mixed connection

Geophysical Research Letters - Tập 18 Số 11 - Trang 2065-2068 - 1991
G. Ryder1
1 Lunar and Planetary Institute

Tóm tắt

Global overturn of a hot, gravitationally unstable lunar mantle immediately following the solidification of a magma ocean [and essentially complete by 4.4 Ga] explains several characteristics of lunar petrology. Lunar mare basalt sources are inferred to be depleted in europium and alumina. These depletions are consensually attributed to complementary plagioclase floating from a magma ocean. However, the connection cannot be so simple and direct: in contrast to the mare basalt source parent magma, the ferroan anorthosite parent magma was more evolved by virtue of its lower Mg/Fe ratio and Ni abundances, although less evolved in its poverty of clinopyroxene constituents, flat rare earth pattern, and lower incompatible element abundances. The europium anomaly in mare sources is inferred to be present at 400 km depth, too deep to have been directly influenced by plagioclase crystallization. Massive overturning of the post‐magma ocean mantle would have carried down clinopyroxene, ilmenite, and phases containing fractionated rare earths, europium anomalies, and some heat‐producing radionuclides. These phases contributed to deep mare basalt sources. Upward‐moving phases would have been magnesian mafic minerals; their immediate melting on pressure release contributed the magnesian suite of plutonic norites and troctolites that post‐date the anorthosites in the highlands crust.

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