Some thoughts on the origin of lunar ANT-KREEP and mare basalts

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A regular correlation between the Sm abundances and the Eu anomalies in all kinds of lunar samples strongly suggests that a series of ANT-KREEP type rocks and the source material for the Apollo 11, 12, 15 and 17 mare basalts may have been derived from a systematic and common magmatic differentiation. In such a differentiation, significant quantities of trapped liquid were occluded with the cumulates and upon partial melting of these source materials, the trapped liquids played a dominant role in the derivation of mare basalts. After increasing the concentration of LIL trace elements to ≈10× chondrites by crystallization of Mg-rich mafic minerals such as olivine, both plagioclase and olivine began to crystallize simultaneously. Varying amounts of melt were trapped with pure anorthosites and yielded overall positive Eu anomalies. These anorthosites remained near the lunar surface. Simultaneously, mafic cumulates and any melt inclusions plus significant amounts of trapped liquid in the cumulate layers settled to yield source matter for the Apollo 15 basaltic rocks with minimum Eu anomalies. With increasing plagioclase and mafic mineral crystallization, the magma changed its chemical composition and the Sm/Eu ratio. Toward the end of the crystallization sequence, the plagioclase differentiation series became KREEP material. The settling mafic cumulates and their trapped liquids account for the sequence of the Apollo 15, 12, 17 and 11 mare basalt source materials. Appreciable amounts of apatite may have settled out with the Apollo 11 and 17 mafic cumulates. Subsequent partial melting of the solidified melt inclusions and trapped liquid, accessory minerals and a fraction of the major mafic minerals from the source materials yielded the respective mare basalts. The melt at the last stage probably had a chemical composition similar to the ilmenite microgabbros.