Petrologic history of the moon inferred from petrography, mineralogy, and petrogenesis of Apollo 11 rocks

Abstract

The mineralogy and petrology of the Apollo 11 rocks are consistent with impact melting of ilmenite and pyroxene crystals plus liquid derived from fractional crystallization of basaltic magama. The complementary plagioclase accumulate should exist in the highlands. The ferrobasaltic magma is derived from a differentiated hot moon of modified chondritic composition. The one-sided distribution of large irregular seas is explained by tidal attraction of the last fraction of liquid to the near side of the moon and release of liquid by meteorite impact. The vesicular ferrobasalt lavas contain ilmenite, clinopyroxene and plagioclase and have textures similar to some terrestrial basalts. Delayed appearance of plagioclase indicates an unusual source of the magma. The iron-enrichment of the coarser microgabbros is extreme at the end of crystallization resulting in a new iron metasilicate, pyroxferroite. The oxygen fugacity at 1050°C of these rocks indicated by the composition of opaque oxides and troilite-iron intergrowths is 10^(-14)-10^(-16) compared to 10^(- 11) for terrestrial basalts and about 10^(16) for ordinary chondrites. Absence of hydrous minerals indicates loss of volatiles at some stage in the moon's history. The breccias and soil have the same mineralogy modified by shock. Plagioclase vitrophyres are probably melted cumulates and may derive from the highlands. Lithification of the breccia results principally from welding of debris discharged from a hot gas cloud created by meteorite impact. Small glass spheres have surface features consistent with a fiery rain of boiling silicate liquid rounded by surface tension and later impacted by high-velocity micrometeorites. Melting experiments of synthetic material of mean Apollo 11 rock composition revealed low liquidus temperature, delayed appearance of plagioclase and narrow crystal-liquid interval. These support the concept of advanced fractional crystallization of a basaltic liquid under reducing conditions leading to high iron enrichment. Flotation of plagioclase and sin king of ilmenite and pyroxene should occur in the liquid basalt. We propose that meteorite impact blasted away a plagioclase-rich crust, melting a mixture of fractionated basalt liquid and ilmenite plus pyroxene crystals. The new liquid formed the Sea of Tranquillity and yielded the near-surface lava flows represented by the ferrobasalts and microgabbros. This new liquid could yield plagioclase only after the extra ilmenite and pyroxene had crystallized. We propose that the original basalt liquid was derived by fractional crystallization of a molten moon of modified chondritic composition yielding a metallic core surrounded by pressure-stable Mg-rich o li vine and pyroxene. The fractionated liquid became Fe-rich resulting in an inverse density stratification. Primitive ultra basic crust should occur in the highlands, along with dominant plagio clase- rich cumulates. The temperature-time relations of the model are discussed qualitatively. Key factors are early removal of radioactive material from the center by fractional crystallization and possible enhancement of radiative heat transfer in volatile-free silicates. The moon rocks should be more refractory and rigid because of the low volatile content.

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