Northwest Africa 12279: Evidence for the Interaction Between Early Lunar Mantle Melt and Anorthositic Crust.

The Mg‐spinel‐bearing rocks are an essential component of the lunar crust. They are widely distributed on the Moon's near and far sides. The study of petrogenesis provides a rare opportunity to understand the early lunar crust‐mantle interaction. Northwest Africa 12279 is a unique Mg‐spinel‐bearing troctolitic anorthosite lunar meteorite that is unbrecciated and pristine. The modal content of the spinel‐bearing troctolitic component is ∼95%. Five hundred sixty spinels larger than 100 μm are investigated in this paper, and 38 of them are found to contain magma melt inclusions. These magma melt inclusions represent the mixed composition between mantle melt and crust material during the crystallization of a spinel. They generally show an immiscible liquid phase separation, which can be divided into a magnesium‐rich and silica‐rich phase. The former is similar in chemical composition to the lunar mantle's peridotite or plutonic ultramafic Mg‐suite. The mantle melt reacts successively with Ferroan Anorthosites in the crust to crystallize Mg‐spinel, plagioclase, olivine, pyroxene, and silica phases. The large grains of euhedral Mg‐spinels with homogeneous chemical composition indicate they are the products of equilibrium crystallization, while plagioclase, olivine, pyroxene, and silica phase are the products of differentiated crystallization of mixed magma. This reaction may represent a typical process of lunar Mg‐suite. This work is the first to find petrological evidence that the melt from the lunar mantle intrudes into the crust and then undergoes a liquid phase immiscible separation to form the Mg‐spinel‐bearing troctolitic anorthosite. Plain Language Summary: Northwest Africa 12279 (weight 1,830 g) is the only known pristine and unbrecciated lunar spinel‐bearing troctolitic anorthosite, including lunar meteorites and returned samples. These pristine samples are indispensable for understanding Moon's history and chemical evolution. The spinel‐bearing rocks are an essential component of the lunar crust. They are widely distributed on the Moon's near and far sides. The study of petrogenesis provides a rare opportunity to understand the early lunar crust‐mantle interaction. Five hundred sixty spinels larger than 100 μm are investigated in three polished thin sections, and 38 contained magma melt inclusions. These magmatic inclusions represent the mixed composition between early mantle melt and crust material during the crystallization of a spinel. This work is the first to find evidence that the melt from the lunar mantle intrudes into the crust. The magnesium‐rich melt from the mantle assimilates with ferroan anorthosite (an igneous rock is composed of >90 vol.% plagioclase rich in calcium with minor iron‐rich pyroxene and olivine) from the early crust to form an Mg‐spinel‐bearing hypabyssal igneous rock. Key Points: Northwest Africa (NWA) 12279 is a unique lunar meteorite that is both unbrecciated and pristineThe magmatic melt inclusion in Mg‐spinel represents the mixed composition between lunar mantle melt and anorthositic crustThe formation of Mg‐spinel in NWA 12279 results from the interaction between the mantle material and the anorthositic crust of the Moon [ABSTRACT FROM AUTHOR]