¹Allan H. Treiman,²Julia Semprich
American Mineralogist 108, 2182-2192 Open Access Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/open_access/AM108P2182.pdf]
¹Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A. 2
²AstrobiologyOU, School of Environment, Earth and Ecosystem Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K.
Copyright: The Mineralogical Society of America

A centimeter-sized fragment of dunite, the first recognized fragment of Moon mantle material, has
been discovered in the lunar highlands breccia meteorite Northwest Africa (NWA) 11421. The dunite
consists of 95% olivine (Fo83), with low-Ca and high-Ca pyroxenes, plagioclase, and chrome spinel.
Mineral compositions vary little across the clast and are consistent with chemical equilibration. Mineral
thermobarometry implies that the dunite equilibrated at 980 ± 20 °C and 0.4 ± 0.1 gigapascal (GPa)
pressure. The pressure at the base of the Moon’s crust (density 2550 kg/m3) is 0.14–0.18 GPa, so the
dunite equilibrated well into the Moon’s upper mantle. Assuming a mantle density of 3400 kg/m3
, the dunite equilibrated at a depth of 88 ± 22 km. Its temperature and depth of equilibration are consistent with the calculated present-day selenotherm (i.e., lunar geotherm).
The dunite’s composition, calculated from mineral analyses and proportions, contains less Al, Ti,
etc., than chondritic material, implying that it is of a differentiated mantle (including cumulates from
a lunar magma ocean). The absence of phases containing P, Zr, etc., suggests minimal involvement
of a KREEP component, and the low proportion of Ti suggests minimal interaction with late melt
fractionates from a lunar magma ocean. The Mg/Fe ratio of the dunite (Fo83) is significantly lower
than models of an overturned unmixed mantle would suggest, but is consistent with estimates of the
bulk composition of the Moon’s mantle