EMP Chemical Analyses: Treiman & Semprich (2023) on lunar mantle dunite in meteorite NWA 11421

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.

在西北非（Northwest Africa, NWA）11421号月球高地角砾岩陨石中，发现了一块厘米级的纯橄岩（dunite）碎片——这是人类首次确认的月球幔物质碎片。该纯橄岩由95%的橄榄石（Fo83）构成，同时含有低钙辉石、高钙辉石、斜长石及铬尖晶石。该岩屑内部的矿物成分差异极小，符合化学平衡特征。通过矿物温压测算可知，该纯橄岩的平衡温度为980 ± 20 ℃，平衡压力为0.4 ± 0.1吉帕斯卡（GPa）。月球地壳底部（密度2550 kg/m³）的压力范围为0.14–0.18 GPa，由此可见该纯橄岩的平衡深度位于月球上地幔深处。若假定月幔密度为3400 kg/m³，则该纯橄岩的平衡深度为88 ± 22 km。其平衡温度与深度与当前测算的月球地热温标（selenotherm，即月球地热剖面）相符。通过矿物分析及占比计算得到的纯橄岩成分显示，其铝、钛等元素含量低于球粒陨石物质，表明其源自分异月幔（包括月球岩浆洋形成的堆积岩）。未发现含磷、锆等元素的物相，说明其受KREEP组分的影响极小；而较低的钛含量则表明其与月球岩浆洋晚期分异熔体的相互作用程度极低。该纯橄岩的镁铁比（Fo83）显著低于倒转未混合地幔模型的预测值，但与月球幔整体成分的估算结果一致。