| The lunar meteorite NWA 12279 is a piece of locally brecciated magnesium-aluminum-bearing spinel anorthosite newly discovered in 2016,consisting of 86 vol.%anorthite and 14 vol.%impact melt breccia.The anorthosite,has a poikilitic texture and porphyritic-like texture is composed of plagioclase?71.0vol.%?,olivine?11.0 vol.%?,pyroxene?10.0 vol.%?and Magnesium-aluminum spinel?7.0 vol.%?,with a small amount of quartz,chromite and ilmenite.The impact melt breccia is mainly composed of lithic clasts such as anorthosite,gabbro,gabbro-troctolite-anorthosite and microporphyritic crystalline impact melt breccias,and such mineral fragments as olivine,pyroxene,plagioclase and spinel,and glass as well as matrix.The anorthosite has the same mineral composition as the breccia:plagioclase(An92.9-98.5),hypersthenes(Fs15.5-32.2Wo2.98-4.22),pigeonite?Fs27.9-53.1Wo7.19-14.7?,augite?Fs8.42-38.9Wo17.0-44.1?,Olivine?Fo53.7-89.4?,magnesium aluminum spinel(Mg4.97Fe0.86)5.83(Al11.4Cr0.61)12.0O24.Based on the petrology,mineralogy,spectroscopy and shock-metamosphism features of the meteorite,we have made Some knowledge regarding to the NWA 12279 lunar meteorite of mineral composition and composition,impact metamorphism,meteoritic origin and sputtering history.?1?According to the meteorite's whole rock principal element composition?FeO=1.74-2.67 wt%,Al2O3=26.9-27.9 wt%?,the Mg#value and An#value of the meteorite mafic minerals?Mg#=55-90,An#=95-100?,Fe/Mn?mol?ratio of olivine and pyroxene?Ol=62-102;Py=37-69?and the principal elemental composition of a glassy substance(MgO/Al2O3=0-0.5%,CaO/Al2O3=0.59-0.75%),we found that the NWA 12279 meteorite comes from highland areas.However,its petrologic features area is different from lunar ferroan anorthosites,Magnesite-suite anorthosite,Mare basalts,and KREEP that have been discovered.Therefore it belong to a new rock type.?2?Anorthite and breccia of this meteorite have different shock metamorphic features.Planar fractures in olivine,maskelynite of plagioclase and impact melt veins containing unmelted pyroxene and olivine fragments were produced in the anorthite,Meanwhile,vitreous veins,impact melt-pockets and rock brecciation were developed in the breccia.The peak shock pressures in anorthosite and breccias are estimated to be?45 GPa and?78 GPa respectively,and the shock temperature peaks to be?1100?and?1890?respectively.The shock stage is S5-6.?3?Combining with the comprehensive analysis of magnesium-aluminum spinel anorthosite fragments in lunar samples,we believe that the content of spinel in magnesium-aluminum-bearing spinel anorthosite or troctolite is 5-30%,the content of olivine+pyroxene is 11-69%,the content of plagioclase is about 15-89%,the Mg#of spinel is between 44-85,and the Al#is between 79-97,However,This is different from the M3 interpretation of the spinel content of up to 30%and the mafic mineral content?5%,mainly due to the relative enrichment of large particles of spinels caused by the Strong impact breccia on the lunar surface,and the spectral characteristics of the smaller particles of mafic minerals are obscured.?4?According to the comparison of petrology and spectral characteristics,it is believed that the NWA 12279 lunar meteorites may have originated in the peak of the central peak or inner ring of the Mare Serenitatis impact crater,it originated from the deep part of the moon,was carried to the lunar surface by impact sputtering,and experienced relatively few impacts,and was ejected from the moon by a low-angle impact event and finally captured by the earth. |
PDF Full Text Request