Petrography and chronology of lunar meteorite Northwest Africa 6950  

作　　者：Zemin BAO Yuruo SHI JLawford ANDERSON Allen KENNEDY Zuokai KE Xiangping GU Peizhi WANG Xiaochao CHE Yuelan KANG Huiyi SUN Chen WANG 

机构地区：[1]Beijing SHRIMP Center,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China [2]Department of Earth and Environment,Boston University,Boston MA 02215,USA [3]John de Laeter Centre,Curtin University,Perth WA 6845,Australia [4]School of Geosciences and Infophysics,Central South University,Changsha 410083,China

出　　处：《Science China(Information Sciences)》2020年第4期178-190,共13页中国科学（信息科学）（英文版）

基　　金：financially supported by National Nature Science Foundation of China(Grant Nos.41842060,41803027);China National Space Administration(Grant No.D020205);China Geological Survey Research Fund(Grant Nos.JYYWF20180104,JYYWF20181702)。

摘　　要：Northwest Africa(NWA)6950 is an olivine-gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts.In this contribution,we report the petrography,mineralogy,and U-Pb geochronology of baddeleyite and phosphate in this lunar meteorite and make a comparison with other NWA 773 paired meteorites.NWA 6950 consists mainly of coarse-grained olivine,pyroxene,and plagioclase,with minor ilmenite,chromite,troilite,baddeleyite,taenite,and apatite.The abundant rounded to euhedral olivine has a limited Fo ranging from 68 to 69.Pyroxenes include both augite and pigeonite,with a narrow range of Mg#=71–80,and composition,respectively,En46-52Fs12-16Wo32-41 and En58-68Fs19-28Wo7-20.Plagioclase is mainly anhedral,conforming to the crystal margins of olivine and pyroxene.The plagioclase grains are elongated and several hundreds of microns in length.Raman spectra shows that plagioclase has been transformed into maskelynite,which is very calcic(An87-95)with low sodium and very low potassium(Ab4-9Or0.6-3.7).The weighted mean 207Pb/206Pb ages of baddeleyite and phosphate measured using a sensitive high resolution ion micro probe(SHRIMP II)are 3119±16 Ma(n=9,MSWD=2.5)(mean squared weighted deviates,MSWD)and 3119±18 Ma(n=15,MSWD=2.1),respectively,giving the crystallization age of the meteorite.NWA 6950 has similar mineral compositions,geochemical characteristics,and an almost identical age of crystallization to the magnesian gabbro of the NWA 773 clan.We conclude that NWA 6950 and NWA 773 are paired meteorites,which crystallize at ca.3119 Ma,and represent magmatic activities in the KREEP(potassium(K),rare earth elements(REE)and phosphorus(P)rich lunar material)region of the Moon.Northwest Africa（NWA） 6950 is an olivine-gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. In this contribution, we report the petrography, mineralogy,and U-Pb geochronology of baddeleyite and phosphate in this lunar meteorite and make a comparison with other NWA 773 paired meteorites. NWA 6950 consists mainly of coarse-grained olivine, pyroxene,and plagioclase, with minor ilmenite, chromite, troilite, baddeleyite, taenite, and apatite. The abundant rounded to euhedral olivine has a limited Fo ranging from 68 to 69. Pyroxenes include both augite and pigeonite, with a narrow range of Mg#=71–80, and composition, respectively, En46-52Fs12-16Wo32-41 and En58-68Fs19-28Wo7-20. Plagioclase is mainly anhedral, conforming to the crystal margins of olivine and pyroxene. The plagioclase grains are elongated and several hundreds of microns in length. Raman spectra shows that plagioclase has been transformed into maskelynite, which is very calcic（An87-95） with low sodium and very low potassium（Ab4-9 Or 0.6-3.7）. The weighted mean207 Pb/206 Pb ages of baddeleyite and phosphate measured using a sensitive high resolution ion micro probe（SHRIMP II） are 3119 ± 16 Ma（n = 9, MSWD= 2.5）（mean squared weighted deviates, MSWD） and 3119 ± 18 Ma（n = 15, MSWD = 2.1）, respectively,giving the crystallization age of the meteorite. NWA 6950 has similar mineral compositions, geochemical characteristics, and an almost identical age of crystallization to the magnesian gabbro of the NWA 773 clan.We conclude that NWA 6950 and NWA 773 are paired meteorites, which crystallize at ca. 3119 Ma, and represent magmatic activities in the KREEP（potassium（K）, rare earth elements（REE） and phosphorus（P） rich lunar material） region of the Moon.

关 键 词：NORTHWEST Africa 6950 lunar METEORITE U-PB CHRONOLOGY BADDELEYITE phosphate 

分 类 号：P597.3[天文地球—地球化学] P185.83[天文地球—地质学]