曼桂陨石的岩石矿物学和冲击变质特征
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摘要
曼桂陨石是2018年6月降落在云南省西双版纳傣族自治州的目击降落型陨石.陨石样品具有新鲜的熔壳,内部呈浅灰色,可见黑色冲击熔融细脉.该陨石主要由橄榄石、辉石、长石以及较少的铁镍金属、陨硫铁、磷灰石等组成.手标本虽可见球粒特征,但背散射电子图像显示球粒界线模糊,残余球粒为斑状橄榄石球粒,基质呈现强烈重结晶特征,次生长石粒度较大,橄榄石和辉石的化学成分均一,可判定其岩石类型为6型.橄榄石的平均Fa值为25.1±0.3(n=71),辉石的平均Fs值为21.1±0.3(n=58),金属含量低,属于L化学群.因此,曼桂陨石是L6型普通球粒陨石.曼桂陨石遭受过强烈的冲击,冲击熔脉和熔融囊广泛发育,熔脉宽度可达600μm.熔脉内及边部的长石已经转变成熔长石,且部分长石转变成硬玉;发现辉石的镁铁榴石高压相,与硬玉伴生.根据这些强烈冲击变质特征,将曼桂陨石的冲击变质强度划分为S5级.尚未在熔脉及边部发现橄榄石、磷酸盐的高压相矿物.这些高压矿物相的缺失,可能是由于该陨石在冲击熔融后的冷却速率较慢,形成的高压相矿物发生了退变质.这些特征表明,曼桂陨石经历过强烈的撞击熔融事件,为研究其母体的撞击历史和高压相的形成机制提供了重要标本.
The Mangui meteorite, also known as the Xishuangbanna meteorite, fell in Menghai, Xishuangbanna Dai Nationality Autonomous Prefecture, Yunnan Province, China at ～9:45 pm on June 1, 2018. More than 500 fragments of Mangui were collected a total mass of ～50 kg. The mass of the largest fragment is ～1228 g. Mangui has a black fusion crust, a light gray exposure surface, and contains many black shock melt veins and pockets. In this work, the classification of Mingui and its shock history are investigated by studying its petrology, mineral chemistry, and the nature of its shock assemblages. Mangui is mainly composed of olivine, pyroxene, feldspar, Fe-Ni metal, troilite, chromite, merrillite and apatite, showing typical petrologic features of an ordinary chondrite. Relict chondrules are observed in hand specimens but their boundaries are less clear under FE-SEM. Electron Probe Micro Analysis(EPMA) shows that the chemical compositions of olivine(Fa25.1±0.3 mol%(n=71)), low-Ca pyroxene(Fs21.1±0.3 Wo1.5±0.2 mol%(n=58)) and plagioclase Ab84.1±0.7 Or7.3±0.4 mol%(n=49)) in Mangui are relatively homogeneous. The petrographic type of Mangui is classified as type 6 by the coarse grains of secondary plagioclase, the homogeneous compositions of olivine and pyroxene, the recrystallization of matrix and blurry boundaries of relict chondrules. The chemical group of Mangui is L based on the average Fa content of olivine(Fe/(Fe+Mg)×100, atomic percentage, 25.1±0.3, n=71) and the average Fs content of pyroxene(Fe/(Fe+Mg+Ca)×100, atomic percentage, 21.1±0.3, n=58). Therefore, the Mangui meteorite is classified as an L6 ordinary chondrite. The Mangui parent body experienced strong shock metamorphism with the widespread distribution of shock melt veins with widths up to 600 μm. Shock induced melt pockets were also observed. Micron-sized olivine and pyroxene in the center of the shock melt veins display chemical zonation, suggesting that they recrystallized from the shock induced melt. Plagioclase in the shock melt veins has transformed into maskelynite. Two high pressure polymorphs, jadeite and majorite, occur in the shock melt veins and melt pockets as measured by Raman analysis. Jadeite usually displays granular textures in these assemblages as euhedral to sub-euhedral crystals. Majorite was only found in one jadeite assemblage, occurring as inclusions in outer rim of an assemblage. The co-occurrence of majorite and jadeite indicate shock pressures from 17–20 GPa at temperatures ranging from 1900–2000°C. However, ringwoodite, whitlockite and other high pressure polymorphs have not been identified in the shock melt veins pockets. The occurrences of maskelynite, jadeite and majorite, and widespread distribution of shock melt veins and pockets suggest the shock stage of Mangui as S5. The absence of ringwoodite and tuite probably attribute to slow post-shock cooling rates, suggesting that the Mangui parent body experienced strong shock metamorphism with a subsequent slow post-shock cooling history. TEM and XRD work will be performed to better constrain the formation mechanisms of jadeite and majorite and their P-T-t histories. Furthermore, we aim on modelling the impact conditions on the Mangui parent body such as impact velocity, and parent body-impactor sizes by further study of the high-pressure minerals in shock-melt veins of the Mangui meteorite.
The Mangui meteorite, also known as the Xishuangbanna meteorite, fell in Menghai, Xishuangbanna Dai Nationality Autonomous Prefecture, Yunnan Province, China at ～9:45 pm on June 1, 2018. More than 500 fragments of Mangui were collected a total mass of ～50 kg. The mass of the largest fragment is ～1228 g. Mangui has a black fusion crust, a light gray exposure surface, and contains many black shock melt veins and pockets. In this work, the classification of Mingui and its shock history are investigated by studying its petrology, mineral chemistry, and the nature of its shock assemblages. Mangui is mainly composed of olivine, pyroxene, feldspar, Fe-Ni metal, troilite, chromite, merrillite and apatite, showing typical petrologic features of an ordinary chondrite. Relict chondrules are observed in hand specimens but their boundaries are less clear under FE-SEM. Electron Probe Micro Analysis(EPMA) shows that the chemical compositions of olivine(Fa25.1±0.3 mol%(n=71)), low-Ca pyroxene(Fs21.1±0.3 Wo1.5±0.2 mol%(n=58)) and plagioclase Ab84.1±0.7 Or7.3±0.4 mol%(n=49)) in Mangui are relatively homogeneous. The petrographic type of Mangui is classified as type 6 by the coarse grains of secondary plagioclase, the homogeneous compositions of olivine and pyroxene, the recrystallization of matrix and blurry boundaries of relict chondrules. The chemical group of Mangui is L based on the average Fa content of olivine(Fe/(Fe+Mg)×100, atomic percentage, 25.1±0.3, n=71) and the average Fs content of pyroxene(Fe/(Fe+Mg+Ca)×100, atomic percentage, 21.1±0.3, n=58). Therefore, the Mangui meteorite is classified as an L6 ordinary chondrite. The Mangui parent body experienced strong shock metamorphism with the widespread distribution of shock melt veins with widths up to 600 μm. Shock induced melt pockets were also observed. Micron-sized olivine and pyroxene in the center of the shock melt veins display chemical zonation, suggesting that they recrystallized from the shock induced melt. Plagioclase in the shock melt veins has transformed into maskelynite. Two high pressure polymorphs, jadeite and majorite, occur in the shock melt veins and melt pockets as measured by Raman analysis. Jadeite usually displays granular textures in these assemblages as euhedral to sub-euhedral crystals. Majorite was only found in one jadeite assemblage, occurring as inclusions in outer rim of an assemblage. The co-occurrence of majorite and jadeite indicate shock pressures from 17–20 GPa at temperatures ranging from 1900–2000°C. However, ringwoodite, whitlockite and other high pressure polymorphs have not been identified in the shock melt veins pockets. The occurrences of maskelynite, jadeite and majorite, and widespread distribution of shock melt veins and pockets suggest the shock stage of Mangui as S5. The absence of ringwoodite and tuite probably attribute to slow post-shock cooling rates, suggesting that the Mangui parent body experienced strong shock metamorphism with a subsequent slow post-shock cooling history. TEM and XRD work will be performed to better constrain the formation mechanisms of jadeite and majorite and their P-T-t histories. Furthermore, we aim on modelling the impact conditions on the Mangui parent body such as impact velocity, and parent body-impactor sizes by further study of the high-pressure minerals in shock-melt veins of the Mangui meteorite.
引文
1 Van Schmus W R,Wood J A.A chemical-petrologic classification for the chondritic meteorites.Geochim Cosmochim Acta,1967,31:747-765
2 Rubin A E.Kamacite and olivine in ordinary chondrites:Intergroup and intragroup relationships.Geochim Cosmochim Acta,1990,54:1217-1232
3 Galimov E M,Kolotov V P,Nazarov M A,et al.Analytical results for the material of the chelyabinsk meteorite.Geochem Int,2013,51:522-539
4 Zucolotto M E,Antonello L L,Varela M E,et al.Varre-sai:The recent brazilian fall.Earth Moon Planets,2012,109:43-53
5 Lodders K,Fegley B.The Planetary Scientist’s Companion.New York:Oxford University Press,1998
6 St?ffler D,Keil K,Scott E R D.Shock metamorphism of ordinary chondrites.Geochim Cosmochim Acta,1991,55:3845-3867
7 Xie X D,Chen M.Investigation of mineral assemblage in the Earth’s mantle on the basis of study of the Suizhou meteorite(in Chinese).Acta Metall Sin,2005,24:277-285[谢先德,陈鸣.从随州陨石研究探讨地幔的高压矿物组成.矿物岩石地球化学通报,2005,24:277-285]
8 Ding M W,Zhang A C,Xu W B.Thermal and shock metamorphism of the Sixiangkou ordinary chondrite(in Chinese).Acta Astronom Sin,2008,49:55-66[丁明伟,张爱铖,徐伟彪.寺巷口普通球粒陨石的热变质和冲击变质历史研究.天文学报,2008,49:55-66]
9 Kondo T,Ohtani E,Goresy A E.Natural occurrence of high-pressure phases,jadeite,hollandite,wadsleyite,and majorite-pyrope garnet,in an H chondrite,Yamato 75100.Meteorit Planet Sci,2010,35(Suppl):A87-A88
10 Kubo T,Kimura M,Kato T,et al.Plagioclase breakdown as an indicator for shock conditions of meteorites.Nat Geosci,2010,3:412-416
11 Ohtani E,Ozawa S,Miyahara M.Jadeite in shocked meteorites and its textural variations.J Mineral Petrol Sci,2017,112:247-255
12 Miyahara M,Ohtani E,Yamaguchi A.Albite dissociation reaction in the Northwest Africa 8275 shocked LL chondrite and implications for its impact history.Geochim Cosmochim Acta,2017,217:320-333
13 Ozawa S,Miyahara M,Ohtani E,et al.Jadeite in Chelyabinsk meteorite and the nature of an impact event on its parent body.Sci Rep,2014,4:5033
14 Bazhan I S,Ozawa S,Miyahara M,et al.“Spherulite-like”jadeite growth in shock-melt veins of the Novosibirsk H5/6 chondrite.Russ Geol Geophys,2017,58:12-19
15 Tomioka N,Miyahara M,Ito M.Discovery of natural MgSiO3 tetragonal garnet in a shocked chondritic meteorite.Adv Sci,2016,2:1-5
2 Rubin A E.Kamacite and olivine in ordinary chondrites:Intergroup and intragroup relationships.Geochim Cosmochim Acta,1990,54:1217-1232
3 Galimov E M,Kolotov V P,Nazarov M A,et al.Analytical results for the material of the chelyabinsk meteorite.Geochem Int,2013,51:522-539
4 Zucolotto M E,Antonello L L,Varela M E,et al.Varre-sai:The recent brazilian fall.Earth Moon Planets,2012,109:43-53
5 Lodders K,Fegley B.The Planetary Scientist’s Companion.New York:Oxford University Press,1998
6 St?ffler D,Keil K,Scott E R D.Shock metamorphism of ordinary chondrites.Geochim Cosmochim Acta,1991,55:3845-3867
7 Xie X D,Chen M.Investigation of mineral assemblage in the Earth’s mantle on the basis of study of the Suizhou meteorite(in Chinese).Acta Metall Sin,2005,24:277-285[谢先德,陈鸣.从随州陨石研究探讨地幔的高压矿物组成.矿物岩石地球化学通报,2005,24:277-285]
8 Ding M W,Zhang A C,Xu W B.Thermal and shock metamorphism of the Sixiangkou ordinary chondrite(in Chinese).Acta Astronom Sin,2008,49:55-66[丁明伟,张爱铖,徐伟彪.寺巷口普通球粒陨石的热变质和冲击变质历史研究.天文学报,2008,49:55-66]
9 Kondo T,Ohtani E,Goresy A E.Natural occurrence of high-pressure phases,jadeite,hollandite,wadsleyite,and majorite-pyrope garnet,in an H chondrite,Yamato 75100.Meteorit Planet Sci,2010,35(Suppl):A87-A88
10 Kubo T,Kimura M,Kato T,et al.Plagioclase breakdown as an indicator for shock conditions of meteorites.Nat Geosci,2010,3:412-416
11 Ohtani E,Ozawa S,Miyahara M.Jadeite in shocked meteorites and its textural variations.J Mineral Petrol Sci,2017,112:247-255
12 Miyahara M,Ohtani E,Yamaguchi A.Albite dissociation reaction in the Northwest Africa 8275 shocked LL chondrite and implications for its impact history.Geochim Cosmochim Acta,2017,217:320-333
13 Ozawa S,Miyahara M,Ohtani E,et al.Jadeite in Chelyabinsk meteorite and the nature of an impact event on its parent body.Sci Rep,2014,4:5033
14 Bazhan I S,Ozawa S,Miyahara M,et al.“Spherulite-like”jadeite growth in shock-melt veins of the Novosibirsk H5/6 chondrite.Russ Geol Geophys,2017,58:12-19
15 Tomioka N,Miyahara M,Ito M.Discovery of natural MgSiO3 tetragonal garnet in a shocked chondritic meteorite.Adv Sci,2016,2:1-5