一块新发现的月球陨石NWA 12279的岩石矿物学、源区和冲击变质作用

Petromineralogy, origin and impact metamorphism of a new lunar meteorite NWA 12279

NWA 12279为2016年发现的一块斜长岩质月球陨石,由86%的斜长岩和14%的冲击熔融角砾岩组成。斜长岩具嵌晶结构,矿物组成主要为斜长石(70. 6%)、橄榄石(11. 3%)、辉石(10. 0%)、镁铝尖晶石(7. 0%),含少量石英、铬铁矿和钛铁矿;冲击熔融角砾岩具角砾状结构,主要由岩屑(斜长岩、辉长-橄长-斜长岩、微斑熔融角砾岩、辉长岩)、晶屑(橄榄石、辉石、斜长石、尖晶石)、玻屑和基质组成。斜长岩和角砾岩的矿物组成基本一致,主要为斜长石(An92. 9~98. 4)、紫苏辉石(Fs15. 5~32. 2Wo2. 98~4. 22)、易变辉石(Fs27. 9~53. 1Wo7. 19~14. 7)、普通辉石(Fs8. 42~38. 9Wo17. 0~44. 1)、橄榄石(Fo53. 7~89. 4)、尖晶石[(Mg4. 97Fe0. 86)5. 83(Al11. 4Cr0. 61)12. 0O24]。通过对陨石的矿物组成、碎屑组成、矿物成分、全岩成分和冲击变质特征进行研究,并和已发现的月球斜长岩进行了对比,认为该陨石可能起源于一个新的富含尖晶石的辉长橄长斜长岩高地。该陨石的斜长岩和角砾岩具有不同的冲击特征,斜长岩区域发育橄榄石面状破裂、斜长石熔长石化及含未熔融的辉石和橄榄石晶屑的冲击熔脉;角砾岩区域发育玻璃质熔脉、冲击熔体及岩石角砾化,这些特征限制了斜长岩区和角砾岩区经历的冲击压力峰值分别约为45 GPa和78 GPa,温度峰值分别约为1 100℃和1890℃,冲击变质阶段为S5~S6。

NWA 12279 is a piece of anorthosite lunar meteorite newly discovered in 2016, consisting of 86% anorthite and 14% impact melt breccia. The anorthosite, has a poikilitic texture and is composed of plagioclase( 70. 6%), olivine( 11. 3%), pyroxene( 10. 0%) and magnesium-aluminum spinel( 7. 0%), with a small amount of quartz, chromite and ilmenite. The impact melt breccia is mainly composed of lithic clasts such as anorthosite,gabbro-troctolite-anorthosite, microporphyritic crystalline impact melt breccias and gabbro, 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. 4), hypersthenes( Fs15. 5 ~ 32. 2 Wo2. 98 ~ 4. 22), pigeonite( Fs27. 9 ~ 53. 1 Wo7. 19 ~ 14. 7), augite( Fs8. 42 ~ 38. 9 Wo17. 0 ~ 44. 1), olivine( Fo53. 7 ~ 89. 4), and magnesium aluminum spinel( Mg4. 97 Fe0. 86)5. 83( Al11. 4 Cr0. 61)12. 0 O24. Based on mineral composition, clastic composition, mineral components, total rock composition and shock-metamosphism features of the meteorite, as well as a comparison with the discovered lunar anorthosite, the authors hold that the meteorite may originate from a new spinel-rich gabbro-troctolite-anorthosite highland. 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, and 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 ~ 1 100℃ and ~ 1 890℃ respectively. The shock stage is S5~ S6.