云南西双版纳(曼桂)目击陨石的热变质和冲击变质研究

Thermal and Shock Metamorphism Study on the Xishuangbanna (Mangui) Ordinary Chondrite

曼桂陨石是新近(2018年6月1日)陨落在云南西双版纳地区的目击球粒陨石,其中的主要矿物为橄榄石(Fa24.3±0.6)、斜方辉石(Fs20.6±0.5Wo0.4±0.2)、长石(An11-12Or2-4)、铁镍金属和陨硫铁,次要矿物为铬铁矿、白磷钙矿等.橄榄石和辉石的化学成分表明曼桂陨石属于L型普通球粒陨石.陨石中球粒很少且轮廓不清晰,基质矿物普遍经历过重结晶,重结晶矿物颗粒粒径较大,高钙辉石粒径25–30μm,长石颗粒大多超过50μm,说明该陨石属于6型岩石类型.橄榄石和辉石颗粒中发育有波状消光、嵌晶块状消光现象、平面裂隙和面状变形构造,长石熔长石化并不完全,陨石中观察到宽度不等且纵横交错的冲击熔融脉,表明曼桂普通球粒陨石受到的冲击变质作用可以达到S5级以上.通过熔融脉中的矿物组合推断出熔融脉中矿物经历的压力在15–16 GPa左右.

Mangui is a recent witnessed fall in Yunnan,China.This meteorite has experienced high level of thermal and shock metamorphism.The major minerals are olivine(Fa24.3±0.6),low-Ca pyroxene(Fs20.6±0.5Wo0.4±0.2),feldspar(An11-12Or2-4),ironnickel metal,and troilite.The minor minerals include chromite,and merrillite.The mineral chemistry of Mangui indicates that it is the L group of the ordinary chondrite.The chondrules in Mangui are rare and poorly defined which set in a recrystallized granular matrix.The matrix is also mainly olivine and pyroxene,with minor high-Ca pyroxene(about 25–30μm)and maskelynite(up to 50μm).The petrological feature of Mangui is consistent with the petrologic type 6.Olivine and pyroxene grains show undulose extinction,planar fractures,mosaicism,and planar deformation features,and the feldspar grains are partly transformed into maskelynite due to shock.Shock-induced melt veins with varying widths are pervasive in the meteorite.Several high-pressure polymorphs(such as ringwoodite and majorite)are also found in the veins.On the basis of these observations,it can be inferred that Mangui has experienced a high shock metamorphism(S5 level)and the peak pressure in the melt veins was around 15–16 GPa.