Petrology,Mineralogy And Shock Metamorphism Of GRV090226 And GRV090228 Meteorites From Antarctica

Shock metamorphism feature is an important characteristic of meteorites and a focus content of meteorites studies.Meteorite parent bodies experienced once or several times impact event in space,that event produced temperature and pressure as high as the P-T conditions of the mantle transition zone,the lower mantle,even the outer core.On the other hand,The initial material of earth exactly is ordinary chondrites.Therefore,the high pressure minerals in meteorites are treasures of understanding mantle mineralogy.GRV090226 and GRV090228 meteorites were collected by 26~thh CHINARE from the Grove Mountains.Petrography and mineralogy of these two meteorites have been studied by micro analysis technologies(EMPA,SEM and LRS).Based on the study results we classified the two meteorites and investigated their shock metamorphism features.GRV090226 and GRV090228 meteorites are two ordinary chondrites(OC)experienced strong shock metamorphism.Chondrules of these two meteorites commonly are relict grains poorly to be delineated.The matrix recrystallized obviously(The matrix has a high degree of recrystallization).The composition of olivine and low-Ca pyroxene are homogeneous.Mineral composition in molecular percent separately are:GRV090226,olivine,av.Fa=23.6mol%(PMD=3.19),low-Ca pyroxene av.Fs=19.9mol%(PMD=1.84);GR090228,olivine,av.Fa=24.0mol%(PMD=2.06),low-Ca pyroxene,av.Fs=20.3mol%(PMD=2.8).We can observe the silicate minerals fracture,wavy extinction,planar deformation features in the host rocks.Feldspar is maskelynite.In the shock-induced melt veins,we discovered olivine-ringwoodite aggregates and majorite high-pressure mineral assemblages.Iron nickel metal and troilite are eutectic.Through the above work,the main conclusions are as follows:(1)GRV090226 and GRV090228 meteorites both are L5 type ordinary chondrites,the stage of shock metamorphism is S5,and the weathering grade is W2.(2)According to the characteristics of mineral deformation and phase change combination in the two meteorites,it is speculated that,during producing the shock-induced melt veins,the formation pressure of the high pressure minerals is between 18GPa~23GPa,and the temperature may be in the range of 1800~2500°C.(3)There are two formation mechanism of ringwoodite in the shock-induced melt veins:solid transformation and melt crystallization.(4)The found location of GRV090226 and GRV090228 meteorites were fairly close,with the same chemical group,petrological type,mineral composition,shock and weathering degree,these characteristics show that the two meteorites may be paired meteorites with strong shock,futher confirm need to determinate their terrestrial age.