Shock effects and the classification of H-chondrites from the Grove Mountains,East Antarctica:Implications for the shock history of H-chondrite parent bodies

The shock metamorphism of 47 H group chondrites (H-chondrites) from the Grove Mountains including undulatory extinction,planar fractures,mosaic extinction,shock veins and pockets,and dendritic eutectic metal-sulfide,is observed through optical microscope.The textures and assemblages of shock veins in these H-chondrites are examined by the scanning electron microscope.Based on observations of the above shock effects,the shock stages of the 47 H-chondrites are classified into S1(5),S2(19),S3(14),S4(8) and S5(1).Of these H-chondrites,GRV 022469 has the highest(S5) shock stage.The comparison of shock stages in these H-chondrites with L group chondrites(L-chondrites) indicates that the shock metamorphism of H-chondrites is relatively low (except for GRV 022469,they are all lower than S5).A scenario for the history of the H-chondrite parent body is proposed that suggests the duration of the shock events in the H-chondrite parent bodies was much shorter than those in L-chondrite parent bodies.Also,the pressure may have been released more quickly,and consequently,the high-pressure phases should be easily preserved.However,the parent bodies of the H-chondrites may have been exposed to high temperatures for a longer time after the shock event,so the high-pressure phases formed by solid transformation might have retro-metamorphosed to low-pressure ones;its peak pressure is estimated to be less than 15 GPa.Wadsleyite was found in a shock vein in GRV022469,as confirmed by the Raman spectrometer.Petrological and mineralogical characteristics support the idea that the wadsleyite was formed by solid-state transformation.