Meteorites are the extraterrestrial rocks,which provide insights into the origin and evolution of the solar system.During the past half century,a great number of meteorites has been discovered on the Antarctic Ice Sheet,confirming that the Antarctica is the most important meteorite concentration area on the earth.Since the first four Antarctic meteorites were found in Grove Mountains in 1998,a total of 9834 meteorites have been collected by four subsequent expeditions.It opens a new field of meteorite study in China,and also accumulates a great deal of scientific samples for China.Recently,classification of Grove Mountains meteorites has been carried out for 6 years,and made following progresses:(1)2433 meteorites,which include many special meteorites,e.g.Martian meteorites,ureilites and carbonaceous chondrites,have been classified.(2)the Antarctic meteorite curation and the sample sharing system are set up preliminarily.(3)the classification procedure,the management of meteorite samples,and the application procedure for the Antarctic meteorites are completed after the systematic classification during these years.(4)young generation researchers on meteorite are trained through the cooperation of many universities and institutes on meteorite classification.
Ureilites are a common group of achondrites with a high abundance of carbon.They probably have a genetic relationship with chondrites,hence provide an insight into origin and evolution of terrestrial planets.A new meteorite-rich region,Grove Mountains(GRV),was found by the Chinese Antarctic Research Expedition,with discovery of 9834 meteorites.Of 2433 meteorites classified,9 ureilites have been identified.In this paper,we report petrography of 6 of these ureilites.Four ureilites contain graphite and exhibit triangle conjunction and common reduced margins of olivine.GRV 052382 probably experienced heavy shock metamorphism followed by fast cooling,as indicated by mosaic texture or fine-grained granular texture of olivine.GRV 022931 was highly reduced of these ureilites,with olivine as isolated grains in abundant carbonaceous matrix.All 9 ureilites are monomict,and are classified into subtype II(with medium FeO content,Fa15-18)and subtype I(with high FeO content,Fa>18)based on compositions of the cores of olivine.The diverse mineral compositions and petrography of these ureilites suggest that they are not paired and reveal a multi-event history of the parent body.Partial melting of the parent body produced carbon-rich magma,followed by crystallization of graphite and silicates.Later,graphite was partially inverted to diamond by shock events.Reburial of the shocked debris experienced various degree of thermal metamorphism.Finally,these rocks were excavated from the parent asteroid and ejected into Earth-cross orbit by another impact event.
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.
