Grove Mountains(GRV)052382,from East Antarctica: Likely a most heavily shocked ureilite.

Grove Mountains(GRV) 052382 is a new highly shocked ureilite found in Grove Mountains,East Antarctica.It is composed mainly of olivine(75%),pigeonite(5%),carbonaceous interstitial(20%) and a little of metal.Olivine is subidiomorphic equigranular texture with the size of 10~20μm,and there is pyroxene melt among them.Based on the distribution of carbonaceous interstitial and grain boundary,the original outlines of olivine grains can be distinguished,and the size of original olivine grains ranges from 0.5 to 1.5mm.The original grains of olivine have reduction zones,which the fine olivine grains in the rim are rich in MgO,while ones in the core are rich in FeO.Due to strong reduction,the core of the original olivine grains varies greatly in composition(Fa12.2-21.8),so the original olivine should be more than 21.8% in Fa.Pigeonite is round-shaped with size of 0.4~0.9mm,and is almost identical in compostion between intergrains(En76.4-82.6Wo4.6-9.8Fs10.9-13.8).But it has wave-varied composition caused by strong shock effect in a single grain.The carbonaceous interstitial between the original olivine grain and pigeonite,is composed mainly of carbon and MgO-rich silicate.The main polymorph of carbon is graphite which occurs as amoiboid-shape and/or net,the size is about 0.2~0.4mm.And the other minor polymorph is diamond which embedded in graphite as small grains(1~3μm).Thus,these petrological characteristics suggest that GRV 052382 has a typical ureilitic texture and is a monomict ureilite.On the basis of the composition of olivine,GRV 052382 is futhurly classed into the FeO subtype(type I).Furthermore,the heavily shocked effects,including(1) shocked fine-grained granulitic texture of olivine;(2) the round outline of fine olivine grains and melt interstitial among the fine grains;(3) the wavy variation of composition in pigeonite;(4) a great deal of irregular shaped vesicles in pigeonite;(5) some kamacite filling in fracture and vesicles,(6) transformation of diamond from graphite,indicate that GRV 052382 has the heavily shock stage of S6.Therefore,GRV 052382 is likely a heavily shocked ureilite,which will probably provide some direct evidence of shock history of ureilites and further to get insight on the history of shock events experienced by the ureilite parent body by the later detail study.