Subdivision of petrologic type of unequilibrated ordinary chondrites from Antarctica

We report results of petrologic and mineralogical studies of 25 unequilibrated ordinary chondrites(19 Chinese Antarctic meteorites)using electron microprobe,scanning electron microscopy(SEM)and energy-dispersive X-ray spectrometry(EDS)techniques.With increasing degree of thermal metamorphism,chemical zoning of olivine was erased;plessite was transformed into intergrowths of coarser-grained kamacite and Ni-rich metal;Cr exsolved from ferroan olivine,produced needle-like crystals and coarsened into equant chromite grains;and feldspar crystallized during devitrification of glass and recrystallization of matrix.These features can help to identify different metamorphic grades.Based on the scheme by Sears et al(1982),we subdivided type 3 Chinese Antarctic ordinary chondrites into petrologic type 3.3 to 4(3 type 3.3,3 type 3.4,1 type 3.5,2 type 3.6,2 type 3.7,5 type 3.8,3 type 4).This classification scheme is a quite effective way to subdivide Antarctic meteorites.Additionally,we propose to revise the chemical groups of GRV 020032 and GRV 020104 to L and H,respectively.