A comparative study of naturally and experimentally shocked chondrites

Samples of the Jilin H5 chondrite were experimentally shock-loaded at the p eak pressures of 12, 27, 39, 53, 78, 83, 93, and 133 GPa. The aim of this s tudy is to compare experimentally shock-induced phenomena with those in nat urally shocked chondrites and to test the feasibility of experimentally cal ibrating naturally induced shock phenomena in Hand L-chondrites. Planar fra ctures, mosaicism, brecciation in olivine and pyroxene, as well as transfor mation of plagioclase into diaplectic glass were observed in the Jilin samp les shocked at pressures lower than 53 GPa. Shock-induced chondritic melts were first obtained at P > 78 GPa and more than 60% of the whole-rock melti ng was achieved at P similar to 133 GPa, and that shook-induced silicate me lt consists of quenched microcrystalline olivine and pyroxene, metal, troil ite and vesicular glass. No high-pressure phases were observed in any of th e experimentally shocked samples, neither in the deformed nor in the molten regions. Deformation features in Jilin samples shock-loaded below 53 GPa a re comparable to those found in H- and L-chondrites. The mineral assemblage s in the molten regions in the shocked Jilin samples are also comparable to those encountered in the heavily shocked Yanzhuang (H6) and some Antarctic H-chondrites, but differ considerably from those found in heavily shocked Sixiangkou and many other L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine, pyroxene, plagioclase and high -pressure liquidus phases, whereas shock melt veins in heavily shocked H-ch ondrites contain mainly low-pressure mineral assemblages. The differences i n the mineral constituents of shock melt veins in L- and H-chondrites clear ly indicate differences in the shock histories of these meteorites. While c rystallization in the shook melt veins in L-chondrites took place at high p ressures, crystallization in shock-induced melt in most H-chondrites took p lace after decompression. It is evident that the thickness and abundance of shock melt veins and size of melt regions is not necessarily a quantitativ e measure of the degree of shock. The duration of the high-pressure regime, the time of the cooling and the P-T regime during the crystallization path , and the post-shock temperatures are stringent parameters that control the evolution of the shock-induced melt. So, scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized aste roids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids. (C) 2001 Elsevier Science B.V. All right s reserved.