Origin of unusual impact melt rocks, Yamato-790964 and 790143 (LL-chondrites)

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We have studied Yamato (Y)-790964 and-790143,which are unusual paired impact-melted LL chondrites. Some petrologic aspects of these impact melt rocks are similar to those of the impact melted L-chondrite, Ramsdorf; these meteorites experienced nearly total melting, yet partly preserve chondritic textures. Olivine and chromite grains in Y-790964 and olivine in Y-790143 are the only relicts of the precursor materials; they were solid clasts during impact melting. All other portions (i.e., chondrules and matrices) were totally melted. It appears that the chondrule ghosts, which are mainly composed of very fine-grained igneous-textured pyroxene, minor olivine and other phases, were melted and crystallized in situ, without significant mixing with less viscous melts. Partial resorption of olivines and chromites suggests that the post-shock temperature could be >∿1600℃. The minor local differences in mineralogy appear to be consistent with small spatial variations in thermal history. Evidence of complex injection of troilite, Fe, Ni-metal and feldspathic glass into the relict minerals suggests that the precursor rock was shocked in situ. The shock pressure might have been >75 or 90GPa for a non-porous precursor rock or >20-40GPa for a porous rock.
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