'Eclogitic' minerals in a shocked basaltic meteorite

Analytical transmission electron microscopy (ATEM) was used to characterize mineral parageneses occurring in melt veins of shock origin in the Martian meteorite Zagami. We discovered a new ultra-high pressure 'eclogite facies ' mineral assemblage consisting of omphacite, stishovite, and KAlSi3O8 holl andite. These tiny (< 200-300 nm) high-pressure minerals are embedded in si licate glass, which forms the groundmass of the shock veins. The microtextu re of the veins and zoning of the clinopyroxene indicate the formation of m inerals by crystallization from a high-pressure melt. Using calibration dat a from shock and multi-anvil experiments we infer that frictional melting o ccurred in veins at minimum temperatures of 2400-2500 degrees C and at a pr essure of ca. 30 GPa. Subsequently, the high-pressure phases crystallized d uring decompression in the pressure interval from 25-5 GPa. (C) 2000 Publis hed by Elsevier Science B.V. All rights reserved.