The nature of maskelynite in shocked meteorites: not diaplectic glass but a glass quenched from shock-induced dense melt at high pressures

Maskelynite, an important constituent of shocked meteorites, once thought t o be diaplectic plagioclase glass formed by shock-induced solid-state trans formation. Our systematic investigation of shocked L-chondrites and SNC met eorites indicates that maskelynite does not contain inherited fractures or cleavage, and shock-induced fractures. We found no evidence for models call ing for melting that initiated in PDFs and affected the whole crystals. Mas kelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene. This is indicative of shock-induced me lting and quenching of the dense melt at high pressure, thus erasing the in herited and shock-induced fractures. This was followed by relaxation of the dense plagioclase glass, which induced the expansion cracks in pyroxene an d olivine. Enrichment in potassium, deviation from stoichiometry, degradati on of igneous zoning, the presence of offshoots of maskelynite in pyroxene, the lack of vesiculation in the melt pockets, melt veins and molten mesost asis are clear evidence for melting and quenching under high pressure. Our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid-state transformation, b ut a dense quenched glass. The duration of the shock pulse in natural event s can be several orders of magnitude longer than in shock experiments. Sinc e kinetic effects are crucial factors in promoting phase transitions, vitri fication and melting, experimentally induced solid-state vitrification of p lagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite-bearing natural samples. (C) 2000 Else vier Science B.V. All rights reserved.