Transmission electron microscopy study of high pressure phases in the shock-induced melt veins of L chondrites: Constraints on the shock pressure and duration.

摘要

Here, this researcher reports the transmission electron microscopy observations of the mineral assemblages in the shock-induced melt veins in L chondrites of shock stages S3 to S6. The mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures. The goal is to see how crystallization pressures are related to the shock pressures inferred from shock classification of Stoffler et al. (1991). Seven L6 chondrites were investigated: Tenham (S6), Umbarger (S4-S6*), Roy (S3-S5*), Ramsdorf (S4), Kunashak (S4), Nakhon Pathon (S4) and La Lande (S4). The melt veins of Tenham include majorite and magnesiowustite in the center of the melt vein, and ringwoodite, akimotoite, vitrified silicate-perovskite and majorite in the edge of the vein. Both mineral assemblages indicate a crystallization pressure ∼25 GPa. The melt veins of Umbarger include: ringwoodite, akimotoite and clinopyroxene in vein matrix, and Fe2SiO 4-spinel and stishovite in SiO2-FeO rich areas of the melt vein. Both mineral assemblages indicate a crystallization pressure ∼18 GPa. The melt veins of Roy include ringwoodite plus majorite, indicating pressure ∼20 GPa; The melt veins of Ramsdorf and Nakhon Pathon contain olivine and clinopyroxene, indicating pressure less than 15 GPa; The melt veins of Kunashak and La Lande include albite and olivine, indicating crystallization at low pressure (2.5 GPa), after shock pressure release. Based upon the assemblages observed, crystallization of shock veins can occur before, during or after pressure release. When the assemblage consists of high-pressure minerals and that assemblage is uniform across a melt vein or pocket, the crystallization pressure represents the equilibrium shock pressure. Equilibrium shock pressures inferred from the mineralogy of shock-induced melt veins suggest that the pressure calibration of Stoffler et al. (1991) is too high by a factor of at least two for highly shocked (S5-S6) chondrites.