Shock compression of a recrystallized anorthositic rock from Apollo 15

Abstract

Hugoniot measurements on 15,418, a recrystallized and brecciated gabbroic anorthosite, yield a value of the Hugoniot elastic limit (HEL) varying from - 45 to 70 kbar as the final shock pressure is varied from 70 to 280 kbar. Above the HEL and to 150 kbar the pressure-density Hugoniot is closely described by a hydrostatic equation of state constructed from ultrasonic data for single-crystal plagioclase and pyroxene. Above - 150 kbar, the Hugoniot states indicate that a series of one or more shock-induced phase changes are occurring in the plagioclase and pyroxene. From Hugoniot data for both the single-crystal minerals and the Frederick diabase, we infer that the shock-induced high-pressure phases in 15,418 probably consist of a 3.71 g/cm^3 density, high-pressure structure for plagioclase (An_(93)) and a 4.70 g/cm^3 perovskite-type structure (En_(64)) for pyroxene. Using the Kelly Truesdell mixture theory we separately calculated the entropy production in each phase, and predict incipient and complete melting in the plagioclase occurs upon release from ~ 500 and ~ 600 kbar. For the pyroxene component, incipient and complete melting occurs upon release from 700 and 850 kbar. The onset of shock-induced vaporization will occur upon release from ~ 1300 kbar and would require the impact of an iron meteoroid traveling at a velocity of ~8 km/sec.