Shock metamorphism and petrography of the Shergotty achondrite

摘要: The Shergotty subsamples 1 and 12 consist of augite pigeonite (67.5%), maskelynite (24%), ilmenite titanomagnetite (2%), pyrrhotite (0.4%), whitlockite (1.8%), apatite (0.1%), quartz (0.5%), baddeleyite (trace), fayalite mesostasis (3%), shock-induced local, polymineralic melt products (0.6%). overall modal composition is similar to other samples except for the rather high content. shock effects observed in mineral constituents include mosaicism, deformation bands, planar fractures, mechanical twin lamellae clinopyroxene; isotropization plagioclase with very rare remnants birefringence; structures, strongly reduced birefringence quartz; twinning ilmenite; localized situ melting neighbouring minerals at contact low density phases. Based on refractive index (average: 1.5467 average An-content 49%) degree an equilibrium pressure 29 ± GPa derived. inferred post-shock temperature 200 20°C. No heating event could have exceeded 400°C (DUKE, 1968). Local stress concentrations reach 60–80 1600–2000°C. can be explained by a single event. A second, weaker as found others appears highly improbable. Equilibrium pressures temperatures known shergottites are 31 ±2 220 50°C (Zagami), 43 2 400–800°C (ALHA 77005). estimate EETA 79001 Lambert (1985) confirmed: 34 GPa; 250 50°C. abundance textural setting these meteorites confirm increasing sequence Shergotty, Zagami, 79001, ALHA 77005. Undoubtedly, melts been formed same which produced (e.g. maskelynitization) meteorites. particle velocities from Hugoniot data basalts 1.5–2.0 km/s range parental rocks shergottites. Ejection therefore order 3–4 km/s. Special ejection mechanisms required exceed escape velocity planet like Mars without producing higher degrees melting) than those