Shock metamorphism and petrography of the Shergotty achondrite

• Published in: Geochimica et Cosmochimica Acta Vol. 50; no. 6; pp. 889 - 903
• Main Authors: Stöffler, D, Ostertag, R, Jammes, C, Pfannschmidt, G, Gupta, P.R.Sen, Simon, S.B, Papike, J.J, Beauchamp, R.H
• Format: Journal Article
• Language: English; Japanese
• Published: Legacy CDMS Elsevier Ltd 01.06.1986; Elsevier BV
• Subjects: Lunar And Planetary Exploration
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/0016-7037(86)90371-6

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