THE LOS ANGELES MARTIAN METEORITE: AN ANALYTICAL ELECTRON MICROSCOPY STUDY OF SHOCK-INDUCED MELT POCKETS

Introduction: A brief description of shock features within Los Angeles, based on optical observations, is given by [1]. Two polished thin sections have been investigated to determine the mineralogy and microtextures of the shock-induced melt pockets using analytical scanning electron microscopy. This instrument is capable of characterizing compositional variations and microtextures of the melt pockets that are unresolvable using optical techniques.

Melt pockets: Two texturally distinguishable types have been observed: (1) Small (0.5 m x 0.70 mm) vesiculated, clast-rich melt pockets with abundant crystallites indicative of quenching. The contact with adjacent host rock minerals is intrusive with abundant stringers and veins of shock-induced melt material extending into them. (2) With increasing volume of melt (3.5 mm x 3.0 mm) textures are dominantly vesicular, clast-poor with flow textures and abundant spheres/blebs of immiscible Fe-sulfides. Severely fractured, melt-intruded clinopyroxenes adjacent to the melt pockets allowed for preferential nucleation of dendritic pigeonite and orthopyroxene crystals that now extend from the contact into glasses. Later devitrification of glasses seeded preferential nucleation of phosphate grains (whitlockite). Opaque deposits and shock-induced melt glasses in irregular and planar fractures of clinopyroxene are observed to have a very close spatial relationship with the melt zones. A high concentration of melt-bearing fractures causes a distinct darkening (opacity) of pyroxene grains in optical microscopy (shock blackening).

Discussion: Vesicular melt pockets are formed during shock compression, which persist in a molten state during pressure release and are thus not exclusively high-pressure [2]. However, this does not preclude the presence of high pressure, high temperature polymorphs within the melt pockets produced by cavitation and bubble implosion [3].

[1] Rubin et al. (2000) Geology, 28, 1011-1014. [2] Kenkmann et al. (2000) Meteorit. Plant. Sci., 35, 1275-1290. [3] Spray, J.G. (1999) Geology, 27, 695-698.