The University of Arizona

Transmission electron microscopy of minerals in the martian meteorite Allan Hills 84001

D. J. Barber, E. R. D. Scott

Abstract


We have studied carbonate and associated oxides and glasses in a demountable section of Allan Hills 84001 (ALH 84001) using optical, scanning, and transmission electron microscopy (TEM) to elucidate their origins and the shock history of the rock. Massive, fracture-zone, and fracture-filling carbonates in typical locations were characterized by TEM, X-ray microanalysis, and electron diffraction in a comprehensive study that preserved textural and spatial relationships. Orthopyroxene is highly deformed, fractured, partially comminuted, and essentially unrecovered. Lamellae of diaplectic glass and other features indicate shock pressures >30 GPa. Bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene. Carbonate crystals are >5 μm in size, untwinned, and very largely exhibit the R3c calcite structure. Evidence of plastic deformation is generally found mildly only in fracture-zone and fracture-filling carbonates, even adjacent to highly deformed orthopyroxene, and appears to have been caused by low-stress effects including differential shrinkage. High dislocation densities like those observed in moderately shocked calcite are absent. Carbonate contains impact- derived glasses of plagioclase, silica, and orthopyroxene composition indicating brief localized impact heating. Stringers and lenses of orthopyroxene glass in fracture-filling carbonate imply flow of carbonates and crystallization during an impact. Periclase (MgO) occurs in magnesite as 30­50 nm crystals adjacent to voids and negative crystals and as ~1 μm patches of 3 nm crystals showing weak preferred orientation consistent with (111)MgO//(0001)carb, as observed in the thermal decomposition of CaCO3 to CaO. Magnetite crystals that are epitaxially oriented at voids, negative crystals, and microfractures clearly formed in situ. Fully embedded, faceted magnetites are topotactically oriented, in general with (111)mag//(0001)carb, so that their oxygen layers are aligned. In optically opaque rims, magnetites are more irregularly shaped and, except for the smallest crystals, poorly aligned. All magnetite and periclase crystals probably formed by exsolution from slightly non-stoichiometric, CO2-poor carbonate following impact-induced thermal decomposition. Any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity.

Keywords


Electron microscopy;Mars;Carbonate;Antarctic meteorites

Full Text:

PDF