Magnetite in ALH 84001: An origin by shock-induced thermal decomposition of iron carbonate

Abstract

In martian orthopyroxenite ALH 84001, pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact(s). Transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad, nanometer-sized magnetite particles with cuboid, irregular, and teardrop morphologies, frequently associated with voids. The fragments of carbonate must have been incorporated into the melt at temperatures of ~900°C, well above the upper thermal stability of siderite (FeCO3), which decomposes to produce magnetite and CO2 below ~450°C. These observations suggest that most, if not all, of the fine-grained magnetite associated with Fe-bearing carbonate in ALH 84001 could have been formed as result of the thermal decomposition of the siderite (FeCO3) component of the carbonate and is not due to biological activity.