Formation and relative ages of maskelynite and carbonate in ALH84001

The morphology and stoichiometry of feldspathic glass in the Martian meteorite ALH84001 indicates it is maskelynite (a diaplectic glass) rather than a flowed glass, although this;glass was heterogeneously affected by a tertiary set of processes. An impact event with shock pressures in excess of 31 GPa was needed to convert the original plagioclase (An(36)Ab(60)Or(4)) to maskelynite. Carbonate is intimately associated with the maskelynite, and the carbonate's radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite. Textures also suggest carbonate was produced at the expense of maskelynite in a dissolution-precipitation reaction that involved a carbonic fluid. This fluid system is tentatively estimated to have been active for at least a few years at temperatures <300 degrees C, based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems (which is the only analogue currently available). This reaction does not need to be mitigated by microbial life. Neither are bacteria needed to produce the radiating textures and globular forms of carbonate, which may instead reflect kinetic phenomena associated with crystal nucleation and growth. Because the carbonate was produced at the expense of maskelynite, it is younger than maskelynite, which has previously been shown to have last degassed 3.92 +/- 0.04 Ga (Turner et al., 1997). However, the specific age of the carbonate and the carbonic fluid system remains unknown. Copyright (C) 1998 Elsevier Science Ltd.