Published in Meteoritics, 29, p. 542.
The ALH 84001 meteorite, long thought to be a diogenite, has recently been reclassified as a martian orthopyroxenite . Unlike the other martian meteorites, ALH 84001 shows evidence for multiple shock events, separated in time by recrystallization of silicates and by aqueous alteration. This history of multiple shock (impact) events suggests that ALH 84001 is older than the other martian meteorites, and might be from an ancient (Hesperian or Noachian) surface on Mars.
The shock features of ALH 84001 must have formed in more than one shock (impact) event. Petrographic evidence of shock in ALH 84001  includes crush/gouge zones with recrystallized cataclastic texture; strain birefringence, deformation features, and fractures in pyroxenes; maskelynite after all original plagioclase; radial fractures surrounding chromite and maskelynite grains; grain boundary offsets along some radial fractures; and fault offsets of zoning in carbonate mineral grains (Fig. 1d of ). These features did not form in a single shock event because (1) the fault offsets of the carbonate zoning postdate formation of the carbonate; (2) the formation of carbonate postdates recrystallization of the crush/gouge zones; and (3) recrystallization of the crush/gouge zones postdates their formation. Thus, ALH 84001 suffered at least two shock events, separated in time by a thermal event (recrystallization) and an aqueous alteration event. Much of the fracturing and deformation in pyroxene can be ascribed to an earlier shock event. The strain features, radial fractures, and maskelynite formation can be ascribed to a later event. The presence of maskelynite is particularly significant as it crystallizes rapidly under modest temperature-time histories .
Evidence for two shock (impact) events in ALH 84001 suggests that it is not likely to be a product of relatively recent magmatism. To accommodate this shock history, the martian source crater for ALH 84001 must be superimposed on (or adjacent to) older impact materials. Of the few craters on young igneous surfaces on Mars , only a small proportion formed on or near earlier impact features (e.g., Figs. 7 and 8 of ). It is much more likely that ALH 84001 was ejected from an old igneous unit (Hesperian or Noachian age), pocked by numerous impact craters over its long exposure at the martian surface.
References:  Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221.  Duke M. B. (1968) in Shock Metamorphism of Natural Materials, 613-621.  Mouginis-Mark P. et al. (1992) JGR, 97, 10213-10225.