Petrogenesis of Allan Hills 84001: Constraints from impact-melted feldspathic and silica glasses

Compositional and textural relationships of shock-melted glasses in the All an Hills (ALH) 84001 meteorite have been examined by optical microscopy, el ectron microprobe analysis, and compositional mapping. The feldspathic and silica glasses exhibit features which constrain the relative timing of shoc k events and carbonate deposition in ALH 84001. The feldspathic glasses are stoichiometric and have compositions plausibly described as forming from i gneous plagioclase (An(27-39)Ab(58-68)Or(3-7)) or sanidine (Or(51)Ab(46)An( 3)), or from a mixture of these phases (mixed-feldspar glasses). These obse rvations argue against prior interpretations of feldspathic glasses as unfl owed maskelynite, hydrothermal precipitates or alteration products, or shoc k melts that: have undergone alkali volatilization. Carbonate was deposited around previously formed mixed-feldspar glass clasts, suggesting that carb onate deposition occurred after the shock event that formed the granular ba nds (crushed zones) in this meteorite. SiO2-rich glasses appear to be silic a remobilized during shock, with little addition of other material. A petrogenetic history of ALH 84001 consistent with the observations of fel dspathic and silica glasses is (1) igneous crystallization and cumulate for mation; (2) a pre-carbonate shock event that formed the granular bands (cru shed zones) and sheared chromites, and melted igneous plagioclase and sanid ine to form mixed-feldspar glasses; (3) carbonate and silica deposition in the granular bands (veining of plagioclase glasses by SiO2 and deposition o f carbonate around mixed-feldspar and plagioclase glass clasts); (4) a post -carbonate shock event that resulted in invasion of carbonate by feldspathi c melts, shock faulting and decarbonation of carbonate, high-temperature mo bilization of silica melts, and minor dissolution of orthopyroxene by silic a melts.