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

Abstract

Compositional and textural relationships of shock-melted glasses in the Allan Hills (ALH) 84001 meteorite have been examined by optical microscopy, electron microprobe analysis, and compositional mapping. The feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in ALH 84001. The feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase (An27-39Ab58-68Or3-7) or sanidine (Or51Ab46An3), or from a mixture of these phases (mixed-feldspar glasses). These observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite, hydrothermal precipitates or alteration products, or shock melts that have undergone alkali volatilization. Carbonate was deposited around previously formed mixed-feldspar glass clasts, suggesting that carbonate deposition occurred after the shock event that formed the granular bands (crushed zones) in this meteorite. SiO2-rich glasses appear to be silica remobilized during shock, with little addition of other material. A petrogenetic history of ALH 84001 consistent with the observations of feldspathic and silica glasses is (1) igneous crystallization and cumulate formation; (2) a pre-carbonate shock event that formed the granular bands (crushed zones) and sheared chromites, and melted igneous plagioclase and sanidine to form mixed-feldspar glasses; (3) carbonate and silica deposition in the granular bands (veining of plagioclase glasses by SiO2 and deposition of carbonate around mixed-feldspar and plagioclase glass clasts); (4) a post-carbonate shock event that resulted in invasion of carbonate by feldspathic melts, shock faulting and decarbonation of carbonate, high-temperature mobilization of silica melts, and minor dissolution of orthopyroxene by silica melts.