Impact‐induced devolatilization of four ungrouped ataxites and the formation of a silica glass spherule in ALHA77255

Abstract

AbstractAllan Hills A77255, Babb's Mill (Blake's Iron), Nordheim, and Chinga are ungrouped ataxitic iron meteorites that are similar to the IAB group of noncarbonaceous‐type irons in their concentrations of common and refractory siderophile elements. Mo‐isotopic data show that ALHA77255, Nordheim, and Chinga are carbonaceous‐type (CC) irons. (The Mo‐isotopic composition of Babb's Mill [Blake's Iron] has not yet been measured, but it also seems likely to be a CC iron.) Relative to mean IAB irons, these four ataxites are severely depleted in moderately volatile elements: Ga, >99%; Ge, >99%; Cu, 79%–97%; As, 70%–96%; P, 76%–90%. These samples were probably devolatilized by major collisions on separate parent asteroids (consistent with fractional crystallization modeling showing they are unlikely to be derived from the same metallic core). Collisionally induced devolatilization of ALHA77255 likely facilitated the formation of a 5‐mm diameter silica–glass spheroid in this meteorite. The spheroid may have formed by a complex process involving impact‐induced vaporization of mantle material in its parent asteroid, followed by fractional condensation.