Crystallisation of the metal in iie irons and possible meteorite analog

Abstract

The metal of the IIE irons has evidence of fractionation in the depths of the asteroid, but the presence of a fine-grained structure is incompatible with its endogenous origin. It was assumed that the metal underwent remelting on the surface of the parent body. Data on the mineragraphy, mineral and chemical composition of IIE irons (Elga, Verkhnodniprovsk, Tobychan, Miles and Watson) indicate that the relatively fine-grained metal structure and xenomorphic schreibersite grains were probably formed by crystallization from the melt. According to the calculated data on the bulk composition of the Elga metal and on the Fe-Ni-P phase diagrams, the crystallization of the first γ-Fe grains began at ~1511°С and ended at ~1060–1100°С with the formation of polygonal crystals of cm-sized taenite and xenomorphic schreibersite along their boundaries. The identical composition of xenomorphic schreibersite, both along the borders of the taenite grains and on the rims around nonmetallic inclusions, indicates their simultaneous formation. Among four generations of schreibersite, the xenomorphic schreibersite is distinguished by a high Fe/Ni ratio. It is also noted that the higher the crystallization temperature of schreibersite, the less nickel content in this schreibersite. Similar metal structures were found in other types of meteorites: in the IAB irons and in metal of some mesosiderites, and the impact mechanism of formation is considered the most likely for them. Thus, the mechanism of formation of the IIE irons by shock remelting of fractionated metal and mixing with silicate fragments in the conditions of the parent surface may have analogues among other types of meteorites.