Metal Crystallization in IIE Irons and Their Possible Meteorite Analogues

Abstract

The metal of IIE irons provides evidence of fractionation in the interiors of an asteroid, but their fine-grained structures are incompatible with their endogenous origin. It was proposed that the metal underwent remelting on the surface of the parent body. Data on the mineragraphy and on the mineral and chemical composition of IIE irons (Elga, Verkhnodniprovsk, Tobychan, Miles, and Watson) indicate that the relatively fine-grained metal structure and anhedral schreibersite grains were probably formed by crystallization from 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 centimeter-sized polygonal crystals of taenite and anhedral schreibersite grains along their boundaries. The identical composition of the anhedral schreibersite, both along the borders of the taenite grains and on the rims around nonmetallic inclusions, indicates that they were formed simultaneously. Among the four generations of schreibersite, the anhedral schreibersite is distinguished for its high Fe/Ni ratio. It was also noted that the higher the crystallization temperature of schreibersite, the lower its Ni content. Similar metal structures were found in other types of meteorites: in IAB irons and in the metal of some mesosiderites, whose impact-related origin is thought to be the most probable. Hence, the mechanism of formation of IIE irons by means of shock remelting of fractionated metal and mixing with silicate fragments on the surface of the parent body may have also produced other meteorite types.