Fayalitic halos around inclusions in forsterites from carbonaceous chondrites

Spindle-shaped fayalitic halos occur in forsterites of the Allende, Kaba, Mokoia, Vigarano, and Ningqiang carbonaceous chondrites. The halos have: 1) ellipsoidal or, less commonly, rounded shapes; 2) long axes parallel to each other within given host crystals; 3) round, bleb-like inclusions of Ni-rich metal and sulfide in their centers; and 4) compositional gradients that are most Fe-rich near the inclusions. The Fe and Mg compositional profiles are well approximately by the error function, which, in combination with halo shapes, indicates a diffusional origin. The high Ni contents, relative to kamacite, of the metal inclusions suggest Fe loss. We propose that the halos formed by oxidation of the Fe-Ni metal in the inclusions and Fe²⁺ diffusion into adjoining fosterite. The halo dimensions suggest they formed between 1000 and 1250 K. Carbonaceous chondrites are characterized by, among other features, minerals and some organic molecules that are only stable at relatively low temperatures, probably less than 500 K and certainly lower than 800 K. The conditions required to form the fayalitic halos by diffusion are inconsistent with such low temperatures. This combination leads us to conclude that the halos formed prior to their incorporation in the parent body(ies) of their host meteorites. Possibilities are that the halos formed in the nebula or within a prior parent body from which material was dispersed and reaggregated.