Metal segregation in planetesimals: Constraints from experimentally determined interfacial energies

Abstract

High temperature experiments have been performed to constrain interfacial energies in a three-phase system (metal–forsterite–silicate melt) representative of partially differentiated planetesimals accreted early in the solar system history, with the aim of providing new insights into the factors affecting the interconnection threshold of metal-rich phases. Experiments were run under controlled oxygen fugacity (ΔNi-NiO=−3) at 1440°C, typically for 24 h. Quantification of the true dihedral angles requires a resolution of at least 30 nm per pixel in order to reveal small-angle wedges of silicate melt at crystal interfaces. At this level of resolution, dihedral angle distributions of silicate melt and olivine appear asymmetric, an observation interpreted in terms of anisotropy of olivine crystals. Based upon the theoretical relation between dihedral angles and interfacial energies in a three-phase system, the relative magnitudes of interfacial energies have been determined to be: γMelt-Ol<γMelt-Ni<γOl-Ni. This order differs from that obtained with experiments using an iron sulfide liquid close to the Fe–FeS eutectic for which γMelt-Sulfide<γMelt-Ol<γOl-Sulfide, implying a lower interconnection threshold for sulfur-rich melts than for pure metallic phases. This dependence of the interconnection threshold on the sulfur content will affect the drainage of metallic phases during melting of small bodies. Assuming a continuous extraction of silicate melt, evolution of the metal volume fraction has been modeled. Several sulfur-rich melts extraction events are possible over a range of temperatures relevant with thermometric data on primitive achondrites (1200–1400°C and 25% of silicate melt extracted). These successive events provide novel insight into the variability of sulfur content in primitive achondrites, which are either representative of a region that experienced sulfide extraction or from a region that accumulated sulfide melt from overlying parts of the parent body.