Percolative core formation in planetesimals

Abstract

The percolation of liquid iron alloy through crystalline silicates potentially played an important role during core formation in small bodies of the early solar system, such as asteroids and planetesimals. This is because heat production by radioactive decay of 26Al and 60Fe, which is believed to be the main heat source in early-formed small planetary bodies, will initially cause Fe–S melts to form, well before the silicates start to melt. In order to test the feasibility of percolation, the effect of pressure on the dihedral angle between Fe–O–S liquid and olivine has been investigated from 1.5 to 5.0 GPa, a pressure range that is relevant for the interiors of large asteroids. Texturally-equilibrated dihedral angles increase from 54° to 98° over this pressure range. The dihedral angle reaches the critical value of 60° at 2–3 GPa depending on the olivine composition (Fe#). This change in dihedral angle is related to the oxygen content of Fe–O–S phase, which decreases with increasing pressure, because oxygen dissolved in the melt reduces the Fe–S melt/olivine interfacial energy. These results show that Fe–O–S liquid can form an interconnected network and percolate through silicate aggregates under conditions of high oxygen fugacity and low pressure, even when the melt fraction is small. Therefore, percolation is likely to have been the dominant core formation mechanism in small relatively-oxidised planetary bodies with a radius less than about 1300 km.