Ferrovolcanism on metal worlds and the origin of pallasites
As differentiated planetesimals cool, their cores can solidify from the outside-in, as evidenced by paleomagnetic measurements and cooling rate estimates of iron meteorites. The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward core growth would likely be achieved by growth of solid FeNi dendrites. Growth of FeNi dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt percent sulfur as FeNi continues to solidify. Here we show that regions of residual sulfur-enriched FeNi melt in the core attain sufficient excess pressures to propagate via dikes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the plantesimals surface. We refer to these processes collectively as ferrovolcanism. Our calculation show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of FeNi metal. Ferrovolcanic eruptions may explain the observations that Psyche has a bulk density inconsistent with iron metorites yet shows evidence of a metallic surface composition.