Differentiation time scales of small rocky bodies

The petrologic and geochemical diversity of meteorites is a function of the bulk composition of their parent bodies, but also the result of how and when internal differentiation took place. Here we focus on this second aspect considering the two principal parameters involved: size and accretion time of the body. We discuss the interplay of the various time scales related to heating, cooling and drainage of silicate liquids. Based on two phase flow modeling in 1-D spherical geometry, we show that drainage time is proportional to two independent parameters: mu m/R2, the ratio of the matrix viscosity to the square of the body radius and mu f/a2, the ratio of the liquid viscosity to the square of the matrix grain size. We review the dependence of these properties on temperature, thermal history and degree of melting, demonstrating that they vary by several orders of magnitude during thermal evolution. These variations call into question the results of two phase flow modeling of small body differentiation that assume constant properties. For example, the idea that liquid migration was efficient enough to remove 26Al heat sources from the interior of bodies and dampen their melting (e.g. Moskovitz and Gaidos, 2011; Neumann et al., 2012) relies on percolation rates of silicate liquids overestimated by six to eight orders of magnitude. In bodies accreted during the first few million years of solar-system history, we conclude that drainage cannot prevent the occurrence of a global magma ocean. These conditions seem ideal to explain the generation of the parent-bodies of iron meteorites. A map of the different evolutionary scenarios of small bodies as a function of size and accretion time is proposed.

Automated summary

The petrologic and geochemical diversity of meteorites is influenced by the bulk composition of the parent bodies as well as how internal differentiation occurred. Factors such as size and accretion time play a role in this differentiation process. The study examines the time scales of heating, cooling, and drainage of silicate liquids and proposes that drainage time is proportional to matrix viscosity and liquid viscosity. The properties of these liquids vary significantly during thermal evolution, challenging the assumption of constant properties in small body differentiation. It is concluded that drainage cannot prevent the occurrence of a global magma ocean in bodies accreted during the early stages of the solar system, which provides insight into the formation of iron meteorite parent bodies.