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Early silica crust formation in planetesimals by metastable silica-rich liquid immiscibility or cristobalite crystallisation : the possible origin of silica-rich chondrules

Faure, F., Scientific Reports

Early silica crust formation in planetesimals by metastable silica-rich liquid immiscibility or cristobalite crystallisation : the possible origin of silica-rich chondrules

Faure, F.

Scientific Reports, 2020, 10:4765

The formation and differentiation processes of planetesimals—small bodies in the solar system— remain actively debated. Planetesimal differentiation is known to have occurred early (<1.5 Myr after the formation of Ca-Al-rich inclusions), as attested by the ages of iron meteorites. Metal-silicate segregation implies global-scale melting, induced by heat released from short-lived radiogenic isotopes, and the consequent generation of a silicate magma ocean. Thermodynamic calculations show that silicate magma crystallisation would have induced silicate-silicate differentiation, leading to the formation of a thick olivine-dominated “mantle” and a thin basaltic “crust”. However, thermodynamic modelling of magma ocean crystallisation does not produce any silica phases. Here I experimentally show that crystallisation of a chondritic liquid does not follow the thermodynamically predicted path. Silica phases are generated early (before 55% differentiation) as a function of initial magma ocean temperature. As cristobalite or liquid silica phases are less dense than residual liquids or olivine, silica phases could have formed proto-crusts that would have acted as buoyant lids at the surfaces of planetesimals, allowing the eventual accretion and preservation of debris (chondrites). Moreover, the destruction of such a crust by impacts could provide an explanation for the origin of the silica reservoir that condensed around some chondrules.

Voir en ligne : https://doi.org/10.1038/s41598-020-...




publié mardi 24 mars 2020