The Chemical Consequences of Magma Ocean Solidification
Abstract
Early Earth likely experienced large-scale melting owing to giant impacts during the last phase of accretion. Numerical studies have suggested the melting of the entire mantle, indicating the possibility of substantial differentiation during the subsequent solidification of a magma ocean. The extent of differentiation, however, is controversial because it depends on various poorly constrained factors including density contrast, viscosity, grain size, and permeability. In this study, we aim to bound the degree of chemical differentiation by creating a model that combines heat and material transfer with Gibbs free energy minimization. We consider the effects of fractional crystallization as well as porous convection. In order to predict the melting behavior of the evolving composition of a solidifying magma ocean, we first create a self-consistent thermodynamic database for a liquid of mantle materials using existing high-pressure melting experiments. Using this thermodynamic database, the composition of a newly fractionating layer can be predicted quantitatively. In case of fractional crystallization but without porous convection, a dense, low Mg# component is likely to remain in the surviving melt near the surface as previously predicted. When porous convection operates throughout the rheologically rigid layer, however, the dense melt can be transported from the surface to the bottom, homogenizing the entire mantle. Considering the Rayleigh number for porous convection, such transport is likely to occur until the fraction of melt decreases below 10%. A magma ocean reaches a rheologically rigid state in 103-104 years, and porous convection may continue for another 104-105 years. The emergence of a basal magma ocean depends critically on the choice of a thermodynamic database. The depth where a magma ocean starts to solidify is a controlling factor, but it is difficult to constrain this depth tightly given the uncertainty of a thermodynamic database originating from published melting experiments.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.P31G3772M
- Keywords:
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- 1027 Composition of the planets;
- GEOCHEMISTRYDE: 3672 Planetary mineralogy and petrology;
- MINERALOGY AND PETROLOGYDE: 6299 General or miscellaneous;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTSDE: 5455 Origin and evolution;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS