Deformation of a crystalline aggregate with a small percentage of high-dihedral-angle liquid: Implications for core–mantle differentiation during planetary formation

• Published in: Earth and planetary science letters 2011-05, Vol.305 (1), p.124-134
• Main Authors: Walte, N.P., Rubie, D.C., Bons, P.D., Frost, D.J.
• Format: Article
• Language: English
• Subjects: analogue experiments; core–mantle differentiation; d-DIA; deformation experiments; melt segregation; olivine
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2011.02.049

Core–mantle differentiation of small planetesimals has been suggested to be initiated by deformation-assisted segregation of molten Fe–S through a crystalline peridotite matrix. In this study we used two different experimental approaches to investigate the effect of varying the strain rate and of varying the dihedral angle of the liquid phase on liquid interconnectivity. (i) High pressure experiments were performed on simplified systems consisting of olivine plus a small percentage of liquid FeS (dihedral angle ~ 75°) or gold (dihedral angle ~ 150°) at a temperature above the FeS/Au solidus but below the silicate solidus. Samples were deformed at high pressure in a deformation-DIA multianvil press at strain rates between 10 − 3 and 10 − 6 s − 1 . (ii) Optical in situ analogue experiments on norcamphor plus H 2O (dihedral angle ~ 86°) were performed at room temperature to complement the d-DIA experiments. The crystalline matrix in both the d-DIA and analogue experiments deforms by grain boundary migration assisted dislocation creep under the experimental conditions. Furthermore, the liquid pockets display a similar strain rate dependent behaviour in both sets of experiments: At high strain rates deformation is strongly localised by the liquid phase, and liquid interconnection and segregation occurs. At low strain rates the geometry of liquid pockets is controlled predominantly by surface tension and little elongation and interconnection occurs. We propose a new empirical scaling approach using the d-DIA results, which indicates that some degree of high dihedral angle liquid interconnection and segregation may be possible down to a few percent residual liquid under natural conditions for moderate dihedral angles (60° < Θ < 90°). This, however, would not suffice for efficient core–mantle segregation and an additional mechanism, such as a magma ocean, would be required. ► Olivine-FeS liquid and norcamphor-H 2O were deformed by axial compression/pure shear. ► Depending on and dihedral angle deformation is stress or surface tension dominated. ► Partial liquid segregation only occurs during stress-dominated deformation. ► Early planetary formation may have occurred in both deformation regimes.