Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p22c0416y&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P22C-0416
Physics
3924 High-Pressure Behavior, 5109 Magnetic And Electrical Properties, 5114 Permeability And Porosity, 5139 Transport Properties, 5724 Interiors (8147)
Scientific paper
The formation a metallic core in a terrestrial planet requires a mechanism for separating and mobilizing Fe-alloy. Measured dihedral angles of well over 60° for molten Fe-alloy in a solid, olivine-rich matrix have apparently precluded metal segregation by grain boundary percolation. However, excess melt over a percolation threshold can create permeability even though the dihedral angle is above the critical value of 60° and some liquid-metal can segregate from the silicate matrix. To determine the percolation threshold for iron alloy melt in crystalline silicate matrix, we performed in situ electrical conductivity measurements on mixtures of olivine and molten FeS compounds with variable volume ratios at high pressure and temperature. Electrical conductivity measurements throughout heating-cooling cycles were conducted at 3GPa using a cubic pressure cell in a DIA-type apparatus. The samples were held at 1473-1573K for over 20 hours to achieve textural equilibrium, which is above the eutectic melting point in the Fe-FeS binary system but below the melting point of Fo90 olivine. After heating at the maximum temperature in runs with metal proportions of 6 vol.% and above, the conductivity was high, nearly constant, and independent of temperature. At lower temperature conditions below the eutectic, preservation of the high conductivity values suggests that the Fe-FeS melt was well connected. In the runs with lower metal proportion (~3 vol.%), conductivities were low and nearly constant up to 873K, but increased considerably as temperature was raised to the maximum temperature. Temperature-conductivity paths in these runs are essentially the same as that without Fe-FeS eutectic melt so we consider that melt was not connected. The percolation threshold of liquid Fe-S compound is approximately 5 vol.%. Dihedral angles determined from these runs were large (95°), consistent with that for pinch-off boundaries predicted by von Bargen and Waff (1986). This suggests that core formation due to the grain boundary percolation can occur when the temperature exceeds the Fe-S melting point. Planetesimals can heat to the FeS melting point within about 3 million years due to radioactive decay of Al26 and Fe60, causing considerable amounts of Fe-alloy to segregate to form a core. This explains the early formation of cores in planetesimals as predicted from Hf-W chronometry, and also provides a mechanism for potential energy release in large proto-planets to form a magma ocean.
Katsura Takashi
Walter Michael J.
Yoshino Taizoh
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