Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr31b..05w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #MR31B-05
Other
1015 Composition Of The Core, 3625 Petrography, Microstructures, And Textures, 3672 Planetary Mineralogy And Petrology (5410), 3914 Electrical Properties
Scientific paper
The formation of a metallic core is one of the most profound events in the early evolution of a planet. There are still on-going controversies on the nature of the physical mechanisms that led to core-formation on the Earth, as well as on the smaller, differentiated planetesimals considered to be building blocks of Earth. Although there is much evidence that favors a very hot and deep magma ocean for the major core formation event on Earth, smaller planetesimals likely never became hot enough to generate the wide-scale melting required for such a scenario. Furthermore, there is evidence that planetesimal cores formed rapidly (within 3My). An inefficient percolative flow mechanism has been suggested to be viable for systems that have a metallic melt fraction in excess of the percolation threshold (approximately 5 vol%), provided that the permeability of these connected melts is high enough to remove the majority of the core liquid from the silicate matrix in such a relatively short time span (eg. Yoshino et al. 2004). More accurate knowledge of the permeability of core forming melts requires a detailed understanding of how the melt is connected in three dimensions, and the complex relationships between melt volume, connectedness and permeability. In this study, we calculated the permeability of core forming metallic liquids (FeS and Fe67S33) within a silicate matrix by lattice-Boltzmann simulations of flow through digital volumes generated from 3- dimensional, synchrotron-based x-ray tomographic images of experimental run samples. Mixtures of San Carlos olivine and sulfide liquids were synthesized in a piston-cylinder apparatus at conditions relevant to core segregation in planetesimals (1400°C and 1GPa for 24 hours). These conditions were determined to be sufficient for the samples to reach micro-textural equilibrium. Upon quench, the recovered samples were imaged at micron scale resolution using the dedicated tomography beam-line(8.3.2)at the Advanced Light Source (Lawrence Berkeley National Laboratory). Electrical conductivity measurements at 1 GPa and temperatures up to 1000°C on the same pre-synthesized samples were also conducted to independently determine the percolation (connectivity) threshold and begin developing a relationship between electrical conductivity and permeability in partially molten samples. The percolation threshold of these samples was determined to be close to earlier measurements (3-6 vol%) through both experimental methods, but the calculated permeability is substantially lower than previously estimated. As a consequence, although percolation still appears viable for some planetesimal sized objects, it may be a secondary mechanism acting in conjunction with flow induced through other processes such as deformation. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Roberts Jeffery J.
Watson Heather C.
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