Metal - Silicate Separation in a Deformation Regime: Implications for Early Differentiation Processes

Details Metal - Silicate Separation in a Deformation Regime: Implications for Early Differentiation Processes Metal - Silicate Separation in a Deformation Regime: Implications for Early Differentiation Processes

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u52a0001r&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #U52A-0001

Other

1507 Core Processes (8115), 3630 Experimental Mineralogy And Petrology, 3662 Meteorites, 5410 Composition, 8125 Evolution Of The Earth

Scientific paper

The segregation of metallic cores from silicate mantles is one of the earliest, and most important, differentiation process involved in the evolution of the Earth and other terrestrial planetary bodies. The physical segregation of Fe-rich metal from silicate imparted a strong geochemical signature on early silicate mantles due to the preferential incorporation of siderophile elements into the core. Reconciling our estimates of primary bulk silicate mantle with candidate planetary bulk compositions requires an understanding of the geochemical consequences of the different regimes in which core forming material may have been mobile. This includes not only the possible differentiation processes that occurred in the terrestrial planets, but also understanding the differentiation processes in the meteorite parent bodies. Although a magma ocean model is possible for efficient core formation, some scenarios call for segregation of the core from solid silicate and the geochemical consequences can be significantly different. Experimental studies are one way in which insight can be gained into the possible geochemical signatures of metal-silicate segregation. Deformation experiments in addition provide a dynamic component, which allows liquid metal to segregate from solid silicate. Starting materials are cored from a slab of the Kernouve fall which is composed of olivine, pyroxene, plagioclase, chromite and chlorapatite; Fe-Ni metal and sulfide form 20-25% of the sample. Experimental conditions are 1.0-1.4 GPa confining pressure with strain rates of 10-4/s to 10-6/s. Temperatures ranging from 900° C to 1050° C produce variable amounts of silicate melt and different mechanisms of metal segregation are observed. In experiments which are below the silicate solidus, mobility of FeS is extensive and deformation textures are cataclastic. Geochemical analyses shows that migration of Fe-S-Ni-O metal through fractures and along grain boundaries produces extensive modification to the solid silicate matrix, particularly at the slower strain rates. New Fe-rich olivine is produced by reaction between Fe and Mg-opx, whereas cpx and primary Mg-olivine become Fe-enriched. At moderate silicate melt fractions (below ~12.5 vol%), we observe preferential segregation of the silicate melt fraction from quench Fe-S, Fe(Ni) and occasionally, Fe-P, by deformation-induced pressure gradients. At the highest silicate melt fractions, metal is fully separated from the silicate melt rich portion of the samples. The silicate solidus is lower than expected and analyses show that silicate glass at 1000°C and 1050°C contains small amounts of Cl (0.01-0.09 wt%), S (0.03-0.07 wt%) and P (0.3 wt%). We suggest the presence of H2O. Chlorapatite, possibly in conjunction with the products of terrestrial weathering may represent a source of Cl, P and OH in the experiments. These results are also providing insight into differentiation processes in meteorite parent bodies which have undergone early differentiation. Different degrees of partial melting concomitant with deformation-enhanced separation of the silicate melt portion may be responsible for the formation of the parent bodies of acapulcoites and lodranites which formed from precursor chondrites. The experimental results contribute to our understanding of dynamic differentiation process, through which these different meteorite types may be linked, and to the formation of some of the earliest planetary compositions.

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