Computer Science
Scientific paper
Apr 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.1181h&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 60, Issue 7, p. 1181-1193.
Computer Science
14
Scientific paper
The effect of iron on the solubilities of Co and Ni in silicate melts was determined at a wide range of oxygen fugacities (from 10-8.1 to 10-12.6 atm) and at temperatures ranging from 1305 1600°C. Four different systems were studied. In system A an FeO-free silicate of anorthite diopside eutectic composition was equilibrated with pure Co- or pure Ni-metal. In system B the same silicate starting material was equilibrated with FeCoNi-alloys. In systems C and D, FeO-containing silicates (basaltic komatiite) were equilibrated with pure Co and pure Ni metal (system C) and with FeCo- and FeNi-alloys (system D). Experiments at temperatures below 1450°C were performed by equilibrating metal loops with silicates; at higher temperature well mixed metal powder and silicates were equilibrated in alumina crucibles. Concentrations of Ni and Co in the silicate and in some of the alloys were determined by neutron activation analysis; most of the alloy compositions were analyzed by electron microprobe. At constant temperature (1400°C) but variable oxygen fugacities, Co, Ni, and Fe metal/silicate partition coefficients calculated from metal solubilities increase with decreasing oxygen fugacities. The extent of the increase suggests that all three metals are dissolved as oxides (CoO, NiO, FeO) in the silicate melt. No evidence for the presence of metallic Co, Ni, or Fe as major species in silicate melt was found within the oxygen fugacity range covered by the experiments, even at oxygen fugacities three log units below the Fe-FeO equilibrium (IW-3). A minor fraction of metallic Ni (below 20%) may be present in the most reducing experiments. No difference in Ni and Co solubility was found for FeO-free and FeO-containing silicates. Depending on temperature all metal/silicate partition coefficients of Co and Ni decrease slightly with increasing temperature along the oxygen fugacity appropriate for core formation, 2.3 log units below the Fe-FeO equilibrium (IW-2.3). The experiments confirm that the relatively high present-day upper mantle concentrations of Co, Ni, and Fe and in particular the chondritic Ni/Co ratio cannot be explained by equilibrium between coreforming metal and silicates of the Earth's mantle in a low pressure environment even at temperatures as high as 4000°C, thus supporting models of inhomogeneous accretion of the Earth. Accumulation of differentiated planetesimals, i.e., with metal cores, as major building blocks of the Earth can also be excluded. In these bodies metal-silicate equilibration would have occurred at low pressures and equilibration of metal cores of accreting planetesimals with the Earth's mantle is unlikely. Homogeneous accretion models may only be possible if equilibration occurred between lower mantle minerals and liquid core forming metal during differentiation of the Earth, and if the relevant Ni and Co partition coefficients are sufficiently high. The Ni and Co partition coefficients between liquid metal and Mg-wüstite and Mg-perovskite are not well known.
Holzheid Astrid
Palme Herbert
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