Partitioning of Ni and Co between Metal and Magnesiasilicate-perovskite under Conditions of the
Lower Mantle

O. Tschauner Hochdruck-Mineralphysik, Max Planck-Institut für Chemie, Mainz, Germany

R. Boehler Hochdruck-Mineralphysik, Max Planck-Institut für Chemie, Mainz, Germany

S. Specht Isotopenchemie, Hochdruck-Mineralphysik, Max Planck-Institut für Chemie, Mainz, Germany

A. Zerr Hochdruck-Mineralphysik, Max Planck-Institut für Chemie, Mainz, Germany

H. Palme Mineralogisch-Petrographisches Institut der Universität zu Köln, Germany

Up to now the overabundance of siderophile elements in the mantle which has been highlighted by metal-silicate partition experiments at 1 bar and high temperature (Rammensee, 1978; Schmitt et al., 1989; Holzheid et al., 1994). remains an unresolved problem in models of accretion and core formation of the earth. To reconcile observation and experiment it has been suggested (Ringwood, 1979) that the partition coefficients Dmet/sil decrease strongly at mantle pressures. Recent studies at moderate pressures show indeed some decrease in Dmet/sil for Ni and Co but the ratio of both does not converge to 1, which is required to explain the nearly chondritic ratio of the Ni and Co abundances in the mantle (Thibault and Walter, 1995; Hillgreen et al., 1994).

We have developed a new technique to measure element partitioning at lower mantle conditions which incorporates diamond cells, CO2-laser-heating (Boehler, 1991) and
chemical analysis using ion-probe. Here we show first measurements on Ni/Co-partitioning between metal and silicate-perovskite between 248 kbar and 585 kbar and 2000-2500 K. Under these conditions both metal and silicate are solid. The experimental conditions have been chosen to minimize pressure and temperature gradients: Single opx crystals were heated in contact to metal foils by a defocussed laser beam which reduces thermal gradients to values lower than 50K/µm. Anisotropic strain and pressure gradients in the cell were reduced to less than 2% by the use of a
soft pressure medium (KBr). Temperatures were measured from the thermal radiation of the sample. Conversion to
the perovskite-structure was checked in situ by Raman-
spectroscopy. The recovered silicate samples were single multidomain-crystals of Si-perovskite.

Our data show lower values of Dmet/sil for Ni and Co than those found at lower pressures and the same temperature. The ratio of the Ni- and Co-partition-coefficients depends strongly on pressure but is always smaller than 1.

Yet the data are to preliminary to rule out homogeneous or inhomogeneous accretion models.


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