PARTITIONING OF MGO, FEO, NIO, MNO AND CR2O3 BETWEEN MAGNESIAN SILICATE PEROVSKITE AND MAGNESIOWUSTITE - IMPLICATIONS FOR THE ORIGIN OF INCLUSIONS IN DIAMOND AND THE COMPOSITION OF THE LOWER MANTLE

Syngenetic mineral inclusions in diamond provide valuable information about the environment in which the diamond originally crystallized. It was earlier proposed that diamonds containing a "forbidden" inclusion assemblage of magnesiowustite (Mg number approximately 85 and NiO approximately 1 wt%) plus enstatite En94-95 may have originally formed in the lower mantle. Enstatite would therefore correspond to the retrogressive transformation product of magnesian silicate perovskite. This hypothesis has been assessed by determining the partition behaviour of MgO, FeO, NiO, MnO and Cr2O3 between perovskite and magnesiowustite at lower mantle pressures (30-50 GPa). Experiments were carried out starting with synthetic olivine that was heated with an infrared laser beam in a diamond anvil high pressure cell. Run products were characterized by transmission electron microscopy and X-ray -microanalysis. Perovskite (Mg number approximately 95) and magnesiowustite (Mg number approximately 86) are produced by the disproportionation of olivine Fo90, whereas Fo85 yields perovskite plus magnesiowustite with Mg number of approximately 94 and approximately 78, respectively. NiO is always strongly partitioned into magnesiowustite (K(NiO) mw/pv (wt%) is 6.3 +/- 5.1, whilst MnO and Cr2O3 show moderate preferential partitioning into magnesiowustite K(MnO) = 2.4 +/- 1.4 and K(Cr2O3) = 2.1 +/- 0.9. The partition behaviour of all five oxide species as observed between the enstatite and magnesiowustite inclusion assemblage in diamond is entirely consistent with equilibration at lower mantle pressures. If these rare minerals did indeed form in the lower mantle, as suggested by the above experiments, then their compositions can be used to assess various classes of lower mantle model bulk compositions. A perovskititic lower mantle would be required to be highly magnesian (Mg number approximately 95), which is unacceptable from a geophysical perspective. A lower mantle possessing significant enrichment of FeO and SiO2, as compared to the upper mantle, would be comprised of perovskite and magnesiowustite with Mg numbers substantially lower than those of their counterparts in diamond. However, mass-balance considerations indicate that the lower mantle could well possess a bulk composition similar to that of a depleted lithology (Mg number approximately 92) derived from "pyrolite". There is therefore no requirement for any profound compositional differences between the upper and lower mantle.