Partitioning of MgO, FeO, NiO, MnO and Cr 2O 3 between magnesian silicate perovskite and magnesiowüstite: implications for the origin of inclusions in diamond and the composition of the lower mantle

Abstract

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 magnesiowüstite (Mg number ∼ 85 and NiO ∼ wt%) plus enstatite En 94–95 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 Cr 2O 3 between perovskite and magnesiowüstite 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 ∼ 95) and magnesiowüstite (Mg number ∼ 86) are produced by the disproportionation of olivine Fo 90, whereas Fo 85 yields perovskite plus magnesiowüstite with Mg number of ∼ 94 and ∼ 78, respectively. NiO is always strongly partitioned into magnesiowüstite ( K NiO mw/pv (wt%) is 6.3 ± 5.1, whilst MnO and Cr 2O 3 show moderate preferential partitioning into magnesiowüstite K MnO = 2.4 ± 1.4 and K Cr 2O 3 = 2.1 ± 0.9. The partition behaviour of all five oxide species as observed between the enstatite and magnesiowüstite 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 ∼ 95), which is unacceptable from a geophysical perspective. A lower mantle possessing significant enrichment of FeO and SiO 2 as compared to the upper mantle, would be comprised of perovskite and magnesiowüstite 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 ∼ 92) derived from “pyrolite”. There is therefore no requirement for any profound compositional differences between the upper and lower mantle.