Lower Mantle Mineral Associations Preserved in Diamonds

Abstract

The composition of the Earth's mantle inferred from carbonaceous chondrites and also from solar energetic particles and spectroscopy, is hugely dominated by SiO2 and MgO, with subordinate but significant contents of FeO, CaO and A1203 (Anderson, 1989). This evidence coupled with geophysical data on mechanical porperties and density, and ultra-high pressure experimental work has led to the consensus that the 670 km seismic discontinuity is associated with the stabilisation of a mineral assemblage of dominantly MgSiO3 (in the perovskite structure) together with MgO (in the rock salt or periclase structure). The FeO component of the lower mantle is expected to be in solid solution in these phases, whilst the CaO is expected to form CaSiO3 in perovskite structure. Whether A1203 is present as oxide or located in some silicate phase has been a matter of uncertainty. To what extent the lower mantle mirrors the upper mantle in details of composition for example, to what extent it is like the upper mantle lithosphere represented by various peridotite xenoliths from basalts and kimberlites (e.g. Harte and Hawkesworth, 1989) or Ringwood's pyrolites (e.g. Ringwood, 1982) is uncertain; but