Majorite fractionation recorded in the geochemistry of peridotites from South Africa

The liquidus phase for ultrabasic rock compositions changes from olivine at low pressures to a pyroxene polymorph at high pressures^1,2. In the range 13-25 GPa, the liquidus phase for a chondritic bulk Earth composition is majorite^3,4, a pyroxene with the garnet crystal structure. For upper-mantle Iherzolite compositions, majorite is also the liquidus phase from ∼16-25 GPa^5-7, and transforms to perovskite at similar or higher pressures⁷. As majorite has a density higher than peridotite magmas^8,9, it could have fractionated during an early differentiation event in the Earth³. Here we argue that this process is recorded in the major-element geochemistry of peridotites from South Africa, and that it has resulted in a lower mantle enriched in silica.