The liquidus phase for ultrabasic rock compositions changes from olivine at low pressures to a pyroxene polymorph at high pressures1,2. In the range 13-25 GPa, the liquidus phase for a chondritic bulk Earth composition is majorite3,4, a pyroxene with the garnet crystal structure. For upper-mantle Iherzolite compositions, majorite is also the liquidus phase from ∼16-25 GPa5-7, and transforms to perovskite at similar or higher pressures7. As majorite has a density higher than peridotite magmas8,9, 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.
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