Nature 332, 823 - 826 (28 April 1988); doi:10.1038/332823a0

Majorite fractionation recorded in the geochemistry of peridotites from South Africa

Claude Herzberg^*, Mark Feigenson^*, Cherylyn Skuba^† & Eiji Ohtani^‡

^*Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA
^†Department of Geology, University of Texas at Arlington, Arlington, Texas 76019, USA
^‡Department of Earth Sciences, Ehime University, Matsuyama 790, Japan

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.

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