Pressure‐temperature phase diagram for the Allende meteorite

Abstract

Piston cylinder and multianvil experiments from 1 to 27 GPa have been performed on the Allende CV3 meteorite to establish a pressure‐temperature phase diagram that includes major phase boundaries and the silicate‐oxide‐sulfide melting intervals. Olivine is the liquidus phase up to ∼14 GPa, followed by garnet up to ∼25 GPa. Near 26 GPa a cotectic exists where garnet and magnesiowüstite are liquidus phases. Magnesiowüstite is likely to be a lower mantle liquidus phase in both chondritic and peridotitic (see also Zhang and Herzberg, 1994) compositions. Hence element partitioning tests that neglect the role of liquidus magnesiowüstite may be incomplete for describing planetary differentiation at pressures >25 GPa. Allende shows immiscibility between (Fe,Ni)‐sulfide melt and FeO‐rich silicate melt. (Fe,Ni)‐sulfide is the lower temperature melt phase and is present at all experimental pressures and temperatures investigated. It is concluded that a terrestrial planet with a radius of ∼3000 km (maximum internal pressure of ∼30 GPa), and a bulk composition of carbonaceous chondrite, will upon magmatic differentiation form an FeO‐rich silicate mantle with an Fe‐Ni‐S core. The silicate fraction of Allende in our high‐pressure experiments is too rich in FeO to be a good match for the composition of peridotite xenoliths from Earth's upper mantle. However, the major elements of a peridotite upper mantle may be derived from an Allende‐like bulk Earth by a combination of lower mantle magnesiowüstite, perovskite, and sulfide fractionation and by upper mantle olivine flotation.