Physics of the Earth and Planetary Interiors, 19 (1979) 31—51 31 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands HIGH-PRESSURE PHASE EQUILIBRIA IN A GARNET LHERZOLITE, WITH SPECIAL REFERENCE TO Mg 2~—Fe2~ PARTITIONING AMONG CONSTITUENT MINERALS M. AKAOGI and S. AKIMOTO Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106 (Japan) (Received June 27, 1978; accepted for publication July 20, 1978) Akaogi, M. and Akimoto, S., 1979. High-pressure phase equilibria in a garnet lherzolite, with special reference to Mg2~—Fe2~ partitioning among constituent minerals. Phys. Earth Planet. Inter., 19: 31—51. Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45—205 kbar and in the temperature range 1050—1200°C.Partition of ele- ments, particularly Mg2~—Fe2~, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(n) to spinel(’y) through modified spinel(~3) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg 2 Si04 —Fe2 Si04. At pressures of 50—150 kbar, a continuous increase in the solid solubiity of the pyroxene com- ponent in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45—75 kbar and 1200°C, the Fe 2~/(Mg + Fe21) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144—146 kbar and 1200°C, garnet exhibits the highest Fe2’/(Mg + Fe2’) value; modified spinel(p) and Ca-poor clinopyroxene follow it. When the modified spinel(~1)—spinel(y) transformation occurred, a higher concentration of Fe2~ was found in spinel(~) rather than in garnet. As a result of the change in the Mg21—Fe2~ partition relation among coexisting minerals, an increase of about 1% in the Fe 2SiO4 component in (Mg,Fe)2SiO4 modified spinel and spinel was observed compared with olivine. These experimental results strongly suggest that the olivine(n)—modified spinel(i3) transformation is responsible for the seismic discontinuity at depths of 380—410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(,3)—spinel(’y) transformation and the pyroxene—garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100—150, 150—380, 380—410,410—500, 500—600 and 600—650 km. 1. Introduction stitution of the mantle transition zone. Since ferro- magnesian olivine, (Mg,Fe)2SiO4, is the most abundant It is widely accepted that the upper mantle consists mineral in the upper mantle, steady efforts have hither- chiefly of a peridotite, and more particularly of a to been focussed on recording the phase diagram for spinel Iherzolite in the uppermost mantle and a garnet the olivine—modified spinel—spinel transformation in llierzolite in the deeper part of the upper mantle. the system Mg2SiO4—Fe2SiO4 (Ringwood and Major, Major constituent minerals of the garnet lherzolite are 1966a, 1970; Akimoto and Fujisawa, 1968; Akimoto, olivine, orthopyroxene, Ca-rich clinopyroxene and 1972; Ito et al., 1974; Suito, 1972, 1977; Kawada, garnet. High-pressure phase transformations of these 1977). Semi-quantitative interpretations of the rapid component silicates and aluminosilicates have been increase in the seismic-wave velocity in the mantle extensively studied to clarify the mineralogical con- transition zone have successfully been made in terms