PYROPE-KNORRINGITE GARNETS IN THE EARTHS MANTLE - EXPERIMENTS IN THE MGO-AL2O3-SIO2-CR2O3 SYSTEM

Pyrope garnets with high chromium contents occur in mantle xenoliths a nd as inclusions in diamonds. We report experimental results on equili bria in the garnet-orthopyroxene-eskolaite and garnet+orthopyroxene+sp inel+forsterite assemblages at 30-60 kbar and 1200-1500 degrees C. Cry stalline starting materials were used to reverse mineral reactions and obtain equilibrium compositions of phases. Experimental products were analysed by electron microprobe. Lattice parameters of all phases wer e obtained by X-ray diffractometry to determine compositional dependen cies of molar volumes. Excess mixing volumes of pyrope-knorringite gar nets can be described by a symmetrical Margules equation with W-V=0.05 6 J/bar. Spinel picrochromite solid solution is best approximated by a n asymmetrical model with W-V,W-Al=.0.0483 and W-VlCr=0/0722 J/bar. Th e volumes of corundum-eskolaite solution are adequately described by a one-parameter equation with W-V=0.038 J/bar. The volumes of ternary e nstatite - tschermakite - Cr-tschermakite solutions suggest almost ide al mixing and a value of 5.96 J/bar for the volume of pure tschermakit e. Slight negative deviation from ideality was found for the enstatite -tschermakite join. The data obtained on the volumes of solid solution s were used to calculate the stability of chrome-bearing garnets at pr essures up to 100 kbar. The distribution of chromium and aluminium bet ween phases suggests that pyrope-knorringite solid solution may be reg arded as ideal at 1 bar with non ideality at higher pressures being at tributed to the excess mixing volume. The calculated lower pressure li mit of pure knorringite stability is 95 kbar at 1000 degrees C and 80 kbar at 1600 degrees C. Cr# isopleths of garnet have negative slopes o n the P-T plane in both systems studied. At pressures above 70-75 kbar garnet in equilibrium with spinel, orthopyroxene, and forsterite is m ore chromian than that from the garnet-orthopyroxene-eskolaite assembl age. This defines the maximum pressure of spinel stability in garnet p eridotite. The results were used to estimate entrapment conditions for chromian garnets in diamonds. The calculated values of pressures (45- 55 kbar) are in good agreement with estimates based on alumina solubil ity in orthopyroxene.