EXPERIMENTAL SIMULATION OF THE GARNET-TO- SPINEL LHERZOLITE TRANSITION IN A NATURAL SYSTEM

A number of experiments on a garnet Iherzolite sample found as xenolit h in alkaline picrite basalt on the Vitim plateau have been carried ou t using the piston-cylinder high-pressure apparatus within the range o f 5-15 kbar and 1000-1350 degrees C. The transition from garnet to spi nel Iherzolite (Gar+O1 = Opx+Cpx+Sp) is accompanied by a concurrent re action of garnet decomposition: Gar = Opx+Cpx+Sp(+P1). Two different r eactions lead to identical associations and are responsible for contra sting composition of the same minerals. Differences are mainly disting uished in Al and Cr contents. The most aluminiferous spinel results fr om garnet decomposition. In general, interaction of garnet with surrou nding minerals leads to an increased chrome content in new-born spinel . The high and maximum chrome contents in spinel are due to the influe nce of water-carbonate fluid phase. Spinel regularly changes with T an d P. Spinel seems to be the most probable mineral for relative geother mobarometric estimations of not exactly equilibrium process of fast up lift of mantle-derived material. The boundary of garnet-free Iherzolit e association is localized at about 10 kbar and 1000-1100 degrees C. G arnet-spinel paragenesis is stable to higher pressures at least to 15 kbar. The lower boundary of polycomponent garnet stability for the sam ple studied is determined below 8-15 kbar at 1100-1250 degrees C.