EXPERIMENTAL-STUDY OF SPINEL-GARNET PHASE-TRANSITION IN UPPER-MANTLE AND ITS SIGNIFICANCE

Experimental study of spinel-garnet phase transition was carried out u sing natural mineral and rock specimens from xenolith of mantle rocks in Cenozoic basalt as starting materials. From the result it was found that the condition of spinel Iherzolite-garnet Iherzolite phase trans ition (T = 1 100 degrees C and P = 1.8-2.0 GPa) is consistent with the P-T equilibrium condition of the five-phase assemblage spinel/garnet Iherzolite in eastern China, suggesting that there may exist a spinel- garnet Iherzolite phase transition zone with the thickness of a few km to several ten km at the depth of 55-70 km in the continental upper m antle of eastern China. The depth of phase transition from spinel pyro xenite to garnet pyroxenite is found to be less than 55 km. Experiment results also show that water promotes metasomatism on one hand but su ppresses phase transition on the other. Zoning of mineral composition was also discussed.