3) Vernadsky Inst. USSR Academy of Sciences, Moscow.Pyropic garnets in Iherzolites and harzburgites from diamond-bearing pipes contain significant amounts of chromium. Similar garnets also occur as disseminated grains in kimberlites. Such garnets are usually low in calcium and chromium must be present mostly as the Mg3Cr2Si302^2 member knorringite. Garnets found as inclusions in diamonds are particularly rich in this component, commonly with 30-40 mol % knorringite (Meyer and Boyd, 1969;Sobolev et al. 1969) and in a few cases with more than 60 mol-% (Meyer, 1975;Sobolev et al., 1973). Experimental work on the join pyropeknorringite was already undertaken more than three decades ago (Coes, 1955) but, because of experimental problems, there remains serious disagreement about the pressure stability limit of knorringite ( Fig. 1).Ringwood (1977) synthesized pure knorringite and solid solutions between pyrope and knorringite at pressures between 60 and 80 kb and temperatures of 1400 -1500 °C. Irifune et al. (1982) and Irifune and Hariya (1983) synthesized pure knorringite at pressures greater than 105 kb and described a stability limit with a positive dP/dT slope. Turkin et al. (1983) brought down the position of the knorringite stability field to 80 -90 kb and found a negative dP/dT slope. The same group investigated the composition of coexisting garnet and spinel in the 4-mineral harzburgite paragenesis Gargg+OpXg +Sp +Fo in the system MASCr from 30 to 70 kb and showed that (i) the knorringite component in garnet increases with increasing pressure and temperature in this paragenesis (negative dP/dT slopes of Cr-isopleths) and (ii) the picrochromite component in spinel increases with pressure and decreases with temperature (positive dP/dT slopes of Crisopleths) . Thus pressure and temperature of crystallisation can be defined uniquely from coexisting garnet and spinel. Moreover, estimated pressures and temperatures are potentially very accurate because the Cr-isopleths for garnet and spinel cut at very steep angles.These prospects for application to geothermobarometry led us to try to clarify the present situation by reversibly determining the compositions of coexisting phases in piston cylinder and belt apparatuses in the pressure range from 27 to 60 kb and at temperatures from 1200 to 1500 °C. Equilibrium composition of coexisting phases were determined reversibly by using mixes of minerals with either high or low Cr/Al ratios. Phase compositions were determined from X-ray properties and by electron microprobe.In a first step we determined the solubility limits of knorringite in pyrope garnets and the composition of coexisting orthopyroxene and corundum-escolaite solid solution and derived thermodynamic mixing parameters for all phases. Extrapolation of our experimental results is consistent with Turkin's et al. (1983) stability limit for pure knorringite with a negative dP/dT slope.
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