Translated from Paleogeotermy i mantiynyye strui, Izvestiya AN SSSR, seriya geologicheskaya, 1986, No. 7, pp. 16-25. The author is with the Institute of the Lithosphere, USSR Academy of Sciences, Moscow. On the basis of his recalibration of xenolith thermobarometers, he distinguishes convective geothermal gra dients associated with mantle plumes from normal conductive gradients. The paleogeothermal conditions in the upper mantle which controlled the course of deep-seated processes is revealed by mantle xenoliths brought by magmas to the earth's surface. The xenoliths often contain garnet (Gr), orthopyroxene (Op), and clinopyroxene (Cp), which are stable even at the deepest horizons of the upper mantle accessi ble to such xenolith sampling, and which are the most suitable phases for thermobarometry, be cause we have a reliable barometer in the solu bility of alumina in orthopyroxene coexisting with garnet and an effective two-pyroxene ther mometer. Measurements on the garnet-two pyroxene equilibrium [15] have been used [11] in two petrogenetic scales: the contents of A (A = Al + Cr) in orthopyroxene against the partition coefficients C-F ' and A-C-F ' (C = Ca, F ' = Mg + Fe + Mn) between garnet and orthopyroxene. This gives two barometers, which enable one to calculate the pressure in the garnet-orthopyroxene equilibrium as the arithmetic mean of the read ings, with a standard deviation of 0.2 GPa as estimated from the calibration curves. The temperature of the garnet-two pyroxene equilibrium is determined from the two twopyroxene thermometers, which are based on the partition coefficients for two (MgSiO3-FeSiO3) or three components (M3MO3-CaSiO3-FeSiO3) in the ternary pyroxene solid solutions with allow ance for the activities, together with the garnetclinopyroxene thermometer from the Mg-Ca-Fe partition coefficient [12]. The latter has been calibrated from experiments in iron capsules, with the result that the ratio F= Fe2O3/FeO (wt%) was virtually zero for both minerals. The twopyroxene equilibrium differs from the garnetclinopyroxene equilibrium in that it has been examined under less reducing conditions (in cap sules of silver, or silver alloyed with palladium), but the thermometers are based on compositions of coexisting pyroxenes with higher iron contents, and consequently with F close to zero.