Thermal and chemical heterogeneity of the upper mantle beneath the Baikal-Mongolia territory

Mineralogical, petrochemical, and geochemical characteristics and the crystallization conditions (T, P, and \(f_{O_2 } \)) of mantle xenoliths in alkaline basalts from the Baikal-Mongolia area indicated: (1) that the compositional heterogeneity of the mantle beneath southeastern and central parts of Mongolia, the Khamar-Daban, and Transbaikalia reflects variations in the degree of melting of the primitive mantle (10% and more for southeastern Mongolia, 0–15% for central Mongolia, and 0–10% for Transbaikalia and the Khamar-Daban) and, perhaps, also the compositional heterogeneity of the mantle source material; (2) the dependence of the concentrations of Y, Zr, Ti, Sc, and REE (from Nd to Lu) on the contents of major oxides (Al₂O₃ and MgO) and on their ratios (MgO/SiO₂ and Al₂O₃/MgO) in the xenoliths testifies that the distribution of trace elements and REE in the mantle was controlled by its partial melting; (3) the thermal state of the mantle varies beneath the Vitim area (geothermal gradient TG = 9.4 ± 0.3°C/km), central Mongolia (TG = 10.2 ± 0.2°C/km), and southeastern Mongolia (TG = 9.4 ± 0.3°C/km); (4) the Pb-Pb model age of the material represented by the primitive mantle xenoliths is 4457 ± 12 Ma and is consistent with the evaluated duration of the development of the core and its complementary primitive mantle calculated for the U-Th-Pb system (Galer and Golddstein, 1996).