Contrasting Diamond Potentials of Kimberlites of the V. Grib and TsNIGRI-Arkhangelskaya Pipes (Arkhangelsk Diamondiferous Province) as a Result of the Different Compositions and Evolution of the Lithospheric Mantle: Data on the Contents of Major and Trace Elements in Garnet Xenocrysts

Abstract

—The paper presents data on the contents of major and trace elements in garnet xenocrysts from kimberlites of the highly diamondiferous V. Grib pipe (1100 grains) and weakly diamondiferous TsNIGRI-Arkhangelskaya pipe (446 grains). We have established that the high diamond potential of the V. Grib kimberlite pipe is due to several factors related to the composition and structure of the lithospheric mantle represented by kimberlite: (1) a “cold” regime, with a heat flow of 36–38 mW/m2; (2) a thick “diamond window” (70–102 km), with the depth level of the lower boundary of the lithospheric mantle estimated at >200 km; (3) the high degree of preservation of diamondbearing peridotites under the P–T conditions of diamond stability despite the high degree of impregnation of the lithospheric-mantle rocks by high-temperature silicate melts. The low diamond content of the TsNIGRI-Arkhangelskaya kimberlite pipe as compared with the V. Grib pipe is due to the following factors: (1) a more intense heat flow in the lithospheric mantle, 38–42 mW/m2; (2) a thinner “diamond window”, 10–60 km, with the depth level of the lower boundary of the lithospheric mantle estimated at <200 km; (3) weak impregnation of the rocks of the middle and lower lithospheric mantle by CHO fluid/melt, which might have induced diamond formation; (4) minimum preservation of diamond-bearing peridotites in the lower lithospheric mantle, partly because of the possible impregnation of this zone by high-temperature silicate melts.