Polygenetic sources of kimberlites, magma composition, and diamond potential exemplified by the East European and Siberian cratons

Abstract

The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO 2 -K 2 O, TiO 2 -(Y, Zr, HREE), SiO 2 -MgO, SiO 2 -Al 2 O 3 , MgO- Ni, MgO-CO 2 , and MgO-H 2 O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial ( 143 Nd/ 144 Nd) i ratio of kimberlites from the Yaku- tian collection is 0.5121-0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: 206 Pb/ 204 Pb of 16.19-19.14, 207 Pb/ 204 Pb of 15.44-15.61, and 208 Pb/ 204 Pb of 34.99-38.55. In the 207 Pb/ 204 Pb- 206 Pb/ 204 Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kim- berlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the astheno- sphere and was enriched in water and other volatile components (especially CO 2 ). During its ascent to the sur- face, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional char- acteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrolog- ical and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks.