COMPARATIVE GEOCHEMISTRY OF WEST-AFRICAN KIMBERLITES - EVIDENCE FOR A MICACEOUS KIMBERLITE ENDMEMBER OF SUBLITHOSPHERIC ORIGIN

A suite of largely unaltered, aphanitic, mica-bearing hypabyssal kimberlites from the Koidu kimberlite complex of the West African Craton have been investigated to determine their geochemical affinity relative to Group I (nonmicaceous) and Group II (micaceous) kimberlites of southern Africa. Comparison is made with altered kimberlites from Liberia, other West African and global kimberlites. Based on major element oxides, the Koidu kimberlites, though mica-bearing, show closest compositional similarity with the Group IA kimberlites of southern Africa. Based on major and trace elements, the Koidu kimberlites show an unusual geochemical signature. This signature is similar to that of the distinctive, micaceous Aries kimberlite of northwest Australia, and includes high Nb/U (most samples >46), Cel Sr(>0.4), Ta/Hf(>2), and Nb/Zr(>1)ratios and low P2O5/Ce*10(4)(<27), Ba/Rb(<32), and U/Th (<0.2) ratios compared with Group I kimberlites. Koidu kimberlites can be readily discriminated from Group II kimberlites by their higher Ti/K (>0.4) and Nb/La (>1) ratios and lower Ba/Nb(<10) and Pb/Ce (<0.06) ratios. The compositions of Liberian kimberlites are leached of mobile incompatible elements, but least affected samples show affinity to Group I. Guinea kimberlites appear to be of two types: one having affinity with Group IA and the other, micaceous variety, having affinity with the Aries kimberlite. Kimberlites with an Aries geochemical signature appear to exist on some other cratons, e.g., the Kundelungu kimberlites(Zaire) and two mica-bearing Group I kimberlites (S. Africa). The Koidu kimberlites exhibit compositionally-dependent isotopic heterogeneity though initial epsilon(Nd) and epsilon(Sr) values are broadly asthenospheric (i.e., near bulk earth) similar to Group I and Aries. A compositional spectrum appears to exist between nonmicaceous Group I kimberlites through mica-bearing Koidu kimberlites to extreme endmembers of the Aries type. This spectrum can be modelled as partial melts of heterogeneous peridotite sources which incorporate a potassic, high-Nb source component. The component may represent a fluoro-K-richterite-bearing peridotite residue derived by melt extraction from subduction-zone metasomatized peridotites. Such materials may have been trapped, together with former oceanic lithosphere, in the Transition Zone of the mantle. In response to lower mantle upwelling, diapiric uprise accompanying reduced volatile degassing of the kimberlite source may occur. Because of differences in oxidation potential across the 400 km discontinuity, reactions in the ascending diapir will lead to redox melting and ultimately segregation of the kimberlite melt at the base of the thermal boundary layer(P similar to 13 GPa) separating the subcratonic lithosphere from the convective mantle.