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

A suite of largely unaltered, aphanitic, mica-bearing hypabyssal kimbe rlites from the Koidu kimberlite complex of the West African Craton ha ve been investigated to determine their geochemical affinity relative to Group I (nonmicaceous) and Group II (micaceous) kimberlites of sout hern Africa. Comparison is made with altered kimberlites from Liberia, other West African and global kimberlites. Based on major element oxi des, the Koidu kimberlites, though mica-bearing, show closest composit ional similarity with the Group IA kimberlites of southern Africa. Bas ed on major and trace elements, the Koidu kimberlites show an unusual geochemical signature. This signature is similar to that of the distin ctive, 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)r atios and low P2O5/Ce10(4)(<27), Ba/Rb(<32), and U/Th (<0.2) ratios c ompared with Group I kimberlites. Koidu kimberlites can be readily dis criminated from Group II kimberlites by their higher Ti/K (>0.4) and N b/La (>1) ratios and lower Ba/Nb(<10) and Pb/Ce (<0.06) ratios. The co mpositions 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 ki mberlite. Kimberlites with an Aries geochemical signature appear to ex ist on some other cratons, e.g., the Kundelungu kimberlites(Zaire) and two mica-bearing Group I kimberlites (S. Africa). The Koidu kimberlit es exhibit compositionally-dependent isotopic heterogeneity though ini tial epsilon(Nd) and epsilon(Sr) values are broadly asthenospheric (i. e., near bulk earth) similar to Group I and Aries. A compositional spe ctrum appears to exist between nonmicaceous Group I kimberlites throug h mica-bearing Koidu kimberlites to extreme endmembers of the Aries ty pe. This spectrum can be modelled as partial melts of heterogeneous pe ridotite sources which incorporate a potassic, high-Nb source componen t. The component may represent a fluoro-K-richterite-bearing peridotit e residue derived by melt extraction from subduction-zone metasomatize d peridotites. Such materials may have been trapped, together with for mer oceanic lithosphere, in the Transition Zone of the mantle. In resp onse to lower mantle upwelling, diapiric uprise accompanying reduced v olatile degassing of the kimberlite source may occur. Because of diffe rences in oxidation potential across the 400 km discontinuity, reactio ns in the ascending diapir will lead to redox melting and ultimately s egregation 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.