Kankan diamonds (Guinea) I: from the lithosphere down to the transition zone

Peridotitic inclusions in alluvial diamonds from the Kankan region of Guine a in West Africa are mainly of lherzolitic paragenesis. Nevertheless, extre me Cr2O3 contents (max. 17 wt%) in some of the exclusively lherzolitic garn ets document that the diamond source experienced a previous stage of melt e xtraction in the spinel stability field. This initial depletion was followe d by at least two metasomatic stages: (1) enrichment of LREE and Sr and (2) introduction mainly of MREE-HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- a nd HFSE-poor character of stage (1) points towards a CHO-rich fluid or carb onatitic melt, the high HFSE in stage (2) favour silicate melts as enrichin g agent. Eclogitic inclusions are derived from a large depth interval rangi ng from the lithosphere through the asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from b oth lithosphere and transition zone suggests a possible relationship to sub ducted oceanic crust. Lithospheric eclogitic inclusions are derived from he terogeneous sources, that may broadly be divided into a low-Ca group with L REE depleted trace element patterns and a high-Ca group representing a sour ce with negative LREE-HREE slope that is moderately enriched in incompatibl e elements relative to primitive mantle. High-Ca inclusions of majoritic pa ragenesis are significantly more enriched in incompatible elements, such as in Sr and LREE. Calculated whole rock compositions require metasomatic enr ichment even if a derivation from MORB is assumed.