The origins of eclogite (clinopyroxene+garnet) and the relative proportions of eclogite in the upper mantle are issues of considerable uncertainty and debate that bear upon the chemical and physical dynamics of petrogenesis, recycling, and remote sensing interpretation. Forty-one upper mantle eclogites from the Koidu Kimberlite Complex, Sierra Leone, were selected for detailed petrographic and chemical examination to bolster an earlier database with a view to the identification of protoliths and possible source regions of eclogite origin. On the basis of MgO contents in garnets, eclogites are divided into a high-MgO suite and a low-MgO suite. High-MgO eclogites contain pyrope (16.5--20.2 wt % MgO), with an average garnet composition of Pyr65Alm20Gross15. Almandine and grossular (5.3--13.2 wt % MgO) are present in low-MgO eclogites and vary from Pyr20Alm60Gross20 to Pyr35Alm30Gross35. Pyroxenes in high-MgO eclogites are diopsidic (Jd11Di89-Jd26Di74); those in low-MgO eclogites range from jadeitic-diopside to omphacite (Jd20Di80-Jd48Di52). Oriented apatite crystals in garnet and clinopyroxene are interpreted to be products of exsolution and, coupled with coexisting rutile, imply that garnet is a major repository for P, Cl, F, OH, and Ti in the upper mantle.

Reconstructed bulk compositions of high- (15.0--18.9 wt%) and low-MgO (7.1--12.2 wt %) eclogites are distinct, and major elements in these xenoliths broadly resemble basalts, picrites, and komatiites. Most high-MgO eclogites equilibrated at 1080 ¿C at 4.7 GPa to 1130 ¿C at 5.2 GPa, whereas most low-MgO eclogites cluster at 880 ¿C at 3.3 GPa to 930 ¿C at 3.8 GPa. Estimated PT and depths or origin of the Koidu eclogites imply that high-MgO eclogites are asthenospheric, low-MgO eclogites are lithospheric, and both are likely products of plume activity. Diamondiferous eclogites, worldwide, have characteristic Na, K, Ti, and IVAl (Si) in garnet-clinopyroxene pairs that point to distinctive source regions and petrogenesis; these chemical variables may be applied to diamond exploration and evaluation. ¿ American Geophysical Union 1995.