PETROGRAPHY AND MINERAL COMPOSITIONS OF ECLOGITES FROM THE KOIDU KIMBERLITE COMPLEX, SIERRA-LEONE

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 Pyr(65)Alm(20)Gross(15). Almandine and grossular (5.3-13.2 wt % MgO) are present in low-MgO eclogites and vary from Pyr(20)Alm(60)Gross(20) to Pyr(35)Alm(30)Gross(35). Pyroxenes in high-MgO eclogites are diopsidic (Jd(11)Di(89)-Jd(26)Di(74)); those in low-MgO eclogites range from jadeitic-diopside to omphacite (Jd(20)Di(80)-Jd(48)Di(52)). 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 degrees C at 4.7 GPa to 1130 degrees C at 5.2 GPa, whereas most low-MgO eclogites cluster at 880 degrees C at 3.3 GPa to 930 degrees C at 3.8 GPa. Estimated PT and depths of 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 Al-IV (Si) in garnet-clinopyroxene pairs that point to distinctive source regions and petrogenesis; these chemical variables may be applied to diamond exploration and evaluation.