Mantle transects in South and Central Africa according to data of mantle xenocrysts and diamond inclusions.

We designed the mantle transects using the PTXFO2 diagrams  (Ashchepkov et al., 2010; 2013; 2017) constructed (Fig. 1) for mantle columns beneath kimberlites of  Kaapvaal and the Congo-Kasai cratons.

The PTXFO2 diagrams (Ashchepkov et al., 2013) in South Africa were constructed using mainly analyses of garnets, eclogitic minerals and inclusions in diamonds in open publications. The sub-calcic type garnets mainly refer to the ancient low-temperature geotherms (35 mv/m2) and higher-temperature inclusions of eclogite-pyroxenite type, giving convective geotherms crossing conductive ones, which reflects the migration process of apparently hybrid melts. 

Roberts Victor is a Mesozoic pipe 95Ma  famous due to the abundance of various eclogite xenoliths. Many eclogites in the SCLM show P-Fe# trends that are typical of ascending and differentiating magmas. Such “basaltic eclogites” may show typical features of their magmatic origin (Fig.1A). They may create channels within the peridotitic lithosphere starting from the deep subduction stages.  These irregularities formed during subduction stages and due to later plumes could explain the irregular distribution of eclogites in kimberlite pipes and abundance in  Roberts Victor (Jacob et al., 2005; Huang et al., 2014) and practical absence in others.

 In the mantle of Luaxe and Cuilo pipes (Fig.1 B, C) the minerals give highly variable conditions representing the multistage metasomatic processes. The oxygen conditions are good for diamonds  The mantle column reveals a long ilmenite trend and the presence of abundant eclogites (Zinchenko et al., 2021; Nikitina et al., 2014; Ashchepkov et al., 2012).

In the sub-meridional mantle transect through the South Kaapvaal and Zimbabwe cratons, mainly dunitic at the basement ancient cores of cratons like in Lesotho and Central part of  Zimbabwe mantle is relatively depleted and low temperature.   In the marginal parts like near Premier pipe, Venetia in Limpopo and Orapa in Magondi belt the amount of the pyroxenitic and eclogitic materials drastically rises and the temperature regimes and oxidation state rise because these zones are more transparent for the melts. These zones are often highly diamondiferous and the largest diamonds are occurring in these regions and pipes (Fig. 2).

In the mantle section through the pipes of the so-called diamond-bearing corridor of the Lucapa within the northeastern part of the Congo craton (Fig. 3), the immersion of the least oxidized and more productive horizon represented mainly by depleted peridotite material and much less oxidized is gradually thinking and in the to the southwest is recorded in the lower part. The temperatures in the lower part are also decreasing. This determines the sharp increase in the diamond grades of kimberlite pipes in this direction. But commonly this transect represents a relatively smooth homogeneous structure, the lithosphere of the craton's mantle distinguishes outflow clusters corresponding to thickenings of pips and kimberlite clusters that have arisen within the limits of separately permeable zones that occur at the intersection of deep faults.

RFBR grant 19-05-00788.  Supported by Ministry of Science and Higher Education.