The concentration of platinum-group elements and gold in southern African and Karelian kimberlite-hosted mantle xenoliths: Implications for the noble metal content of the Earth's mantle

We have determined the concentrations of the platinum-group elements and gold in 111 mantle xenoliths from more than 20 kimberlite pipes in southern Africa and the Karelian craton, Fennoscandian Shield, with the aim to constrain the composition of the Archean- early Proterozoic primitive mantle. Average noble metal contents of the southern African peridotite xenoliths (in ppb) are 3.67 Ir, 6.54 Ru, 0.93 Rh, 4.3 Pt, 1.84 Pd, and 1.09 Au, and for the Karelian xenoliths 3.75 Ir, 6.58 Ru, 0.79 Rh, 4.84 Pt, 2.28 Pd, and 1.46 Au. The distribution of PGE-bearing phases is heterogenous, as in previously published datasets of lithospheric mantle from the Kaapvaal and other cratons. We argue that the heterogenous noble metal content of the cratonic lithospheric mantle is partly due to sluggish equilibration of the Archean mantle with late veneer (Maier et al., 2009, Nature, v. 460, 620-623). As a result, the noble metal content of the primitive mantle cannot be accurately determined using cratonic mantle samples. Younger mantle rocks (e.g., orogenic and oceanic peridotites) show more homogenous noble metal contents, but since they formed after the onset of crust formation it is debatable whether their composition accurately defines the noble metal content of PUM.