Platinum-group element abundances in xenoliths from Marsabit Volcanic field (Kenya Rift).

PGE have been determined in 14 petrologically well-charaterized spinel peridotite xenoliths from the Marsabit volcanic field (Kenya Rift). The samples pertain to 3 groups previously distinguished based on their textural characteristics: porphyroclastic (I) and recryztallized (II) spl lherzolite and porphyroclastic harzburgite (III). The overall total PGE contents of Marsabit xenoliths are between 14 and 32 ppb. Whether coarse-grained, recrystallized, deformed, virtually anhydrous or modally metasomatized, all peridotites have Pd-depleted PUM-normalized PGE patterns probably inherited from an old (Archean? Proterozoic?) partial melting event. This is confirmed by the crude correlation existing between the clinopyroxene modal content of the rocks and their total PGE or the anti-correlations existing between total PGE and olivine modal content or Pd/Ir ratios and clinopyroxene modal content. Only one rock, the richest in total PGE and displaying clear signs of modal metasomatism, falls outside the trend, having high Ol and low Cpx contents. However, the shape of PGE patterns can greatly vary between or within different peridotite groups. Only group II shows coherent PGE patterns. The variability of PGE patterns and concentrations probably reflects recent magmatic history and/or ancient metasomatism. Preliminary Sr-Nd-Pb isotopic data show that the most metasomatized lherzolites display unradiogenic signatures (clinopyroxene separates and whole rock).