Lead and strontium isotopes in Cretaceous kimberlites and mantle-derived xenoliths from Southern Africa

Abstract

Concentrations of lead, uranium and thorium and isotopic compositions of lead are reported for twelve Cretaceous kimberlites and five Cretaceous nucleated autoliths. The samples are from Lesotho and from the area around Kimberley (Cape Province, South Africa). In the case of the autoliths potassium, rubidium and strontium concentrations and 87Sr/ 86Sr ratios were also measured. Work reported on clinopyroxenes from mantle-derived xenoliths in kimberlites includes lead isotopes for twelve samples and strontium isotopes for nine of these. The autoliths have initial 87Sr/ 86Sr ratios between 0.7035 and 0.7095. A large spread in initial lead isotope ratios ( 206Pb/ 204Pb: 17.6–20; 208Pb/ 204Pb: 37.7–39.5) was found in the matrix kimberlites and autoliths. In the 207Pb/ 204Pb vs. 206Pb/ 204Pb plot, the initial lead isotope ratios of the kimberlite and autolith samples roughly define a slope of 0.10, corresponding to an age of 1575 m.y. With respect to the spread of initial ratios as well as with respect to this slope, the kimberlite and autolith lead isotopic pattern is comparable to patterns obtained from carbonatites and ocean island volcanics. The xenoliths studied include coarse-granular and porphyroclastic material from the Kimberley area and coarse-granular samples from Lesotho. Their 87Sr/ 86Sr ratios are generally between 0.704 and 0.706 but a value of 0.713 was found in one sample. They show a surprisingly large spread in lead isotope ratios ( 206Pb/ 204Pb: 17.5–20; 208Pb/ 204Pb: 37.3–39.4). The isotopic patterns of the xenolithic material and of the kimberlites and autoliths are considered to provide a strong indication that the upper mantle beneath Southern Africa is isotopically heterogeneous on a regional scale.