In situ U–Pb Dating and Sr–Nd Isotopic Analysis of Perovskite: Constraints on the Age and Petrogenesis of the Kuruman Kimberlite Province, Kaapvaal Craton, South Africa

Abstract

The Kuruman Kimberlite Province contains the oldest known kimberlites in the Kaapvaal Craton ( 1·6^1·8 Ga), but only limited geochronological and isotopic data exist for this province.To provide better constraints on the sources of the kimberlite magmas and the timing of magmatism, we have carried out in situ U^Pb dating and Sr^Nd isotopic analysis of groundmass perovskite from four of the Kuruman kimberlites (Bathlaros, Elston, Helpmekaar and Zero).We also have measured the Sr-isotope composition of groundmass apatite and carbonate in several of the kimberlites, for comparison with published whole-rock isotopic data. Attempts to date perovskite using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were hindered by extensive Pb loss, even in apparently unaltered grains, yielding anomalously young emplacement ages. The assumption of a Mesoproterozoic common-Pb component allows the extraction of intercept ages from the least altered grains (Bathlaros, 1640 13 Ma; Elston, 1663 49 Ma; Helpmekaar, 1638 Ma; Zero, 1845 45 Ma). The smaller sampling volume of secondary ion mass spectrometry and the ability to measure Pb, thus allowing more precise common-Pb corrections, produces concordant ages.These reveal that kimberlite intrusion occurred over a time span of 200 Myr (Bathlaros, 1647 27 Ma; Zero, 1835 25 Ma), with emplacement ages younging to the east.The results of this study suggest that alteration may disrupt the U^Pb system in perovskite. In situ Sr and Nd isotope analyses [LA-multi-collector (MC)-ICP-MS] of perovskite show a moderate range in values (Sr/ Sr(i)1⁄4 0·70383^0·70507; eNd(i)1⁄4 ^2·7 to 1·5), but similar mean values among the four kimberlites (Sr/ Sr(i)1⁄4 0·70396^0·70425; eNd(i)1⁄4 ^0·8 to 0·8). The Nd-isotope compositions of perovskite are similar to those of whole-rock samples of the host kimberlite (eNd(i)1⁄4 1·0 to 3·0), whereas Sr in perovskite and apatite (Sr/ Sr(i)1⁄4 0·70424^0·70427) is more radiogenic than the whole-rock values (Sr/ Sr(i)1⁄4 0·70293^0·70382). However, groundmass carbonates generally yield lower values (Sr/ Sr(i)1⁄4 0·70348^0·70405), closer to those of the kimberlite samples, indicating that the whole-rock Sr isotope compositions are largely controlled by carbonates, probably assimilated from the immediate country rocks. The isotopic data suggest that the Kuruman kimberlite magmas were initially derived from a deeper sub-lithospheric source region, but their distinctive compositional characteristics were imposed through assimilation, first of metasomatized sub-continental lithospheric mantle and later of crustal material, during magma ascent.