A novel approach to double-spike Pb–Pb dating of carbonate rocks: examples from Neoproterozoic sequences in southern Africa

Application of the 207Pb–204Pb double-spiking technique to correct for mass-fractionation led to marked improvement in the quality of Pb isotope data and resulting ages obtained on low-grade metamorphosed Neoproterozoic carbonate rocks from the Pan-African Gariep and Saldania Belts in southern Namibia and South Africa. Carbonate rock powders were dissolved using 1 N acetic acid. Residue fractions were separated from the supernatant (carbonate fractions) and analysed separately. Marbles from the lower Pickelhaube Formation, Gariep Belt, yielded distinctly different 207Pb/206Pb isochron ages of 728±32 (MSWD=1.5) and 545±13 Ma (MSWD=7.4) for the carbonate and residue fractions, respectively. The former is interpreted as dating early diagenesis, whereas the latter is in excellent agreement with the timing of regional metamorphism. In contrast, both the carbonate and residue fractions from the Bloeddrif Member, Holgat Formation, Gariep Belt, and from the Kombuis Member, Matjies River Formation, Saldania Belt, yielded indistinguishable 207Pb/206Pb isochron ages. The isotope data from both fractions combined define an isochron age for the Bloeddrif Member marbles of 555±28 (MSWD=22.8). A whole rock isochron age of 553±30 Ma (MSWD=14.3) is given by the weighed sums of separately analysed carbonate and residue fractions data of the Kombuis Member. Both ages are interpreted to reflect early diagenesis. Our novel approach to double-spike 207Pb/206Pb dating of carbonate rocks highlights not only the importance of dating carbonate and residue fractions separately, but also led to a major revision of stratigraphic correlation between the Gariep and Saldania Belts.