Recent studies have demonstrated the potentially robust nature of U-Pb and Pb/Pb systematics within certain sedimentary and metamorphic carbonates (e.g. Moorbath et al., 1987; Jahn, 1988; Jahn et al., 1990; DeWolf and Halliday, 1991). During the course of this work, the Pb/Pb dating technique has been applied successfully to the direct dating of Proterozoic stromatolitic carbonates from Western Australia and India, Silurian stromatoporoidal carbonates from Sweden and Archaean marbles from India, permitting the direct age determination of depositional/early diagenetic, late diagenetic and metamorphic events. Results indicate that large variations in μ value (238
Pb) and virtually homogeneous initial Pb isotopic compositions are a recurrent feature of sedimentary and metamorphic carbonates. Authigenic marine carbonates may incorporate U and Pb through a variety of geochemical mechanisms;
- organic complexing;
- crystal lattice substitution;
- adsorption onto particulate oxyhydroxides and
- early diagenetic reduction.
Since modern and ancient carbonates have U and Pb concentrations of the order of ppm, whereas dissolved U and Pb in the oceans occur at 3.2 ppb and 0.003 ppb, preconcentration within the water column must be an important factor in the establishment of appropriate geochemical conditions. The rapid scavenging of Pb, compared to rates of U fixation under suboxic conditions, means that depositional μ values seldom approach the sea water figure of c.80,000. Owing to the largely independent geochemical behaviour of U and Pb, early diagenetic, late diagenetic and metamorphic recrystallisation may either partially disturb Pb/Pb and U-Pb systematics or effect complete resetting of radiometric ages. Consequently, results from geochronological studies should be interpreted only after due consideration of all available geological information. The extensive distribution of metamorphic and sedimentary carbonates throughout the geological record, coupled with the apparent robustness of Pb/Pb systematics, means that this technique can offer an effective means of event dating, stratigraphic correlation and time scale calibration, particularly in the Precambrian where independent age constraints are limited. In addition, the identification of late diagenetic recrystallisation ages offers exciting potential for constraining the diagenetic histories of sedimentary basins.