PbPb and UPb geochronology of carbonate rocks: an assessment
Journal
Chemical Geology
Journal Volume
115
Journal Issue
1-2
Pages
125-151
Date Issued
1994
Author(s)
Abstract
Recent application of carbonate PbPb and UPb geochronology has made an important contribution to the dating of primary deposition (strictly speaking, early diagenesis) or metamorphism of sedimentary sequences. The present article summarizes some highlights of this development, discusses the functioning of carbonate UPb and PbPb chronometers, and outlines major advantages, drawbacks and some problems to be resolved in the future. Like in all classic dating methods (RbSr, KAr, Sm1bNd, U1bPb zircon), the validity of UPb and PbPb ages of non-metamorphosed carbonates rests upon the closed-system behavior of U and Pb. Carbonate UPb or PbPb ages alone do not yield conclusive geologic meanings. However, for metamorphic carbonates, PbPb isochron ages of marbles generally reflect faithfully the time of metamorphic recrystallisation. Additional information on the thermal histories and UPb fractionation of carbonates may be obtained usin the three-dimensional approach of Wendt (1984). The available data for carbonates from 16 localities, with ages ranging from Archean to Mesozoic, show that except for one case (Mushandike Limestone) all model μ1-values have a small range of 8.4 ± 0.4. Very high μ1-value as would be expected from a closed-system isotopic homogenization in metamorphic carbonates has never been observed, suggesting: (1) in diagenetic or metamorphic conditions, Pb was homogenized in an open system containing both carbonate and extraneous Pb; and (2) the extraneous Pb had a signature of major terrestrial reservoir, which dominated the Pb budget in the renewed carbonate UPb isotopic systems. The published UPb and PbPb ages so far are not sufficiently precise to useful in the calibration of geologic time scale for the Phanerozoic. Despite many carbonate rocks have been reasonably dated by the UPb and PbPb methods, there are cases of failure (no isochron obtained) and results of ambiguous geological significance (disturbed isotopic systems). Several important problems remain to be answered: (1) modes of carbonate formation and UPb incorporation; (2) processes of dolomitization and their relation with U and Pb redistribution; (3) time interval between deposition and diagenesis/dolomitization; (4) Pb isotopic homogenization and UPb redistribution during all stages of diagenesis and metamorphism; and (5) identification of uranium carriers in carbonates. © 1994.
Type
journal article