The Pb isotopic evolution of the Earth: inferences from river water suspended loads

Abstract

Pb and U concentrations and Pb isotopic variations in the suspended loads of major rivers draining regions with a range of ages of crustal formation are reported. The 206Pb/ 204Pb , 207Pb/ 204Pb and 208Pb/ 204Pb ratios are positively correlated with 87Sr/ 86Sr and negatively correlated with ε Nd values in the samples. A coherent negative correlation of the Pb isotope ratios with ε Nd values allows us to estimate the average Pb isotope composition of the upper continental crust at 206Pb/ 204Pb= 19.32 ± 0.28 , 207Pb/ 204Pb= 15.76 ± 0.09 and 208Pb/ 204Pb= 39.33 ± 0.39 . The 206Pb/ 204Pb ratios are positively correlated with their Nd model ages, reflecting the higher upper continental crust μ values relative to mantle μ values. The upper continental crust, as well as the MORB and OIB sources, plot to the right of the geochron. Assuming that the lower crust is the missing low-μ reservoir, we have calculated a range of possible lower crust Pb isotope compositions similar to many granulite terrains. Overall, the upper continental crust has somewhat higher 207Pb/ 204Pb than MORB or OIB, while 206Pb/ 204Pb overlaps with the MORB values and is less radiogenic than some OIB values. Typical measured μ values for the suspended loads of large rivers are ∼ 4, while the μ values of their crustal source rocks (based on the Pb isotope composition) are ∼ 12, allowing for up to 65% of U from continental weathering being carried in the dissolved load to the oceans. The near-overlap of the 206Pb/ 204Pb values of MORB and continental crust is consistent with the suggestion that U is recycled into the mantle through altered oceanic crust. The exception to these trends are the samples with the oldest Nd model ages (∼ 3.5 Ga). The Pb isotope ratios show drastic deviation from the Pb and Sr correlation trend suggesting an inherent low μ value in the mantle source of the early Archean continental crust.