The Pb, Sr and Nd isotopic composition of the upper continental crust: An Australian perspective.

A bulk composition for the upper continental crust (UCC) is often constructed using sediments and associated river waters, which are both assumed to efficiently average contributions from a multitude of bedrock components. Many potential biases, however, exist in sedimentary systems, rendering this approach less than optimal. In order to further investigate these phenomena, we estimate the bulk Pb, Sr, and Nd isotopic composition of the Australian UCC using archived National Geochemical Survey of Australia (NGSA) regolith samples, collected so as to represent 1186 catchments covering an area approaching 6.17 million km2. The size of the dataset and its systematic nature provide a high level of confidence that it is statistically representative, while the relative tectonic stability of the Australian continent, preserving a wide range of sample ages and lithologies, suggests that the data may form a reliable proxy for the global bulk UCC composition. Calculated median values for the Pb isotope system based on 1229 NGSA samples (some catchments were sampled in duplicate) representing an area of 5.65 million km2 (206Pb/204Pb = 18.838, 207Pb/204Pb = 15.686, 208Pb/204Pb = 38.989, 207Pb/206Pb = 0.83210, 208Pb/206Pb = 2.0669) are comparable to previous estimates of the global UCC, providing a strong validation of our methodology. We believe these new global bulk UCC values to be considerably more robust, however, given the much larger and more spatially representative dataset they derive from. In contrast, however, our Sr and Nd isotope results (median 87Sr/86Sr = 0.7377, 143Nd/144Nd = 0.511817), derived from significantly smaller subsets of the NGSA sample suite (576 and 93 samples, respectively, in turn representing areas of 2.6 million and 490 thousand km2), are considerably more radiogenic/evolved than previous UCC estimates. We discuss potential reasons for these discrepancies and conclude that the present data appear to confirm the possibility of multiple potential biases in current global assessments of bulk UCC isotopic values, related to methodological approach, over-sampling of younger, tectonically active regimes, and significant non-gaussian behaviour in sedimentary datasets. • Continental-scale isotopic analysis of regolith samples. • Provide a good proxy for the Upper Continental Crust (UCC) composition. • Pb isotope ratios in agreement with previous global estimates. • Sr and Nd isotopes show more isotopically evolved ratios than previous estimates. • Further investigation of global UCC compositions appears to be warranted. [ABSTRACT FROM AUTHOR]