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.

中文翻译：

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上部大陆地壳的 Pb、Sr 和 Nd 同位素组成：澳大利亚的观点


上陆壳 （UCC） 的主体成分通常是使用沉积物和相关的河水构建的，这两者都被认为可以有效地平均多种基岩成分的贡献。然而，沉积系统中存在许多潜在的偏差，这使得这种方法不是最佳方法。为了进一步研究这些现象，我们使用澳大利亚国家地球化学调查局 （NGSA） 存档的风化层样本估计澳大利亚 UCC 的体量 Pb、Sr 和 Nd 同位素组成，这些样本收集起来代表了 1186 个集水区，覆盖面积接近 617 万平方公里。数据集的大小及其系统性为其具有统计代表性提供了高度的信心，而澳大利亚大陆的相对构造稳定性，保留了广泛的样本年龄和岩性，表明数据可能构成全球整体 UCC 组成的可靠代理。根据 1229 个 NGSA 样本（一些集水区一式两份采样）计算出的铅同位素系统的中位数值，面积为 565 万平方公里（206Pb/204Pb = 18.838,207Pb/204Pb = 15.686,208Pb/204Pb = 38.989,207Pb/206Pb = 0.83210,208Pb/206Pb = 2.0669）与之前对全球 UCC 的估计相当，为我们的方法提供了有力的验证。然而，我们认为这些新的全局体 UCC 值要稳健得多，因为它们来自更大且空间更具代表性的数据集。然而，相比之下，我们的 Sr 和 Nd 同位素结果（中位数 87Sr/86Sr = 0.7377,143Nd/144Nd = 0.511817）来自 NGSA 样品组的明显较小的子集（分别为 576 和 93 个样品，依次代表 2.600 万和 49 万平方公里），比之前的 UCC 估计的放射性/进化性要高得多。我们讨论了这些差异的潜在原因，并得出结论，目前的数据似乎证实了当前对整体 UCC 同位素值的全球评估中存在多种潜在偏差的可能性，这些偏差与方法学方法、对较年轻的、构造活跃的制度的过度采样以及沉积数据集中重要的非高斯行为有关。