Major-element composition of sediments in terms of weathering and provenance: Implications for crustal recycling

Abstract

The elemental composition of a sediment is set by the composition of its protolithand modified by weathering, sorting, and diagenesis. An important problem is deconvolving these contributions to a sediment’s composition to arrive at information about processesthat operate on the Earth’s surface. We approach this problem by developing a predictive andinvertible model of sedimentary major-element composition. We compile a dataset of sedimentary rock, river sediment, soil, and igneous rock compositions. Principal componentanalysis of the dataset shows that most variation can be simplified to a small number of variables. We thus show that any sediment’s composition can be described with just two vectorsof igneous evolution and weathering. We hence define a model for sedimentary compositionas a combination of these processes. A 1:1 correspondence is observed between predictionsand independent data. The log-ratios ln(K 2 O/MgO) and ln(Al 2 O 3 /Na 2 O) are found to besimple proxies for respectively the model’s protolith and weathering indices. Significant deviations from the model can be explained by sodium-calcium exchange. Using this approach,we show that the major-element composition of the upper continental crust has been modified by weathering and we calculate the amount of each element that it must have lost tomodify it to its present composition. By extrapolating modern weathering rates over the ageof the crust we conclude that it has not retained a significant amount of the necessarily produced weathering restite. This restite has likely been subducted into the mantle, indicating acrust-to-mantle recycling rate of 1.33 ± 0.89 × 10 13 kg yr −1 .