The elemental composition of a sediment is set by the composition of its protolith and modified by weathering, sorting, and diagenesis. An important problem is deconvolving these contributions to a sediment's composition to arrive at information about processes that operate on the Earth's surface. We approach this problem by developing a predictive and invertible model of sedimentary major element composition. We compile a data set of sedimentary rock, river sediment, soil, and igneous rock compositions. Principal component analysis of the data set 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 vectors of igneous evolution and weathering. We hence define a model for sedimentary composition as a combination of these processes. A 1:1 correspondence is observed between predictions and independent data. The log ratios ln(K 2 O∕MgO) and ln(Al 2 O 3 ∕Na 2 O) are found to be simple 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 to modify it to its present composition. By extrapolating modern weathering rates over the age of 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 a crust-to-mantle recycling rate of 1.33 ± 0.89 ×10 13 kg•year −1 .

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<div> <div> <div> </div> </div> </div><div> <div> <div> <p>The volume and composition of Earth's earliest continental crust is a matter of ongoing debate, but is an essential component of solid-Earth evolution, relating to mantle dynamics and the origin of plate tectonics. The isotopic composition of titanium within sedimentary rocks, a suggested proxy for protolith composition, suggests the early emergence of an evolved continental crust. Other geochemical proxies such as Ni/Co and Cr/Zn ratios suggest a more mafic early crust.  Important to understanding the differences between these proxy-based interpretations of crustal growth and composition is the mechanism of crustal chemical evolution.  Two key processes may occur: weathering, whereby cations are selectively removed from the continents and transported to the oceans; and igneous differentiation.  Resolving these processes is hampered by the ability to deconvolve their compositional effects. To overcome this, we derive a predictive and invertible model of sedimentary major-element composition that reconstructs protolith composition, and hence that of the crust, whilst accounting for the effect of weathering. </p> <p>We compile a dataset of sedimentary rock, river sediment, soil, and igneous rock compositions. By applying principal component analysis to the log-ratio transformed compositional dataset we show that any composition can be well described by considering just two linear vectors of igneous evolution and weathering. We thus define a model for sedimentary composition as a linear combination of these two processes, which allows us to undo the compositional effect of weathering to reconstruct the major element composition of protoliths and thereby average upper continental crust through time.</p> <p>We find that the major-element composition of the modern upper continental crust has been modified by weathering relative to pristine igneous rocks. We calculate the amount of each element that must be lost to sufficiently modify the crustal composition. By extrapolating modern weathering rates over the age of the crust we conclude that a significant amount of weathering restite has likely been subducted into the mantle indicating a crust-to-mantle recycling rate of 1.47  ± 1.00  ×10<sup>13</sup>kg yr<sup>-1</sup>. Secondly we apply our model to the extensive dataset of sedimentary rocks compiled by the Sedimentary Geochemistry and Paleoenvironments Project from across the stratigraphic record so as to reconstruct the composition of the ancient crust. We find that the Archean upper continental crust is more mafic than present day, but stabilised into the present evolved composition by 2.5 - 2.0 Ga. </p> </div> </div> </div>

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 .