UC Santa Barbara

The continental crust is an important geochemical reservoir, and its average composition provides critical insights into the processes of its formation, crust-mantle recycling, and interactions with other reservoirs such as the atmosphere, oceans, and mantle. However, due to its inherent heterogeneity, the existing estimates are subject to significant uncertainties, whether for major or trace elements. By systematically examining element concentrations and ratios in crustal igneous, sedimentary, and select metamorphic rocks, I reevaluated the concentrations of major elements and halogens in the continental crust, as well as their behavior during geological processes related to crust formation, differentiation, and evolution.Chapter I presents a study focusing on the major elements of the upper continental crust (UCC). Based on a suite of newly analyzed loess samples, as well as compiled data from fine-grained siliciclastic sedimentary rocks (loess, glacial diamictites, and shales) and a large dataset of continental igneous rocks, this study explored a new approach to estimate the major element composition of the UCC. Compared to previous methods, this new approach avoids biases such as oversampling of mafic rocks when averaging igneous rocks and the incorporation of weathering signatures in estimates based on sedimentary rocks and/or large-scale sampling. The new results confirm an evolved, granodiorite composition for the UCC and also suggest that continental weathering exerts a significant control on seawater composition for many elements.Chapter II focuses on the analysis of halogen (F, Cl, Br, and I) concentrations in a suite of ancient glacial diamictite composites. With systematically low I concentrations in most composites, these samples are inferred to have predominantly sampled the crystalline bedrocks. Based on linear correlations between halogens and other elements, a new estimate for the present-day crystalline UCC is derived. The new results are used to quantify magmatic degassing during continental formation and the loss of Cl from continents through chemical weathering. Further calculations suggest that weathering alone is insufficient to account for the Cl inventory in the oceans, which may instead be primarily derived from mantle outgassing and/or late volatile accretion.Chapter III examines halogen (F, Cl, Br, and I) concentrations in a set of worldwide loess samples. With systematically higher concentrations than those of glacial diamictite composites, the loess samples reflect a systematic enrichment of halogens on the continental surface, with estimated enriched following the trend: F ≈ Cl < Br << I. Additionally, the halogen ratios (Br/Cl, I/Cl, and Br/I) of loess samples are similar to those of organic-rich soils/sediments but distinct from those of glacial diamictites, suggesting that another process (beyond biological influence) may be responsible for fractionating halogens in the glacial diamictites. Using a mixing model, it is estimated that the materials composing loess are predominantly (>80–90%) sourced from crystalline bedrocks.Chapter IV investigates the behavior of halogens (F, Cl, Br, I, and Cl isotopes) during high-grade metamorphism and partial melting in the deep continental crust, based on a well characterized suite of samples from the Ivrea Zone, Northwestern Italy, which experienced amphibolite- to granulite-facies metamorphism. The results show that F, I, and possibly Br are lost during prograde metamorphism. By contrast, Cl appears to be influenced by the input of externally derived Cl-rich fluids during granulite-facies metamorphism, possibly introduced by underplating magmas. These halogens, present in the lower continental crust, may have played a crucial role in the formation of granitic magmas, thereby contributing to the differentiation of the continental crust.