Chemical Composition Of Continental Crust Research Articles

Phosphorus deficit in continental crust induced by recycling of apatite-bearing cumulates

Abstract Trace-element anomalies in the continental crust (e.g., Nb-Ta-Pb) are useful to decipher the formation and evolution of continents. We found that the trace-element patterns of upper, middle, and lower crust are all marked by strongly negative phosphorus anomalies, which were not likely to have been generated directly by mantle melting. In order to address this “crust composition paradox,” we compiled major- and trace-element data from global arc magmas, igneous minerals, and deep arc cumulates and found that the phosphorus deficit in continental crust is coupled with an elevated Th/La ratio. This feature can be best explained by the fractional crystallization of apatite during arc magma evolution rather than the influence of a subducted slab. Accumulation of apatite-bearing mafic cumulates in deep arcs followed by foundering into the upper mantle causes the mismatch in the phosphorus systematics between the mantle and the continental crust. This process plays a critical role in shaping the chemical composition of continental crust.

Composition and Evolution of Continental Crust at Orogenic Belts: Constraints From a 3‐D Crustal Model of Southeast China

AbstractThe chemical composition of continental crust is fundamental and crucial to understanding its origin and evolution. Orogenic belts make up about 30% of the global continental crust, yet their chemical compositions are still poorly constrained. In this study, we report an overall estimate for the continental crust composition of a collisional orogenic belt terrane: the Southeast China. The estimate is obtained through a 3‐D crustal model combining a lithology proportion model and probability density function models for element concentrations of various lithologies. Based on our model, the Southeast China crust is amongst one of the most evolved continental crust composition estimates (71.0% SiO2 content for the upper crust and 64.7% SiO2 for the bulk crust). A strong correlation between SiO2 and the chemical index of alteration (CIA) of different composition models for upper continental crust (UCC) indicates that chemical weathering is a controlling factor for the development of a felsic UCC. In contrast, weak correlations of SiO2 with Ninorm (15.9*Ni/Al2O3) and CIA suggest that igneous differentiation and chemical weathering act together to modify the composition of the bulk continental crust. Orogenic belt terranes like the Southeast China can develop a highly evolved crustal composition, especially for the upper crust, due to extensive intracrustal reworking and differentiation, enhanced chemical weathering, and potential subsequent delamination and removal of thickened crust root. Therefore, an overall estimation of the average composition of continental crust should take into account the more evolved composition of orogenic belts.

Heat Flow and the Chemical Composition of Continental Crust

Recent models for the chemical composition of the continental crust rely on various geophysical, geological, and petrological constraints and display a wide range in composition, with $$SiO_{2}$$ varying between 57 and 64 wt % and $$K_{2}O$$ varying between 1.1 and 2.4 wt %. Models that predict relatively high levels of the most incompatible elements imply extreme depletion (>70%) of these elements in the "depleted mantle." Crustal heat flow constrains the abundances of heat-producing elements (HPE: K, Th, U) within the continental crust and thus provides a relatively unambiguous and important test for these various models. The crustal radiogenic component of continental heat flow must lie within the range of $$18-48 mW/m^{2}$$ and almost certainly lies within the range of $$21-34 mW/m^{2}$$. The latter constraints limit the abundances of HPE within the crust (assuming K/U = 10,000, Th/U = 3.8, and a crust 41 km thick) to $$K_{2}O = 0.96-1.57\%$$; Th = 3.0-4.94 ppm, and U = 0.80-1.30 ppm. Models of crustal composition based largely on seismological or geological mapping data, and that predict relatively siliceous and incompatible element enriched compositions, exceed the upper limits of these constraints.