Abstract

The mass, age, and chemical composition of the continental crust are fundamental data for understanding Earth differentiation. The inaccessibility of most of the volume of the crust requires that inferences be made about geochemistry using seismic and heat flow data, with additional constraints provided by scarce lower crustal samples (Rudnick and Fountain, Rev. Geophys., 1995; Rudnick and Gao, Treatise on Geochem., 2003). The global crustal seismic database CRUST2.0 (Bassin, et al., EOS, 2000; Mooney, et al., JGR, 1998; hereafter C2) provides a useful template with which the size and composition of the continents can be assessed, and may be a useful vehicle to organize and analyze diverse geochemical data. We have used C2 to evaluate the modern mass and composition of the continental crust and their uncertainties, and explored our results in the context of global mass balances, such as continents versus depleted mantle. The major source of uncertainty comes from the definition of "continent." The ultimate constraint is the total mass of Earth's crust (oceanic + continental), which, from C2, is 2.77 (in units of 10²² kg). Using crustal thickness as a definition of continent, the mass of continental crust (CC) is 2.195 if the minimum thickness is 12-18km, 2.085 for 22.5km, 2.002 for 25km, and 1.860 for 30km. These numbers include all sediment as continental crust. Using C2 definitions to distinguish oceanic and continental crust (and including oceanic plateaus which contain some continental crust), we calculate the CC mass as 2.171. To estimate chemical composition, we use the C2 reservoir masses. For minimum thickness of 22.5km, C2 yields the proportions 0.016 oceanic sediment, 0.038 continental sediment, 0.321 upper crust, 0.326 middle crust, 0.299 lower crust. Upper, middle, and lower crust are assigned compositions from Rudnick and Gao (2003), continental sediments are assigned upper crust composition, and oceanic sediments are assigned GLOSS composition (Plank and Langmuir, Chem. Geology, 1998). Because the C2 model gives a larger proportion of upper versus lower crust, the resulting CC composition is enriched in P₂O₅, K₂O, Rb, Pb, Th, U, Hf and LREE, and depleted in MgO and CaO, relative to Rudnick and Gao (2003). The mass and chemical composition of CC figure prominently in calculations of the mass of depleted mantle (DM) that is required to balance the crustal enrichments in incompatible elements. However, because thin crust also tends to be young crust, and is less enriched in incompatible elements, uncertainties in the mass of CC have little effect on the calculated size of DM, which must be considerably smaller than the whole mantle. Using different approaches to estimating crustal composition, and adding age provinces as well as other types of geochemical and geochronological data, could make CRUST2.0 an important geochemical resource.