The composition of the Earth

This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

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Composition of the Continental Crust

The composition of the continental crust

The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology

Zircon is the main mineral in the majority of igneous and metamorphic rocks with Zr as an essential structural constituent. It is a host for significant fractions of the whole-rock abundance of U, Th, Hf, and the REE (Sawka 1988, Bea 1996, O’Hara et al. 2001). These elements are important geochemically as process indicators or parent isotopes for age determination. The importance of zircon in crustal evolution studies is underscored by its predominant use in U-Th-Pb geochronology and investigations of the temporal evolution of both the crust and lithospheric mantle. In the past decade an increasing interest in the composition of zircon, trace-elements in particular, has been motivated by the effort to better constrain in situ microprobe-acquired isotopic ages. Electron-beam compositional imaging and isotope-ratio measurement by in situ beam techniques—and the micrometer-scale spatial resolution that is possible—has revealed in many cases that single zircon crystals contain a record of multiple geologic events. Such events can either be zircon-consuming, alteration, or zircon-forming and may be separated in time by millions or billions of years. In many cases, calculated zircon isotopic ages do not coincide with ages of geologic events determined from other minerals or from whole-rock analysis. To interpret the geologic validity and significance of multiple ages, and ages unsupported by independent analysis of other isotopic systems, has been the impetus for most past investigations of zircon composition. Some recent compositional investigations of zircon have not been directly related to geochronology, but to the ability of zircon to influence or record petrogenetic processes in igneous and metamorphic systems.

Sedimentary rocks may also contain a significant fraction of zircon. Although authigenic zircon has been reported (Saxena 1966, Baruah et al. 1995, Hower et al. 1999), it appears to be very rare and may in fact be related to …

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The Composition of Zircon and Igneous and Metamorphic Petrogenesis

Geophysical, petrological, and geochemical data provide important clues about the composition of the deep continental crust. On the basis of seismic refraction data, we divide the crust into type sections associated with different tectonic provinces. Each shows a three-layer crust consisting of upper, middle, and lower crust, in which P wave velocities increase progressively with depth. There is large variation in average P wave velocity of the lower crust between different type sections, but in general, lower crustal velocities are high (>6.9 km s−1) and average middle crustal velocities range between 6.3 and 6.7 km s−1. Heat-producing elements decrease with depth in the crust owing to their depletion in felsic rocks caused by granulite facies metamorphism and an increase in the proportion of mafic rocks with depth. Studies of crustal cross sections show that in Archean regions, 50–85% of the heat flowing from the surface of the Earth is generated within the crust. Granulite terrains that experienced isobaric cooling are representative of middle or lower crust and have higher proportions of mafic rocks than do granulite terrains that experienced isothermal decompression. The latter are probably not representative of the deep crust but are merely upper crustal rocks that have been through an orogenic cycle. Granulite xenoliths provide some of the deepest samples of the continental crust and are composed largely of mafic rock types. Ultrasonic velocity measurements for a wide variety of deep crustal rocks provide a link between crustal velocity and lithology. Meta-igneous felsic, intermediate and mafic granulite, and amphibolite facies rocks are distinguishable on the basis of P and S wave velocities, but metamorphosed shales (metapelites) have velocities that overlap the complete velocity range displayed by the meta-igneous lithologies. The high heat production of metapelites, coupled with their generally limited volumetric extent in granulite terrains and xenoliths, suggests they constitute only a small proportion of the lower crust. Using average P wave velocities derived from the crustal type sections, the estimated areal extent of each type of crust, and the average compositions of different types of granulites, we estimate the average lower and middle crust composition. The lower crust is composed of rocks in the granulite facies and is lithologically heterogeneous. Its average composition is mafic, approaching that of a primitive mantle-derived basalt, but it may range to intermediate bulk compositions in some regions. The middle crust is composed of rocks in the amphibolite facies and is intermediate in bulk composition, containing significant K, Th, and U contents. Average continental crust is intermediate in composition and contains a significant proportion of the bulk silicate Earth's incompatible trace element budget (35–55% of Rb, Ba, K, Pb, Th, and U).

Nature and composition of the continental crust: A lower crustal perspective

3.01 – Composition of the Continental Crust

Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

Recycling lower continental crust in the North China craton

The continental crust has an andesitic bulk composition, which cannot have been produced by the basaltic magmatism that dominates sites of present-day crustal growth—at both convergent margins and within plates. These observations suggest that there may have been a different mode of continental crust generation in Archaean times, and may point to delamination of the lower crust as an important recycling process.

Roberta L. Rudnick

Papers

Composition of the Continental Crust

Nature and composition of the continental crust: A lower crustal perspective

3.01 – Composition of the Continental Crust

Recycling lower continental crust in the North China craton

Making continental crust