There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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.

/pdf/composition-of-the-continental-crust-2hxz49vsod.pdf

Composition of the Continental Crust

The composition of the continental crust

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

How mafic is the lower continental crust

Development of damage and permeability in deforming rock salt

https://hal.univ-reunion.fr/hal-01389712/document

Seismic anisotropy and shear-wave splitting in lower-crustal and upper-mantle rocks from the Ivrea Zone—experimental and calculated data

Velocities and Q values of P and S waves as functions of pressure and temperature (at 100 and 600 MPa) are presented for a serpentinite and an amphibolite. Both rocks exhibit a strong lattice preferred orientation (LPO) of the major mineral phases antigorite and hornblende, respectively. Velocities and Q values increase with pressure; the rate of increase is different in the three orthogonal directions (normal and parallel to foliation and lineation) and closely related to progressive closure of microcracks. Increasing temperature decreases velocities and Q values only slightly as long as thermal cracking is prevented by the applied confining pressure. Substantial anisotropy of velocities and Q in P and S waves is observed in both rocks but is found to be different in origin. Anisotropy of P and S wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the LPO of major minerals. Increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate. The residual anisotropy of P and S wave velocities (shear wave splitting) at high confining pressure is mainly a result of preferred mineral orientation. By contrast, anisotropy of Q is very low at low confining pressure and markedly enhanced as pressure is increased. At high confining pressure, substantial anisotropy of Q in P waves is apparent but reversed from that of P wave velocities: Qp is highest in the direction normal to the foliation plane whereas Vp (and VS) is lowest in this direction. The generation of a pronounced anisotropy of Qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation. The increase of Q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain. The reverse is true for the X and Y directions (serpentinite) and X direction (amphibolite) parallel to the foliation plane.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB03392

Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite

The directional dependence of P and S wave velocities have been measured at pressures (up to 600 MPa) and temperatures (up to 700°C) in rocks from the Santa Rosa Mylonite Zone (southern California). During tectonism, these were progressively deformed from granodiorite protolith to mylonite and ultimately phyllonite. The mineralogical and chemical composition of protolith and mylonite is nearly identical. Thus these rocks provide excellent material for documenting the effect of microstructural and textural changes on rock anisotropy. Velocity anisotropy increases significantly with the degree of deformation, whereas average velocities and densities do not change. At low pressure (50 MPa) the velocity anisotropy ranges from 1.7% in granodiorite up to 19% in phyllonite and is due to both oriented microfractures and crystallographic preferred orientation. At high pressure (600 MPa), the residual anisotropy up to 12% is mainly due to preferred mineral orientation, in particular of biotite. Significant shear wave splitting is measured parallel to the foliation plane and shows a good correlation with the biotite texture. These observations confirm that oriented microcracks and preferred orientation of minerals should be taken into account in the interpretation of seismic reflection and refraction data in terranes with deformed rocks.

Hartmut Kern

Papers

How mafic is the lower continental crust

Development of damage and permeability in deforming rock salt

Seismic anisotropy and shear-wave splitting in lower-crustal and upper-mantle rocks from the Ivrea Zone—experimental and calculated data

Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite

Fabric-related velocity anisotropy and shear wave splitting in rocks from the Santa Rosa Mylonite Zone, California