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

This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

The Mechanics of Earthquakes and Faulting

3.01 – Composition of the Continental Crust

The overall mechanics of fold-and-thrust belts and accretionary wedges along compressive plate boundaries is considered to be analogous to that of a wedge of soil or snow in front of a moving bulldozer. The material within the wedge deforms until a critical taper is attained, after which it slides stably, continuing to grow at constant taper as additional material is encountered at the toe. The critical taper is the shape for which the wedge is on the verge of failure under horizontal compression everywhere, including the basal decollement. A wedge of less than critical taper will not slide when pushed but will deform internally, steepening its surface slope until the critical taper is attained. Common silicate sediments and rocks in the upper 10-15 km of the crust have pressure-dependent brittle compressive strengths which can be approximately represented by the empirical Coulomb failure criterion, modified to account for the weakening effects of pore fluid pressure. A simple analytical theory that predicts the critical tapers of subaerial and submarine Coulomb wedges is developed and tested quantitatively in three ways: First, laboratory model experiments with dry sand match the theory. Second, the known surface slope, basal dip, and pore fluid pressures in the active fold-and-thrust belt of western Taiwan are used to determine the effective coefficient of internal friction within the wedge,/x = 1.03, consistent with Byerlee's empirical law of sliding friction,/at, = 0.85, on the base. This excess of internal strength over basal friction suggests that although the Taiwan wedge is highly deformed by imbricate thrusting, it is not so pervasively fractured that frictional sliding is always possible on surfaces of optimum orientation. Instead, the overall internal strength apparently is controlled by frictional sliding along suboptimally oriented planes and by the need to fracture some parts of the observed geometrically complex structure for continued deformation. Third, using the above values of/at, and/x, we predict Hubbert-Rubey fluid pressure ratios X = Xt, for a number of other active subaerial and submarine accretionary wedges based on their observed tapers, finding values everywhere in excess of hydrostatic. These predicted overpressures are reasonable in light of petroleum drilling experience in general and agree with nearby fragmentary well data in specific wedges where they are available. The pressure-dependent Coulomb wedge theory developed here is expected to break down if the decollement exhibits pressure-independent plastic behavior because of either temperature or rock type. The effects of this breakdown are observed in the abrupt decrease in taper where wedge thicknesses exceed about 15 km, which is the predicted depth of the brittle-plastic transition in quartz-rich rocks for typical geothermal gradients. We conclude that fold-and-thrust belts and accretionary wedges have the mechanics of bulldozer wedges in compression and that normal laboratory fracture and frictional strengths are appropriate to mountain-building processes in the upper crust, above the brittle-plastic transition.

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Mechanics of fold-and-thrust belts and accretionary wedges

Major and trace element composition of the depleted MORB mantle (DMM)

The solubility of hydrogen in Mg-Fe olivines as a function of temperature and iron concentration was investigated by hydrothermally annealing single crystals of synthetic forsterite and San Carlos olivine. Experiments were performed at temperatures between 1,273 and 1,573 K on samples with compositions between Fa0 and Fa16.9 under a confining pressure of 300 MPa in a gas-medium apparatus with oxygen fugacity, % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! <![CDATA[% MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSn0BKvguHDwzZbqefeKCPfgBGuLBPn % 2BKvginnfarmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy % 0Hgip5wzaebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq % -He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaea % aakeaacaWGMbWaaSbaaSqaaiaab+eadaWgaaadbaGaaeOmaaqabaaa % leqaaaaa!3C2D! $$ f_{{{\text{O}}_{{\text{2}}} }} $$
 , buffered by the Ni:NiO solid-state reaction and silica activity, % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSn0BKvguHDwzZbqefeKCPfgBGuLBPn % 2BKvginnfarmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy % 0Hgip5wzaebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq % -He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaea % aakeaacaWGHbWaaSbaaSqaaiaabofacaqGPbGaae4tamaaBaaameaa % caaIYaaabeaaaSqabaaaaa!3DF1! $$ a_{{{\text{SiO}}_{2} }} $$
 , buffered by the presence of enstatite. Hydroxyl concentrations were determined from infrared spectra obtained from polished thin sections in crack-free regions ≤50 µm in diameter. Hydroxyl solubility increases systematically with increasing temperature and with increasing iron content. Combined with published results on the dependence of hydroxyl solubility on water fugacity and pressure, the present results can be summarized by the relation % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSn0BKvguHDwzZbqefeKCPfgBGuLBPn % 2BKvginnfarmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy % 0Hgip5wzaebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq % -He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaea % aakeaacaWGdbWaaSbaaSqaaiaab+eacaqGibaabeaakiabg2da9iaa % dgeacaWGMbWaa0baaSqaaiaabIeadaWgaaadbaGaaeOmaaqabaWcca % qGpbaabaGaaGymaaaakiGacwgacaGG4bGaaiiCamaadmaabaGaeyOe % I0YaaeWaaeaacqGHuoarcaWGfbWaa0baaSqaamaacmaabaaacaGL7b % GaayzFaaaabaGaae4BaaaakiabgUcaRiaadcfacqGHuoarcaWGwbWa % a0baaSqaamaacmaabaaacaGL7bGaayzFaaaabaGaae4BaaaaaOGaay % jkaiaawMcaaiaac+cacaWGsbGaamivaaGaay5waiaaw2faaiGacwga % caGG4bGaaiiCamaabmaabaGaeqySdeMaamiwamaaBaaaleaacaqGgb % GaaeyyaaqabaGccaGGVaGaamOuaiaadsfaaiaawIcacaGLPaaaaaa!63CF! $$ C_{{{\text{OH}}}} = Af^{1}_{{{\text{H}}_{{\text{2}}} {\text{O}}}} \exp {\left[ { - {\left( {\Delta E^{{\text{o}}}_{{{\left\{ {} \right\}}}} + P\Delta V^{{\text{o}}}_{{{\left\{ {} \right\}}}} } \right)}/RT} \right]}\exp {\left( {\alpha X_{{{\text{Fa}}}} /RT} \right)} $$
 with A=90±10 H/106Si/MPa, α=97±4 kJ/mol, % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! <![CDATA[% MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSn0BKvguHDwzZbqefeKCPfgBGuLBPn % 2BKvginnfarmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy % 0Hgip5wzaebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq % -He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaea % aakeaacqGHuoarcaWGfbWaa0baaSqaamaacmaabaaacaGL7bGaayzF % aaaabaGaae4Baaaaaaa!3ED8! $$ \Delta E^{{\text{o}}}_{{{\left\{ {} \right\}}}} $$
 =50±2 kJ/mol, and % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! <![CDATA[% MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSn0BKvguHDwzZbqefeKCPfgBGuLBPn % 2BKvginnfarmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy % 0Hgip5wzaebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq % -He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaea % aakeaacqGHuoarcaWGwbWaa0baaSqaamaacmaabaaacaGL7bGaayzF % aaaabaGaae4Baaaaaaa!3EE9! $$ \Delta V^{{\text{o}}}_{{{\left\{ {} \right\}}}} $$
 =(10.0±0.1)×10−6 m3/mol. The subscript {} indicates that hydroxyl ions are incorporated primarily as defect pairs, probably of the type % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSn0BKvguHDwzZbqefeKCPfgBGuLBPn % 2BKvginnfarmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy % 0Hgip5wzaebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq % -He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaea % aakeaadaGadaqaamaabmaabaGaae4taiaabIeaaiaawIcacaGLPaaa % daqhaaWcbaGaae4taaqaaiabgkci3caakiabgkHiTiaabAfadaqhaa % WcbaGaaeytaiaabwgaaeaacaGGNaGaai4jaaaakiabgkHiTmaabmaa % baGaae4taiaabIeaaiaawIcacaGLPaaadaqhaaWcbaGaae4taaqaai % abgkci3caaaOGaay5Eaiaaw2haamaaCaaaleqabaGaaeiEaaaakiab % ggMi6oaabmaabaGaaGOmaiaabIeaaiaawIcacaGLPaaadaqhaaWcba % GaaeytaiaabwgaaeaacaqG4baaaOGaaiOlaaaa!568F! $$ {\left\{ {{\left( {{\text{OH}}} \right)}^{ \bullet }_{{\text{O}}} - {\text{V}}^{{''}}_{{{\text{Me}}}} - {\left( {{\text{OH}}} \right)}^{ \bullet }_{{\text{O}}} } \right\}}^{{\text{x}}} \equiv {\left( {2{\text{H}}} \right)}^{{\text{x}}}_{{{\text{Me}}}} . $$
 Under similar thermodynamic conditions, the water content in olivine in the martian mantle and in olivine from gabbros may be as much as 5 to 25 times larger than in the less iron-rich olivine dominant in Earth’s mantle.

Solubility of hydrogen in olivine: dependence on temperature and iron content

Specimens of naturally deformed peridotite were annealed and observed under the transmission electron microscope to study the kinetics of the recovery process in dry olivine. The lattice diffusion constant appropriate to climb, which we tentatively identify with 0−, was obtained at 1290° and 1450°C from the observed collapse of sessile dislocation loops. The data are represented by D = 3 × 104 exp [(—135 kcal/mole)/kT] cm2/sec. The climb of dislocations into subboundaries occurs by diffusion along dislocation cores with an activation energy of 140 ± 30 kcal/mole. The nature and distribution of dislocations in olivine grains from a Salt Lake crater lherzolite xenolith are incompatible with a Nabarro-Herring or subgrain creep model but consistent with a model based on dislocation climb.

Laboratory study of dislocation climb and diffusion in olivine

Recent experiments demonstrate that solution-precipitation (pressure-solution) processes in the liquid basalt-olivine system are exceedingly rapid. The effect of liquid basalt on the diffusional (Newtonian) creep rate of polycrystalline olivine is significant; fine-grained, chemically and texturally equilibrated, partially molten assemblages of olivine plus basalt deform at rates which are a factor of 2–5 faster than polycrystalline olivine specimens without the liquid basalt second phase. Measured values of the activation energy for creep suggest that while the kinetics of deformation in this partially molten system are affected by short-circuit diffusion through melt-filled triple junctions, the deformation process is rate-limited by matter transport through melt-free grain boundaries. A simple geometrical model, based on a dynamic equilibrium partial-melt morphology which consists of melt-free olivine grain boundaries coupled with an interconnected melt network along triple junction channels, adequately describes our deformation results. The extent to which the liquid phase penetrates from the triple junctions into the grain boundaries of texturally equilibrated partial melts determines the contribution of solution-precipitation mechanisms to the creep rate; the critical physical parameters, therefore, are those which affect the solid-liquid and solid-solid interfacial energies, such as various composition-related chemical activities in the liquid phase and the composition and spatial distribution of mineral phases in the crystal residuum.

Rheology and structure of olivine‐basalt partial melts

WATER plays an important role in geodynamic processes in the Earth's upper mantle: for example, hydrogen (as water) affects the amount and composition of magma generated by partial melting1 and a trace amount of hydrogen ( ∼0.001 wt% water) can markedly weaken the dominant upper-mantle mineral, olivine2,3. Migration of hydrogen ions may be responsible for the anomalously high electrical conductivity of the asthenosphere4. The quantitative importance of hydrogen in mantle processes must depend on how much water or hydrogen can be stored in nominally anhydrous olivine, and on where hydrogen resides in the olivine lattice. Here we report the results of hydrothermal experiments on olivine single crystals, which show that at 1,573 K and 50–300 MPa, olivine can accommodate as much as 0.0034 wt % water. Hydrogen solubility depends on hydrogen fugacity and oxygen fugacity to the first and the one-half powers, respectively, indicating that hydrogen ions are associated with either oxygen interstitials or magnesium vacancies. Extrapolation of our data to a depth of ∼ 100 km under oceanic areas yields a substantial hydrogen solubility (0.03 wt % water), demonstrating that olivine may indeed be a primary sink for hydrogen in the upper mantle.

Substantial hydrogen solubility in olivine and implications for water storage in the mantle

To investigate the solubility and the sites of incorporation of hydrogen in olivine as a function of point defect concentration, two-stage high-temperature annealing experiments have been carried out. The first annealing stage (the dry preannealing stage) was conducted at a total pressure of 0.1 MPa, a temperature of 1300° C and various oxygen fugacities in the range 10−11–10−4 MPa for times > 12 h. In these heat treatments, the samples were buffered against either orthopyroxene or magnesiowustite, or they remained unbuffered. The second annealing stage (the hydrothermal annealing stage) was performed at 300 MPa and 900–1050 ° C under a hydrogen fugacity of ∼ 158 MPa for 1–5 h. Infrared spectra from the annealed samples revealed two distinct groups of bands. Group I bands occurred at wavenumbers in the range 3450–3650 cm−1, while Group II bands occurred in the range 3200–3450 cm−1. The hydrogen solubility associated with Group I bands is proportional to f
O
2 to the 1/6 power for samples preannealed in contact with orthopyroxene, to the 1/3 power for samples preannealed in contact with magnesiowustite, and to the 1/13 power for samples preannealed in the absence of a solid-state buffer. The hydrogen concentration for Group II bands varies with f

 o
 2
 to the 1/3 power for opxbuffered samples, to the 1/2 power for mw-buffered samples, and to the 1/3 power for unbuffered samples. The dependence of hydrogen solubility on oxygen fugacity and orthopyroxene activity suggests that hydrogen is incorporated into the olivine structure via association with point defects. The presence of two distinct groups of absorption bands indicates that hydrogen is associated with two distinct lattice defects. The following point defect model for the mechanism of incorporation of hydrogen in olivine is consistent with these results: Hydrogen ions responsible for the Group I bands are associated with doubly charged oxygen interstitials, while hydrogen ions responsible for the Group II bands are associated with singly charged oxygen interstitials. Furthermore, the infrared bands observed in naturally derived olivines are present in spectra from our hydrothermally annealed crystals. Thus, the mechanisms of incorporation of hydrogen in olivine under geological conditions are the same as those operative under laboratory conditions. The maximum solubility reached in these experiments was ∼ 360H/106Si, which corresponds to ∼ 0.002 wt% of H2O. This value is a lower bound for the solubility of hydrogen in olivine under upper mantle conditions.

David L. Kohlstedt

Papers

Solubility of hydrogen in olivine: dependence on temperature and iron content

Laboratory study of dislocation climb and diffusion in olivine

Rheology and structure of olivine‐basalt partial melts

Substantial hydrogen solubility in olivine and implications for water storage in the mantle

Effects of chemical environment on the solubility and incorporation mechanism for hydrogen in olivine