Self-gravity, self-consistency, and self-organization in geodynamics and geochemistry

The results of seismology and geochemistry for mantle structure are widely believed to be discordant, the former favoring whole-mantle convection and the latter favoring layered convection with a boundary near 650 km. However, a different view arises from recognizing effects usually ignored in the construction of these models, including physical plausibility and dimensionality. Self-compression and expansion affect material properties that are important in all aspects of mantle geochemistry and dynamics, including the interpretation of tomographic images. Pressure compresses a solid and changes physical properties that depend on volume and does so in a highly nonlinear way. Intrinsic, anelastic, compositional, and crystal structure effects control seismic velocities; temperature is not the only parameter, even though tomographic images are often treated as temperature maps. Shear velocity is not a good proxy for density, temperature, and composition or for other elastic constants. Scaling concepts are important in mantle dynamics, equations of state, and wherever it is necessary to extend laboratory experiments to the parameter range of the Earth's mantle. Simple volume-scaling relations that permit extrapolation of laboratory experiments, in a thermodynamically self-consistent way, to deep mantle conditions include the quasiharmonic approximation but not the Boussinesq formalisms. Whereas slabs, plates, and the upper thermal boundary layer of the mantle have characteristic thicknesses of hundreds of kilometers and lifetimes on the order of 100 million years, volume-scaling predicts values an order of magnitude higher for deep-mantle thermal boundary layers. This implies that deep-mantle features are sluggish and ancient. Irreversible chemical stratification is consistent with these results; plausible temperature variations in the deep mantle cause density variations that are smaller than the probable density contrasts across chemical interfaces created by accretional differentiation and magmatic processes. Deep-mantle features may be convectively isolated from upper-mantle processes. Plate tectonics and surface geochemical cycles appear to be entirely restricted to the upper ~1,000 km. The 650-km discontinuity is mainly an isochemical phase change but major-element chemical boundaries may occur at other depths. Recycling laminates the upper mantle and also makes it statistically heterogeneous, in agreement with high-frequency scattering studies. In contrast to standard geochemical models and recent modifications, the deeper layers need not be accessible to surface volcanoes. There is no conflict between geophysical and geochemical data, but a physical basis for standard geochemical and geodynamic mantle models, including the two-layer and whole-mantle versions, and qualitative tomographic interpretations has been lacking

New Theory of the Earth

New Theory of the Earth is an interdisciplinary advanced textbook on all aspects of the interior of the Earth and its origin, composition, and evolution: geophysics, geochemistry, dynamics, convection, mineralogy, volcanism, energetics and thermal history. This is the only book on the whole landscape of deep Earth processes that ties together all the strands of the subdisciplines. This book is a complete update of Anderson’s Theory of the Earth (1989). It includes dozens of new figures and tables. A novel referencing system using Googlets is introduced that allows immediate access to supplementary material via the internet. There are new sections on tomography, self-organization, and new approaches to plate tectonics. The paradigm/paradox approach to developing new theories is developed, and controversies and contradictions have been brought more center-stage. As with the Theory of the Earth, this new edition will prove to be a stimulating textbook for advanced courses in geophysics, geochemistry, and planetary science, and a supplementary textbook on a wide range of other advanced Earth science courses. It will also be an essential reference and resource for all researchers in the solid Earth sciences

The Statistics of Helium Isotopes Along the Global Spreading Ridge System and the Central Limit Theorem

An unbiased estimate of the arithmetic mean of the ^3He/^4He ratio (R) of basalts from the global spreading ridge system is 9.14 ± 3.59 Ra (n=503), where Ra is the atmospheric ratio and n is the number of data points. The ± two standard deviation (2σ) range covers much of the oceanic island and continental flood basalt data that have been attributed to ‘plume’ or ‘lower mantle’ components. The highest R values along the ridge system are associated with new ridges, backarc basins and near‐ridge seamounts. Low values are associated with long‐lived or abandoned ridges. The median, a more robust measure of the average, is 8.51 Ra

Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies

Pressure-induced solid-solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle. Lateral variations in temperature cause a similar effect; abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion. Temperature-induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure-induced phase changes. In cold slabs the equilibrium mineral assemblage contains high-density, high-velocity phases which are not stable in hotter mantle. In particular, the ilmenite form of MgSiO_3 and the γ-spinel form of Mg_2SiO_4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling. Recent arguments for slab penetration into the lower-mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature-induced phase changes. When these effects are taken into account, the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection. The seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection. Mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions, and this is due to the presence of a melt phase and lower-velocity, high-temperature phase assemblages. Deep, > 200 km, long-lived continental roots, differing in chemistry from “normal” mantle, are not required when isobaric phase changes are taken into account. High-velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high-temperature phase assemblages. The whole mantle convection, thick continental root, and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis. Phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition. The lack of correlation of the geoid with ridges, shields, heat flow, and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle, consistent with layered convection

Chemical Composition of the Mantle

The composition of primitive mantle (54 elements) is estimated by a mass balance approach that does not make a priori assignments of basalt:peridotite ratios or LIL contents of these components. It is also not necessary to assume that such ratios as Rb/Sr and K/U are the same as in the crust. Primitive upper mantle is treated as a four-component system: crust, peridotite, LIL-depleted basalt (MORB), and an LIL-enriched component. These are combined to give chondritic ratios of the oxyphile refractory trace elements. The composition of the whole mantle is estimated by requiring chondritic ratios of the major elements as well. In this way one can estimate the volatile and siderophile content of the mantle. The primitive mantle has K = 152 ppm, U = 0.020 ppm, Th = 0.078 ppm, K/U = 7724, and Rb/Sr = 0.025. The ratios are significantly less than previous estimates. The inferred steady state heat flow, 0.9 μ cal/cm^2s, implies a substantial contribution of cooling to the observed heat flow. The crust and upper mantle may contain most of the terrestrial inventory of the incompatible elements, including K, U, and Th. There is no evidence that the chalcophiles are strongly partitioned into the core

Evolution of Earth Structure and Future Directions of 3D Modeling

It is no longer adequate to treat the Earth as a nearly spherically symmetric body with simple receiver, source and attenuation corrections tacked on. The aspherical velocity structure is now being determined by surface wave and body wave tomographic techniques and it has been found that heterogeneities are present at all levels. In the upper mantle the lateral variations in velocity are as large as the variations across the radial discontinuities. There is good correlation of velocity with surface tectonic features in the upper 250 km but the correlation rapidly dimishes below this depth. The focusing and defocusing effect of these lateral variations can cause large amplitude anomalies and these effects can be more important than attenuation. Velocity variations in the mantle can be caused by temperature, mineralogy and anisotropy, or crystal orientation. The largest variations are caused by anisotropy and relaxation phenomena such as partial melting and dislocation relaxation. There is increasing evidence for anisotropy in the upper mantle and this must be taken into account in Earth structure modeling. Both azimuthal and polarization effects are important. Layering or fabric having a scale length less than a wavelength will show the statistical properties of the small scale structure. Global maps of heterogeneity and anisotropy show that if anisotropy is ignored the data will be mapped into a false heterogeneity. Azimuthal anisotropy compounds the off-great-circle problem. The absorption band concept predicts that Q should be higher at short periods than at long periods and that there should be large lateral and radial variations in Q. The t* controversy is probably related to shifts in the absorption band. If velocity is anisotropic then Q should be as well. Evidence is starting to suggest that there is a Love wave, Rayleigh wave discrepancy in Q, suggestive of Q anisotropy

Petrology: The Study of Igneous, Sedimentary, and Metamorphic Rocks

No abstract

The Inside of Earth: Deep-Earth Science from the Top Down

Earth is really several planets. Which planet you see depends on where you view it from. Looking at it from outside, from space, stripped of clouds, you can see that Earth has two quite different hemispheres—a continent hemisphere and an ocean hemisphere. The latter, the Pacific hemisphere, is underlain almost entirely by one gigantic tectonic plate—a continuous chunk of Earth’s crust— which is diving under what is called the ring of fire because of the volcanoes that line the plate boundary along Oregon, Washington State, British Columbia, the Aleutians, the Kuriles, Japan, the Marianas, Tonga-Fiji, South America, and Central America. There are also volcanoes in other places: along other plate boundaries on the sea-floor and in island chains throughout the Pacific. One of the unanswered questions in geology is: why are there volcanoes in some places? In my own work, I turn the question around and ask: why aren’t there volcanoes everywhere? For seismology tells us that there is a semimolten layer underneath the plate almost everywhere. Something is keeping it down

Structure and Composition of the Mantle

The last four years have been a period of increased emphasis on the problems of discontinuities, lateral variations, and shear velocities in the mantle. The presence of discontinuities near 400 km and 600 km has been verified by travel time, apparent velocity, and reflection and amplitude studies; it has been shown by refraction and reflection amplitudes that these discontinuities are extremely sharp, 4 km or less. Other discontinuities, or abrupt changes in velocity gradient, have been found in the upper and lower mantle. It now appears that there are relatively few large radial stretches of the mantle that are truly homogeneous. A summary of the locations of discontinuities in the mantle is given in Johnson [1967] and Whitcomb and Anderson [1970]. A second-order discontinuity has been found near 500 km by Helmberger and Wiggins [1971]

Planet Earth

A planets surface provides geologists with clues as to what is happening inside. But many of these clues are ambiguous because so many other processes (impacts and erosion, for example) contribute to surface characteristics. Most of the surface of the Earth is less than 100 million years old, and even its oldest rocks are less than 4 billion years old, so the record of the origin of our planet has been erased many times. Part of this erasure is due to erosion by wind and water, and part is due to the continual recycling of material back into the interior and the repaving of the ocean basins by seafloor spreading