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Über dieses Buch

Igneous petrology was to some extent essentially a descriptive sci­ ence until about 1960. The results were mainly obtained from field work, major element analyses, and microscopical studies. During the 1960's two simultaneous developments took place, plate tectonics became generally accepted, and the generation of magmas could now be related to the geodynamic features like convection cells and subduction zones. The other new feature was the development of new analytical apparatus which allowed high accuracy analyses of trace elements and isotopes. In addition it became possible to do ex­ perimental studies at pressures up to 100 kbar. During the 1970's a large amount of analytical data was obtained and it became evident that the igneous processes that control the compositions of magmas are not that simple to determine. The composition of a magma is controlled by the compositions of its source, the degree of partial melting, and the degree of fractionation. In order to understand the significance of these various processes the relationship between the physical processes and their geochemical consequences should be known. Presently there are several theories that attempt to explain the origin of the various magma types, and these theories can only be evaluated by turning the different ideas into quantitative models. We will so to speak have to do some book keeping for the various theories in order to see which ones are valid. the present book is intended as an introduction to the more fun­ damental aspects of quantitative igneous petrology.

Inhaltsverzeichnis

Frontmatter

I. Monary Systems

Abstract
By comparing the compositions of minerals and rocks with the compositions obtained from experimental work it becomes possible to estimate the PT conditions that control the igneous processes. The estimation of the phase relations of magmas and the magmatic sources, like the mantle and the subducted oceanic crust, therefore, form the basis for an understanding of the generation of igneous rocks. After the PT conditions have been estimated the next step is an analysis of the dynamic processes that control the temperature and pressure conditions. Such an analysis will involve the calculation of the temperature distribution in mantle plumes and ascending convection currents, an estimation of the velocity of dyke propagation, and calculations of the cooling rates of intrusions.
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II. Binary Systems

Abstract
The binary system is the fundamental unit within the phase theory for multicomponent systems; the higher ordered systems are built up from binary joins, and the binary phase diagrams display the basic phase relations in a convenient manner as both compositions and temperatures are read directly from the diagrams.
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III. Ternary Systems

Abstract
There is a variety of ternary phase diagrams. However, the analysis of the crystallization behavior by equilibrium crystallization and fractional crystallization as well as by partial melting can be done with the aid of a few basic principles, which will be considered here. Most helpful in this connection are the crystallization vectors dealt with in this chapter.
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IV. Pseudobinary Systems

Abstract
The pseudobinary systems differ from the true binary ones by the presence of one or more phases with compositions lying outside those of the binary join, i.e., the compositions of all the phases cannot be expressed as a linear combination of the two end member components. In the system albite-orthoclase, sanidine melts incongruently to leucite plus liquid, but the leucite composition cannot be expressed from those of the two feldspars. Similarly spinel occurs in the binary join anorthite-forsterite, but has a composition outside the join. The pseudobinary systems occur frequently in silicate systems and interpretation of these systems is required for an evaluation of important phase relationships. The pseudobinary systems also bear a relationship to P-T diagrams of rock compositions, since these P-T diagrams may be regarded as distorted pseudobinary systems.
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V. P-T Diagrams

Abstract
The pressure-temperature diagrams of rock compositions display the stability fields for the different phase assemblages at various pressures and temperatures. These diagrams afford information about the possible natural phase relations for rocks and magmas, and indicate the possible combinations of phenocrysts that might constitute a fractionate, and might as well show the P-T regions for the generation of a particular composition by partial melting. The disadvantage of the P-T diagrams for natural compositions is that the particular phase relations estimated, is only valid for the composition investigated. If an incongruent reaction occurs for a given composition, it is not granted that the reaction also occurs for a similar but slightly different composition. The influence of a small difference in composition on the phase relations is especially evident from systems with congruent compounds and thermal divides. The synthetic systems with pure end member compositions will clarify the fundamental phase relations, and indicate the compositional ranges for which a particular phase relation is valid. On the other hand, the synthetic systems do not define accurate P-T values for the natural magmas. Thus, the two types of systems supplement each other, the fundamental phase relations are evident from synthetic systems, while accurate estimates of pressure and temperatures may be obtained from experiments on natural compositions at different P-T values.
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VI. Schreinemakers’ Phase Theory

Abstract
The classic thermodynamic theory was worked out by Gibbs (1876), who also considered various phase relations. The thermodynamic theory of Gibbs was applied in detailed studies of phase relations by Roozeboom (1893, 1901), and one of Roozeboom’s co-workers, F. A. H. Schreinemakers evaluated the methods and theorems required for an estimation of the systematic relationships of univariant equilibria and P-T diagrams. The works of Schreinemakers (1912–1925)are highly original and his methods very powerful, as only a limited amount of information lead to an estimate of fundamental phase relations. Schreinemakers largely applied geometrical methods, while the analytical treatment of heterogeneous phase equilibria was worked out by Morey and Williamson (1918) and Morey (1936). The petrological application of Schreinemakers’ theorems have been considered by Niggli (1937), and their experimental applications have been demonstrated in a series of fundamental phase studies by Wyllie (1966, 1976, 1977).
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VII. Gas-Bearing Systems

Abstract
The addition of a gas component to silicate melts has pronounced effects for the phase relations. The melting temperature decreases, and the phase boundaries are shifted away from those of the dry systems. The composition of the melts formed in the presence of a gas phase is, therefore, different from those of the dry system. The addition of water results in melts with a higher SiO2 content, and by the addition of carbon dioxide the melts become less silica saturated. The effect of water and carbon dioxide for the melting temperature of diopside is shown in Fig. VII-1 for vapor-saturated melts. The lowering of the melting temperature is most pronounced for water, but the addition of CO2 also has a significant effect. The variation in the composition of melts by the addition of water and carbon dioxide is shown in Fig. VII-2 for the ternary system forsterite-diopside-silica.
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VIII. Oxygen Fugacity

Abstract
The FeO/Fe2O3 ratio of magmas will depend on the oxygen fugacity, and the oxygen fugacity will itself be dependent on the composition of the magma and its gas content. The stability of the iron-bearing minerals depended on the FeO/Fe2O3 ratio, and the oxygen fugacity will, therefore, exert control on the fractionation trends of magmas. The oxygen fugacity is also significant for experimental work, as the fugacity of the charges should be the same as the fugacity of the magma (cf. Ulmer 1971).
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IX. Partial Melting

Abstract
The magmas extruded on the surface of the Earth originated by partial melting of a solid source that was transformed into a primary magma and a residuum. It appears that magmas were formed by partial melting during the main part of the Earth’s history, however, it is possible that the very first continental crust was formed by fractionation of magmas stemming from the oceans of magma that initially covered the Earth.
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X. Fractional Crystallization

Abstract
The chemical composition of igneous rocks from many different igneous provinces displays systematic variation. When the chemical compositions of the rocks are plotted in a diagram, it becomes evident that they apparently define either linear or curved trends. Comparisons of trends for similar rock types have shown that the trends for different provinces are quite similar, neither age nor geographic origin seem to have a major influence on the character of the trends. This consistency suggests that the compositions of igneous rocks are controlled by specific processes that operate under certain circumstances.
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XI. Magma Kinetics

Abstract
Nearly all petrological processes imply changes in the phase assemblages where new phases are formed and the old ones change their compositions. This applies for anatexis, partial melting, and fractionation as well as for metamorphic reactions. All these processes require that diffusion can take place over a short distance of the order of centimeters. The compositions of the new phases will be determined by the thermodynamic properties of the phases and the P-T conditions, but the diffusion rate will also influence the composition of the new phases as the rate of equilibration is directly dependent on the diffusion rate. Fast diffusion will, thus, ensure perfect equilibrium, while slow diffusion rates might hinder the attainment of equilibria. As an example the exsolution in solids may be mentioned. All the exposed rocks have been cooled down to temperatures near 0°C. Nevertheless, the composition of minerals might suggest equilibration temperatures between 600° and 1200°C. These temperatures are the temperatures at which the reaction ceased because the diffusion practically stopped. Probably the most conspicuous feature about the diffusion in magmas and rocks is the slow rate by which the diffusion occurs. As will be demonstrated later, the distances by which diffusion can occur in the span of a million years is less than a few meters. The slow diffusion consequently shows that material transport over long distances has to occur by other processes, like flow or gravitative fractionation. A comprehensive treatment of diffusion has been given by Crank (1975), while experimental aspects have been considered in detail by Frischat (1975). A detailed review of the diffusion processes in rocks and the diffusion coefficients of rocks and magmatic liquids have been given by Hofmann (1980).
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XII. Magma Dynamics

Abstract
The compositions of magmas are governed by a series of processes which occur at different pressures and temperatures. The very first stage in the development of magmas is the formation of the, initial magma by the partial melting of some source rock. The initial magma may then either ascend to the surface of the Earth without any fractionation or it may undergo one or several stages of fractionation. At any stage during its development the composition of the magma is controlled by the phase relations and the P-T conditions. The P-T conditions are governed by the dynamic processes, and it is, therefore, the dynamic processes which exert the ultimate control on the composition of magmas. The period of time that the magma stays at different P-T conditions before its final eruption is related to the geodynamic processes, and a determination of the petrogenesis of magmas, therefore, implies estimates of both phase relations and dynamic processes. Until the 1970’s the greatest importance was attached to the phase relations of magmas, and the petrogenetic theories were mainly dealing with phase relations, while the dynamic aspects were dealt with in less detail. After the fundamental phase relations were determined, the dynamic aspects gradually began to attract attention, and the relationships between the dynamic processes and the compositions of magmas became more into focus. The present chapter gives an introduction to some of the more basic aspects of magma dynamics and will deal with the pressure relationships of magmas, the flow of magma in dykes, the ascent of plumes and convection currents, and the flow of magma in a permeable mantle. The density and viscosity of magmas control the dynamic behavior of melt accumulation and the flow of magma in dykes and we will, therefore, briefly consider these features first.
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XIII. Isotope Geology

Abstract
The initial significance of isotopes was their potential for precise age determinations, and isotopic systematics still provide the most accurate age determinations. After iso­topic data had been accumulated for some years it became evident that the source of igneous rocks also may be estimated from the isotopic ratios. The reason for this additional advantage is that isotopes with atomic numbers higher than 40 are not fractionated during partial melting and fractional crystallization, so that any un­contaminated magma will obtain exactly the same isotopic ratios as its source. As an example, the granite controversy may be mentioned, which has lasted several de­cades. Two major types of origin have been considered for the granites. The granitic magmas may either have originated by anatexis from a gneiss or sediment, or by fractional crystallization from an andesitic magma. Both these theories have proven correct, as granites may form in both manners. The combined evidence from the 87Sr/86Sr, 143Nd/144Nd ratios and the lead isotopes will now provide the fun­damental information about the origin of a granite.
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Backmatter

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