The albite-anorthite system
Most of the common minerals found in igneous rocks are solid-solution phases. These include olivine, pyroxene, amphibole, biotite, and plagioclase feldspars. Crystallization behaviour is illustrated best by using the NaAlSi3O8 (albite or Ab)–CaAl2Si2O8 (anorthite or An) plagioclase system shown in Figure 4. Consider a liquid of composition L (60 percent An + 40 percent Ab) which is at an initial temperature of 1,500 °C. On cooling it will begin crystallizing plagioclase with 85 percent An (point P on the solidus) at the liquidus temperature of about 1,470 °C. As cooling continues further, the liquid will move down the liquidus toward B while simultaneously reacting continuously with the early-formed plagioclase to convert it to a homogeneous plagioclase that is more albitic and in equilibrium with the liquid. For example, when the liquid has reached A, at 1,400 °C, about 65 percent plagioclase with about 73 percent An (point O on the solidus) has crystallized from the liquid, which is now at about 36 percent An and 64 percent Ab. Finally, when the temperature of about 1,330 °C is reached (B in the figure), the last small amount of the liquid of composition 20 percent An + 80 percent Ab is consumed in the reaction and a homogeneous plagioclase of 60 percent An + 40 percent Ab remains (point S). Now consider the case in which the liquid is prevented from reacting with the early-formed plagioclase. This may be achieved by physically removing the plagioclase immediately after its formation or by cooling the liquid faster than the reaction process can consume the plagioclase. The liquid could theoretically reach the pure Ab composition at 1,100 °C, where it will disappear into the crystallizing albite. A whole range of plagioclase compositions from An84 to An00 will be preserved in the cooling process.
Bowen’s reaction series
These two examples illustrate two principal reactions that occur during crystallization of common magmas, one discontinuous (the olivine-liquid-pyroxene reaction) and the other continuous (the plagioclase-liquid reaction). This was recognized first by the American petrologist Norman L. Bowen, who arranged the reactions in the form shown in ; in his honour, the mineral series has since been called the Bowen’s reaction series. The left branch of the Y-shaped arrangement consists of the discontinuous series that begins with olivine at the highest temperature and progresses through pyroxene, amphibole, and biotite as the temperature decreases. This series is discontinuous because the reaction occurs at a fixed temperature at constant pressure wherein the early-formed mineral is converted to a more stable crystal. Each mineral in the series displays a different silicate structure that exhibits increased polymerization as the temperature drops; olivine belongs to the island silicate structure type; pyroxene, the chain; amphibole, the double chain; and biotite, the sheet. On the other hand, the right branch is the continuous reaction series in which plagioclase is continuously reacting with the liquid to form a more albitic phase as the temperature decreases. In both cases, the liquid is consumed in the reaction. When the two reaction series converge at a low temperature, minerals that will not react with the remaining liquid approach eutectic crystallization. Potash feldspar, muscovite, and quartz are crystallized. The phases that are crystallized first are the common minerals that compose basalt or gabbro, like bytownite or labradorite with pyroxene and minor amounts of olivine. Andesite or diorite minerals, such as andesine with either pyroxene or amphibole, crystallize next and are followed by orthoclase and quartz, which are the essential constituents of rhyolite or granite. A basaltic liquid at the top of the Y can descend to the bottom of the series to crystallize quartz only if the earlier reactions are prevented. As demonstrated above, complete reactions between early-formed minerals and the liquid depletes the supply of the liquid, thereby curtailing the progression down the series. One means by which basaltic magma can be transformed to rocks lower in the series is by fractional crystallization. In this process, the early-formed minerals are removed from the liquid by gravity (such minerals as olivine and pyroxene are denser than the liquid from which they crystallized), and so unreacted liquid remains later in the series.
Assimilation
Another method of creating different daughter magmas from a parent is by having the latter react with its wall rocks. Consider a magma that is crystallizing pyroxene and labradorite. If the magma tears from its wall minerals, say, olivine and anorthite, which are formed earlier than pyroxene and labradorite in the series, they will react with the liquid to form these same minerals with which the magma is in equilibrium. The heat for driving this reaction comes directly from the magma itself. More pyroxene and labradorite will crystallize during the reaction and will release their latent heats of crystallization. On the other hand, if a mineral (quartz, for example) formed at a later stage than pyroxene or labradorite falls from the rock wall into the magma, the latent heat provided by further crystallization of pyroxene and labradorite will cause it to dissolve. This situation will occur only if the quartz from the wall rock is at a lower temperature than the magma. It will cause the magma to transfer its heat to the quartz in a cooling process. The cooling of the magma will necessarily be accompanied by the crystallization of the minerals already present. In both cases, the composition of the parent magma will be changed by the xenolithic (foreign rock) contamination. The contaminant need not belong to the reaction series in order for it to cause reactions or dissolution. In most cases, the end result will be a shift from the original composition of the parent magma toward that of the contaminant. This process in which wall rocks are incorporated into the magma is called assimilation. Because assimilation is accompanied by crystallization, it is likely that both fractional crystallization and assimilation will take place simultaneously. This combined process, referred to as AFC for assimilation–fractional crystallization, has been proposed as the mechanism by which andesites are produced from basalts.
Volatile constituents and late magmatic processes
Effects of water and other volatiles
Water and most other volatile substances profoundly influence the properties and behaviour of magmas in which they are dissolved. They reduce viscosity, lower temperatures of crystallization by tens to hundreds of degrees, and participate directly in the formation of minerals that contain essential hydroxyl (OH) or elements such as the halogens. They also increase rates of crystallization and reaction, especially when they are present as a fluid phase distinct from the magma. In general, however, they have only a limited influence on the sequence of magmatic crystallization, except in the latest stages of the reaction series.
The relatively low confining pressures in volcanic environments permit ready escape of volatile constituents, which nonetheless leave their imprint in the form of special mineral assemblages and a variety of textural and structural features among the volcanic rocks. Under the higher pressures of plutonic environments, these constituents tend to be maintained in magmatic solution and to be increasingly concentrated as crystallization progresses with falling temperature. Few members of the reaction series require them as compositional contributors; water, for example, is not thus used until amphiboles or micas begin to form, and even then the amounts removed from the melt rarely are large. Escape of volatiles from the system can occur “osmotically” if the enclosing rocks are pervious to them but not to the magma, but in general they are fractionated in favour of the residual melt until their concentration reaches the limit of solubility under the prevailing conditions of temperature and effective confining pressure. When this happens, normally at a very late stage of magmatic crystallization, they are exsolved from the melt as a separate fluid phase that under most circumstances is a supercritical gas. This process has been referred to as resurgent boiling, a somewhat misleading term because the exsolved fluid is not necessarily expelled from the system.
Pegmatites and late-stage mineralization
Coexistence of residual magma and a volatile-rich fluid (generally aqueous) promotes the partitioning and segregation of constituents, as well as the growth of very large crystals. The exsolved fluid, with its very low viscosity, not only can move readily through open spaces in the nearly solid igneous rock and in adjacent rocks but also serves as a medium through which various substances can diffuse rapidly in response to concentration gradients. Thus, it plays an important role in the formation of such special rock types as the pegmatites and lamprophyres, special features such as miaroles and plumose mineral aggregates, and many kinds of ore deposits whose constituents are derived from the original magma.
Most plutonic systems remain at elevated temperatures for long periods of time after all magma has been used up, and during these periods hydrothermal conditions normally obtain. These depend upon the continued presence of a typically aqueous fluid that further facilitates crystallization and exchanges of materials. It speeds up exsolution within homogeneous solid phases and devitrification of any glass that may be present, and it is a potent agent in the alteration, leaching, and replacement of minerals. Rock textures thereby are modified, especially along boundaries between original mineral grains, and details of composition also can be much changed. In some instances the bulk chemistry of the rock is markedly affected.
The hydrothermal alterations favour development of phases such as albite, carbonates, chlorites, clay minerals, epidotes, iron oxides, micas, silica minerals, talc, and zeolites, and many of them are accompanied by gross changes in volume.
Richard H. Jahns Albert M. KudoForms of occurrence
Extrusive igneous rocks
Extrusive igneous rocks are the products of volcanic activity. They appear at the surface as molten lava that spreads in sheets and hardens, or they are made up of fragments of magma ejected from vents by violent gaseous explosions. Large-scale extrusive features include stratovolcanoes (composite cones), shield volcanoes, lava domes, and cinder cones. Smaller extrusive features include lava flows known as pahoehoe and aa. For a detailed description of the principal forms of extrusive igneous rocks, see volcano: Volcanic landforms: Major types of volcanic landforms.