Laboratory experiments on the cooling of silicate magmas consistently demonstrate that the magma will solidify over a range of temperatures and that the crystallizing minerals will almost always have a different chemical composition than the magma (see Di-An). Furthermore, the compositions of solid solution minerals that crystallize depend on the composition of the magma, with the compositions of Mg-Fe silicate minerals more Mg-rich than the magma (see olivine), and the compositions of plagioclase more An-rich than the magma norm. Therefore, if some of the minerals that grow from a magma are separated from the magma, the magma composition will be changed. Separation of crystals from a magma as they grow is called fractional crystallization.
Figure 8.01. Crystal Settling Movie. Sylvite crystals nucleating and growing from a cooling H2O-KCl solution sink to the bottom of a model magma chamber in this movie. Click on the diagram to see the movie and for more information.
Figure 8.02. Igneous Layering. Click on the image to see a larger version and for more information.
The simplest and most likely mechanism for separating crystals and magma is the sinking or floating of crystals or a crystal-rich mush because of a density contrast with the liquid. You can see a laboratory analog example of this in Figure 8.01, a brief movie of crystal settling in a H2O-KCl model magma chamber. Click on the figure to watch the movie. There is considerable evidence that crystal settling happens on earth, particularly for magmas of comparatively low viscosity such as basaltic liquids. During the 19th century, geologists such as Charles Darwin (Pearson, 1996) wrote about field evidence of mineral accumulations. The geologic literature includes many descriptions of mafic intrusions that have a layered structure where the layers contain minerals that appear to have settled to the bottom of a magma chamber, forming the layers. An example of magmatic layering in the Kiglapait layered mafic intrusion is shown in Figure 8.02. In an ideal case, the layers are a record of the minerals that formed, in sequence, as the magma crystallized, with the earliest-formed minerals at the bottom. Detailed studies of Layered Mafic Intrusions have confirmed the importance of crystal settling, and have revealed many other details of the processes that take place in magma chambers.
