The above results show that when the content of S02 in melt is low (e.g. S02 of ultrabasic rocks is lower than 45wt%), the crystallization of the equilibrium system will be near the nodal point (i.e. 5-7) in Fig. 5-7 (a) as long as the time is sufficient.
(b) at the end of the p-point, it is impossible to reach the co-junction point e, and there will be no quartz crystallization. Therefore, we usually see that ultrabasic rocks do not contain quartz, Fo and quartz can not coexist.
The Fc ^ Si02 system can explain the types of rocks formed by different crystallization processes and some structures of rocks. For example, when the initial melt composition is at point a, pyroxenite will eventually be formed; at point (a), porphyry basalt or quartz gabbro with silicate supersaturation will be formed. When the initial melt composition is at/point, if/point is close to the refractory pyroxene, pyroxene or even peridotite is formed. At this time, refractory pyroxene mostly grows around the crystal of forsterite, forming the characteristic reaction edge structure. When the composition of the melt is more biased towards the original refractory pyroxene, the less Fo is generated, and the amount of refractory pyroxene in the final product is more than that of olivine, forming olivine pyroxene, and the last remaining is less. Quantity Fo is often reactively melted into circular crystals and encapsulated in primary pyroxene to form olivine structure.
Under natural conditions, the equilibrium crystallization of Fo~Si02 system can be destroyed by two factors: separation crystallization and barrier of reaction edge of olivine pyroxene. As a result, the residual melt gradually becomes more silicon-rich. As the early crystallized Fo minerals have been removed from the melt after P-point, Fo will not react with the residual melt to form En, and the crystallization of the system will end at the co-crystallization point E. Thus, a set of rock assemblages with increasing acidity are formed: from bottom to top, from pure peridotite and pyroxene to gabbro, forming a layered igneous accumulation complex.
The evolution path of residual melt composition during melt crystallization is also called liquid line of descent. From the above analysis, it can be seen that the separation crystallization of melts in the same system has a wider range of composition changes than the equilibrium crystallization process under the same conditions. Correspondingly, the product of separation crystallization has a larger range of composition changes than that of equilibrium crystallization.
In addition, for the melt with lower Si02 content, the final product of equilibrium crystallization is forsterite and pyroxene, which reflects the unsaturated characteristics of Si02. However, separate crystallization can produce another crystalline phase, namely, cristobalite, which ultimately crystallizes the supersaturated Si02 assemblage of "cristobalite + persistent pyroxene".
In Fo-Si02 system, forsterite and pyroxene form a "reaction pair" at the near node. In natural rocks, pyroxene forms the reaction edge around a single grain of forsterite. On a larger scale, the "reaction pair" can also be seen in a layered intrusion into the human body, where layered olivine-bearing rocks are often covered by orthopyroxene-bearing rocks (left figure 5-8 (b)). In evolutionary neutral calc-alkaline magma, the reaction of clinopyroxene and melt can produce amphibole. When the reaction is incomplete, amphibole reaction edges appear around clinopyroxene in diorite. At low temperatures, amphibole reacts with more evolved melts to form biotite. Two or more reaction pairs are formed in the differentiated multi-component magma, forming a discontinuous reaction series. Many different reaction series may occur in magma, which mainly depends on their whole rock chemical composition and crystallization properties.
As mentioned earlier, the crystallization process of a melt consisting of pyroxene (containing 59.85 boundaries 1% Fen 2 and 40.15 boundaries 1% 8) can be used to explain some rules of the "reaction relationship". Conversely, if pure pyroxene crystals melt under latm, unexpected melting behavior will be found. The inconsistent melting of xenopyroxene at 1557 C produces a slightly Si02-rich melt with a crystal of forsterite. With further heating of the system, forsterite also dissolves in silicate melt, and finally melts completely at about 1600 C, resulting in a melt with the same composition as refractory pyroxene.