Model Answer
0 min readIntroduction
Igneous rocks, formed from the cooling and solidification of magma or lava, provide valuable insights into Earth’s interior processes. The crystallization of magma is a complex process governed by temperature, pressure, and composition. Phase diagrams, graphical representations of thermodynamically stable phases under different conditions, are essential tools for understanding this process. The Mg2SiO4 - Fe2SiO4 - SiO2 system is particularly important in understanding the crystallization of basic magmas, as it represents the olivine-pyroxene-quartz assemblage commonly found in these rocks. This diagram helps predict the order in which minerals crystallize from a cooling magma, influencing the final rock composition and texture.
The Mg2SiO4 - Fe2SiO4 - SiO2 Ternary Diagram
The Mg2SiO4 - Fe2SiO4 - SiO2 system is a ternary diagram representing the compositions of forsterite (Mg2SiO4), fayalite (Fe2SiO4), and quartz (SiO2). It’s a simplified system, but crucial for understanding the crystallization of mafic and intermediate magmas. The diagram is a triangle, with each apex representing a pure end-member. The sides of the triangle represent binary systems (e.g., Mg2SiO4-Fe2SiO4, Mg2SiO4-SiO2, Fe2SiO4-SiO2).
Here's a description of the key features and fields within the diagram:
- Forsterite (Mg2SiO4) Field: Represents compositions rich in magnesium silicate.
- Fayalite (Fe2SiO4) Field: Represents compositions rich in iron silicate.
- Quartz (SiO2) Field: Represents compositions rich in silica.
- Olivine Solid Solution Series: The area between forsterite and fayalite represents solid solutions of Mg and Fe in the olivine structure.
- Pyroxene Field: Located in the central portion of the diagram, representing compositions of magnesium-iron silicates with a different crystal structure than olivine.
- Tie Lines: Horizontal lines connecting points of coexisting phases at a given temperature. These lines indicate the compositions of the minerals that are in equilibrium with the melt.
- Eutectic Point: The lowest temperature at which a liquid can exist in equilibrium with all three solid phases (olivine, pyroxene, and quartz).
Implications for Crystallization of Basic Magma
Basic magmas (e.g., basaltic magmas) are typically rich in iron, magnesium, and silica. As a basic magma cools, the crystallization sequence is dictated by Bowen’s Reaction Series. This diagram helps visualize the early stages of crystallization.
Early Stage Crystallization: Olivine and Pyroxene
Initially, as the magma cools, olivine (specifically, a magnesium-rich olivine) begins to crystallize. The composition of the olivine that crystallizes will be determined by the magma’s composition and the position of the tie line. As the magma cools further, the olivine becomes more iron-rich (approaching fayalite) as magnesium is depleted from the melt. Simultaneously, pyroxene begins to crystallize. The pyroxene composition will also evolve with decreasing temperature and changing melt composition.
Intermediate Stage Crystallization: Continued Olivine and Pyroxene Evolution
As crystallization progresses, the remaining melt becomes increasingly enriched in silica. The olivine continues to evolve towards fayalite, and the pyroxene composition changes as well. The tie lines shift, indicating the changing equilibrium compositions of the coexisting phases.
Late Stage Crystallization: Quartz and Final Mineral Assemblages
At lower temperatures, quartz begins to crystallize. The presence of quartz indicates that the magma has reached a silica-saturated state. The final mineral assemblage will depend on the overall magma composition and the extent of crystallization. Residual melt may become enriched in volatile components and form late-stage minerals like feldspars and oxides.
Role of Fractional Crystallization
Fractional crystallization, the removal of early-formed crystals from the melt, significantly influences the evolution of the magma. By removing olivine and pyroxene, the remaining melt becomes more silica-rich, promoting the crystallization of quartz and other silica-rich minerals. This process can lead to the formation of differentiated igneous rocks with varying compositions.
Application to Natural Magmas
The Mg2SiO4 - Fe2SiO4 - SiO2 system, while simplified, provides a fundamental understanding of the crystallization processes in many natural magmas. Basaltic magmas from mid-ocean ridges and oceanic hotspots often follow a similar crystallization sequence, resulting in rocks with olivine, pyroxene, and plagioclase feldspar as dominant minerals. The specific mineral compositions and proportions will vary depending on the magma’s initial composition and the conditions of crystallization.
Conclusion
The Mg2SiO4 - Fe2SiO4 - SiO2 phase diagram is a powerful tool for understanding the crystallization behavior of basic magmas. It illustrates the sequence of mineral formation, the evolution of mineral compositions, and the influence of fractional crystallization. By applying this knowledge, geologists can interpret the origin and evolution of igneous rocks and gain insights into the processes occurring within Earth’s mantle and crust. Understanding these phase relationships is crucial for deciphering the complex history of our planet.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.