Model Answer
0 min readIntroduction
The phase rule, formulated by Gibbs, is a fundamental tool in petrology for understanding the conditions under which different phases can coexist in a system. It’s expressed as F = C - P + 2, where F is the degrees of freedom, C is the number of components, and P is the number of phases. In igneous petrology, it helps predict mineral assemblages and textures during magma crystallization. The diopside-anorthite system provides a classic example of a binary eutectic system, demonstrating how compositional variations influence crystallization behavior and ultimately, the textures observed in basaltic rocks. Understanding this system is crucial for interpreting the origin and evolution of basaltic magmas.
Phase Rule Application to the Diopside-Anorthite System
The diopside-anorthite system is a simple binary system consisting of two end-member minerals: diopside (CaMgSi2O6) and anorthite (CaAl2Si2O8). Here, C = 2 (two components – Ca and Mg/Al-Si-O). The phase rule dictates the number of degrees of freedom at different points in the system.
The Phase Diagram
The phase diagram for the diopside-anorthite system exhibits a eutectic point. At the eutectic point, three phases (liquid, diopside, and anorthite) coexist in equilibrium. The eutectic composition represents the lowest melting temperature for the system. Above the eutectic temperature, only liquid exists. Below the eutectic temperature, the system crystallizes into a mixture of diopside and anorthite.
The phase diagram can be visualized as follows:
| Temperature Range | Phases Present | Degrees of Freedom (F) |
|---|---|---|
| Above Eutectic Temperature | Liquid | 1 |
| At Eutectic Temperature | Liquid + Diopside + Anorthite | 0 |
| Below Eutectic Temperature | Diopside + Anorthite | 0 |
Eutectic Reaction
The eutectic reaction can be represented as:
Liquid → Diopside + Anorthite
This reaction occurs at a specific temperature and composition (the eutectic point). The resulting mixture of diopside and anorthite will have a characteristic texture, as discussed below.
Textures Produced During Crystallization of Basaltic Magmas
Basaltic magmas often contain both diopside and anorthite components. The textures formed during their crystallization depend on several factors, including cooling rate, magma composition, and the order of crystallization.
Slow Cooling – Cumulate Textures
If a basaltic magma rich in diopside and anorthite cools slowly, the minerals will have time to grow to a relatively large size. Early-formed crystals of diopside and anorthite may settle out of the magma due to gravity, forming cumulate layers. These layers exhibit cumulate textures, characterized by large, well-formed crystals of diopside and anorthite in a finer-grained matrix. The crystals may show zoning, reflecting changes in magma composition during crystallization.
Intermediate Cooling – Subophitic to Ophitic Textures
Intermediate cooling rates lead to the development of subophitic to ophitic textures. In these textures, plagioclase (anorthite component) laths are partially enclosed by pyroxene (diopside component) crystals. The degree of enclosure determines whether it's subophitic (partial enclosure) or ophitic (complete enclosure). This texture indicates that pyroxene started crystallizing later than plagioclase, but eventually grew to encompass the plagioclase laths.
Rapid Cooling – Microcrystalline/Glassy Textures
Rapid cooling, such as during subaerial eruptions or quenching in water, results in microcrystalline or glassy textures. In this case, there isn't enough time for large crystals to grow. The magma solidifies into a fine-grained aggregate of microscopic crystals or, if cooling is extremely rapid, into a glass. The diopside and anorthite components may be present as tiny, indistinguishable crystals within the glassy matrix. Pilotaxitic texture, where plagioclase laths are randomly oriented in a glassy groundmass, is also common.
Other Textures
- Glomeroporphyritic texture: Clusters of early-formed diopside and anorthite crystals.
- Skeletal crystals: Rapidly grown crystals with hollow or incomplete forms.
Conclusion
The diopside-anorthite system, governed by the phase rule, provides a valuable framework for understanding the crystallization behavior of basaltic magmas. The resulting textures – from cumulate to glassy – are direct consequences of cooling rate and magma composition. Analyzing these textures allows geologists to infer the cooling history and petrogenetic evolution of basaltic rocks, providing insights into the processes occurring within the Earth's mantle and crust. Further research involving trace element analysis and experimental petrology can refine our understanding of these complex systems.
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.