Explain using phase rule the binary eutectic nature of diopside-anorthite system. Comment upon textures produced during crystallization of basaltic magmas rich in diopside and anorthite components.

Phase Rule and Binary Eutectic Systems

The phase rule, as stated above, dictates the conditions under which a system can exist in equilibrium. In the context of igneous petrology, it helps predict which minerals will crystallize at a given temperature and pressure, and in what proportions. A binary eutectic system, like diopside (CaMgSi₂O₆) – anorthite (CaAl₂Si₂O₈), consists of two components and exhibits a characteristic phase diagram.

The Diopside-Anorthite Phase Diagram

The phase diagram for the diopside-anorthite system shows:

• Solid Solution Series: Diopside and anorthite form a continuous solid solution series at high temperatures.
• Eutectic Point: A specific composition and temperature (the eutectic point) where the liquid solidifies directly into a mixture of diopside and anorthite, without forming an intermediate solid solution.
• Liquidus Line: The line above which the system is entirely liquid.
• Solidus Line: The line below which the system is entirely solid.

Applying the phase rule at the eutectic point (C=2, P=2 – liquid and two solid phases), F = 2 - 2 + 2 = 2. This means that at the eutectic temperature, the composition of the liquid and the proportions of the two solid phases are fixed. Away from the eutectic point, the degrees of freedom change, allowing for variations in composition and temperature during crystallization.

Textures Produced During Crystallization of Basaltic Magmas

Basaltic magmas, often enriched in diopside and anorthite components, exhibit a range of textures depending on the cooling rate and magma composition. The crystallization path, dictated by the phase diagram, significantly influences these textures.

Slow Cooling – Cumulate Textures

Slow cooling allows for the development of larger crystals. Early-formed diopside and anorthite crystals can settle due to gravity, forming cumulate textures. These textures are characterized by:

• Modal layering: Distinct layers of different minerals, reflecting changes in magma composition or settling rates.
• Orthocumulate textures: Minerals are aligned due to flow during crystallization.
• Adcumulate textures: Minerals grow in place, forming skeletal or intergrown structures.

These textures are commonly found in layered intrusions.

Intermediate Cooling – Subophitic to Ophitic Textures

Intermediate cooling rates result in subophitic to ophitic textures. In these textures, plagioclase (anorthite component) laths are partially enclosed or completely enclosed (ophitic) by pyroxene (diopside component) crystals. This indicates that pyroxene crystallized later and grew around the earlier-formed plagioclase. The degree of enclosure reflects the relative crystallization rates and the viscosity of the magma.

Rapid Cooling – Microcrystalline/Glassy Textures

Rapid cooling, such as during subaerial eruptions or quenching in water, inhibits crystal growth, leading to microcrystalline or glassy textures. In this case, the magma may not reach the eutectic point, and the resulting rock will contain very small crystals of diopside and anorthite, or even be entirely glassy (obsidian). Pilotaxitic texture, common in basalt, is a type of microcrystalline texture where plagioclase laths are randomly oriented in a fine-grained groundmass.

Other Textures

Zoning: Crystals may exhibit compositional zoning, reflecting changes in magma composition during growth. This is particularly common in plagioclase.

Skeletal Crystals: Rapid crystallization can lead to the formation of skeletal crystals, which are incomplete and hollow.