Model Answer
0 min readIntroduction
Igneous rocks often exhibit textural features that provide clues about their cooling history. Porphyritic textures, characterized by large, well-formed crystals (phenocrysts) embedded in a finer-grained matrix (groundmass), are indicative of a two-stage cooling process. Understanding the principles of phase equilibria, as represented by binary eutectic systems, is crucial for deciphering the formation of such textures. A binary eutectic system describes the phase relationships between two end-member components during cooling and crystallization. This answer will illustrate a binary eutectic system and explain how it can model the formation of a porphyritic basic rock with the specified mineralogy.
Binary Eutectic System (1 atm)
A binary eutectic system represents the phase relationships between two components, A and B, at a constant pressure (here, 1 atm). The system is typically represented graphically as a temperature-composition diagram.
Key features of the diagram:
- Liquidus Line: The line above which the system is entirely liquid.
- Solidus Line: The line below which the system is entirely solid.
- Eutectic Point: The lowest temperature at which a liquid mixture of A and B can exist. At this point, the liquid transforms directly into a mixture of solid A and solid B.
- Compositional Ranges: The areas representing the compositions of the solid and liquid phases.
Formation of Porphyritic Basic Rock
To explain the formation of a porphyritic basic rock with plagioclase phenocrysts and a groundmass of plagioclase and clinopyroxene, consider a binary eutectic system involving plagioclase (A) and clinopyroxene (B). Basic rocks are typically rich in Ca-Na plagioclase and pyroxenes.
Stage 1: Phenocryst Formation (Slow Cooling at Depth)
Initially, the magma resides at depth, experiencing slow cooling. If the magma composition falls within the compositional range where plagioclase is the first phase to crystallize (towards the A-rich side of the system), plagioclase crystals will begin to nucleate and grow. Due to the slow cooling rate, these crystals have ample time to develop into large, well-formed phenocrysts. This is because of the lower nucleation rate at lower temperatures and the availability of ions for crystal growth over a prolonged period.
Stage 2: Ascent and Rapid Cooling (Groundmass Formation)
As the magma ascends towards the surface, the pressure decreases, and the cooling rate increases significantly. This rapid cooling shifts the cooling path on the phase diagram. The remaining liquid, now enriched in clinopyroxene (B), enters the eutectic region. Here, simultaneous crystallization of plagioclase and clinopyroxene occurs, but at a much faster rate. This results in the formation of a fine-grained groundmass consisting of intergrown plagioclase and clinopyroxene crystals. The higher viscosity of the melt, due to the presence of already crystallized plagioclase, also contributes to the formation of a fine-grained texture.
Cooling Path Representation
The cooling path on the binary eutectic diagram would start at a high temperature and composition favoring plagioclase crystallization. It would then traverse the liquidus line, allowing plagioclase phenocrysts to grow. As the magma ascends and cools rapidly, the cooling path would cut across the eutectic region, leading to the simultaneous crystallization of plagioclase and clinopyroxene in the groundmass.
The specific composition of the magma and the rate of cooling determine the relative proportions of phenocrysts and groundmass minerals. A magma closer to the plagioclase end-member will produce more plagioclase phenocrysts, while a faster cooling rate will result in a finer-grained groundmass.
Conclusion
In conclusion, the formation of a porphyritic basic rock with plagioclase phenocrysts and a groundmass of plagioclase and clinopyroxene can be effectively explained using a binary eutectic system. The two-stage cooling history – slow cooling at depth leading to phenocryst growth, followed by rapid cooling during ascent resulting in a fine-grained groundmass – is key to understanding this textural feature. This model highlights the importance of phase diagrams in interpreting the petrogenesis of igneous rocks and understanding their cooling histories.
Answer Length
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