Model Answer
0 min readIntroduction
Mineral zoning refers to the systematic variation in chemical composition within a single mineral grain. This phenomenon is a common feature in igneous and metamorphic rocks, providing valuable insights into the crystallization history and the physicochemical conditions prevailing during mineral formation. Zoning arises due to changes in the environment during crystal growth, leading to compositional variations that are preserved within the mineral structure. Understanding these zoning patterns is crucial for deciphering the petrogenetic evolution of rocks. The plagioclase feldspar series, particularly the Albite-Anorthite system, serves as an excellent model for illustrating the processes responsible for zoning due to its well-defined solid solution behavior and sensitivity to changes in temperature and pressure.
Types of Zoning Observed in Minerals
Mineral zoning can be categorized based on the nature and distribution of compositional variations. The primary types include:
- Normal Zoning: The most common type, where the core of the crystal is more calcium-rich (in plagioclase) or magnesium-rich (in olivine) compared to the rim. This indicates a gradual decrease in temperature or a change in melt composition during growth.
- Reverse Zoning: The opposite of normal zoning, with the core being less calcium-rich or magnesium-rich than the rim. This suggests an increase in temperature or a change in melt composition during growth, often associated with late-stage magmatic evolution.
- Oscillatory Zoning: Alternating bands of different compositions, creating a rhythmic pattern. This is typically caused by periodic fluctuations in temperature, pressure, or melt composition during crystal growth.
- Sector Zoning: Compositional variations concentrated in specific sectors or directions within the crystal, often related to the crystal's growth morphology and the influence of external factors like stress.
- Spotty Zoning: Irregularly distributed compositional variations, often associated with inclusions or localized changes in the environment.
Processes of Formation of Zoning in Plagioclase: The Albite-Anorthite System
The Albite (NaAlSi3O8) – Anorthite (CaAl2Si2O8) system is a solid solution series, meaning that Albite and Anorthite can substitute for each other in the plagioclase structure. The composition of plagioclase is expressed as a percentage of Anorthite (An) content. Zoning in plagioclase arises from several processes:
1. Equilibrium Zoning (Crystallization from a Changing Melt)
As a magma cools, the equilibrium composition of plagioclase changes. According to the plagioclase solvus (phase diagram), the solubility of Anorthite in Albite decreases with decreasing temperature. Therefore, early-formed plagioclase crystals will be more calcium-rich (higher An content) as they crystallize from a hotter melt. As the magma cools, subsequent growth layers will incorporate less Anorthite, resulting in normal zoning. This is governed by the phase rule and the changing equilibrium conditions.
2. Disequilibrium Zoning (Rapid Cooling & Diffusion)
Rapid cooling of magma doesn't allow for complete equilibration between the melt and the growing crystal. This leads to disequilibrium zoning. Two main mechanisms contribute to this:
- Diffusion: Atoms diffuse within the crystal lattice. If cooling is rapid, diffusion is limited, and the core retains the composition of the initial melt, while the rim reflects the composition of the later, cooler melt.
- Zone Refining: During rapid cooling, a liquid layer can form between the crystal and the melt. As the crystal continues to grow, this liquid layer is swept along, creating compositional variations.
3. Reaction Zoning (Interaction with Melt)
If a plagioclase crystal is introduced into a melt with a different composition, reaction zoning can occur. The crystal will react with the melt, leading to changes in its composition near the surface. For example, a calcium-rich plagioclase crystal in a sodium-rich melt will undergo exsolution of sodium, resulting in a sodium-rich rim.
4. Subsolidus Zoning (Exsolution)
Below the plagioclase solvus temperature, plagioclase becomes unstable and can undergo exsolution. This involves the separation of the solid solution into two distinct phases: Albite and Anorthite. This process can create lamellar or patchy zoning, where alternating layers or patches of Albite and Anorthite are visible.
Illustrative Example: Consider a basaltic magma cooling slowly. Initially, calcium-rich plagioclase (An80-90) crystallizes. As the magma cools and its composition evolves due to fractional crystallization, the plagioclase composition shifts towards sodium-rich (An50-60). This results in a plagioclase crystal with a core of An90 gradually decreasing to An60 towards the rim – a classic example of normal zoning.
| Zoning Type | Process | Albite-Anorthite System Example |
|---|---|---|
| Normal | Cooling from a changing melt; decreasing temperature | Core: An90, Rim: An60 |
| Reverse | Increasing temperature or changing melt composition | Core: An60, Rim: An90 (rare) |
| Oscillatory | Periodic fluctuations in temperature/composition | Alternating bands of An70 and An50 |
Conclusion
Mineral zoning, particularly in plagioclase, is a powerful tool for understanding magmatic and metamorphic processes. The Albite-Anorthite system provides a clear illustration of how changes in temperature, pressure, and melt composition can lead to diverse zoning patterns. Analyzing these patterns allows geologists to reconstruct the thermal history and evolution of igneous and metamorphic rocks, providing valuable insights into Earth’s dynamic processes. Further research utilizing advanced analytical techniques will continue to refine our understanding of zoning mechanisms and their implications for petrogenesis.
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.