Magmatic Evolution and Differentiation: Fractional Crystallization

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## **Magmatic Evolution**

* The process by which a **parent magma** changes its composition during cooling, crystallization, and interaction with rocks.
* Leads to the **formation of diverse igneous rocks** from a limited number of magma types.

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## **Magmatic Differentiation**

* The set of processes that cause **one parent magma** to give rise to magmas (and rocks) of different compositions.
* **Fractional crystallization** is the **most important mechanism**.

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## **Fractional Crystallization**

### **Definition**

Fractional crystallization is the process where **early-formed crystals are physically separated from the remaining liquid magma** and therefore cannot re-equilibrate with it.

* This removal prevents reaction between crystals and melt.
* As a result, the **residual melt becomes progressively enriched** in certain elements.

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### **Mechanism**

* Minerals crystallize in a definite sequence known as **Bowen’s Reaction Series**:

  * **Discontinuous series:** olivine → pyroxene → amphibole → biotite.
  * **Continuous series:** Ca-rich plagioclase → Na-rich plagioclase.
* Early minerals (olivine, Ca-plagioclase, pyroxene) crystallize from a mafic melt.
* If they are removed (by settling, floating, or filter pressing), the residual liquid evolves toward a **more felsic composition**.

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### **Conditions for Fractional Crystallization**

* Presence of large **magma chambers**.
* Ability of crystals to **settle under gravity** (depends on density, viscosity).
* Magma must be capable of **convection currents** or other processes to separate crystals.

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### **Effects on Magma Composition**

1. **Depletion in early-forming minerals** (Fe, Mg, Ca).
2. **Enrichment of residual melt** in Si, Na, K, and volatiles.
3. Leads to the **progressive evolution**:

   * Basalt → Andesite → Dacite → Rhyolite.

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### **Examples**

* **Layered intrusions** (Bushveld Complex, Skaergaard Intrusion): show crystal layers of olivine, pyroxene, chromite.
* **Oceanic crust**: Gabbroic layers formed by fractional crystallization of basaltic magma.

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### **Geological Significance**

* Explains **igneous rock diversity** from common parental magmas.
* Responsible for formation of **layered igneous complexes**.
* Leads to **concentration of ore minerals** (chromite, magnetite, platinum-group elements).
* Important for reconstructing **petrogenesis** of igneous provinces.

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