Evolution of Magmas PDF

**Evolution of Magmas: How Magma Changes Over Time** Magma evolves as it rises through the crust, interacts with surrounding rocks, and cools. This evolution results from physical and chemical processes that alter its composition and texture. These changes are reflected in the **textures** of the r...

**Evolution of Magmas: How Magma Changes Over Time** Magma evolves as it rises through the crust, interacts with surrounding rocks, and cools. This evolution results from physical and chemical processes that alter its composition and texture. These changes are reflected in the **textures** of the rocks it forms and in its **geochemical signature**. **1. Processes of Magma Evolution** **a. Fractional Crystallization** - **What happens**: As magma cools, minerals crystallize out of the melt in a specific sequence (Bowen's Reaction Series). The remaining liquid becomes enriched in elements that don\'t fit into the crystalline structure of early-forming minerals (incompatible elements). - **Result**: - Early crystals (e.g., olivine, pyroxene) remove magnesium (Mg) and iron (Fe), making the remaining magma richer in silica (Si). - Produces a progression from mafic (basalt) to intermediate (andesite) to felsic (rhyolite) compositions. **b. Magma Mixing** - **What happens**: Two magmas of different compositions mix. This can happen when a more mafic magma intrudes into a chamber with more evolved felsic magma. - **Result**: - Produces intermediate compositions with mixed textures. - Evident in the presence of disequilibrium textures like resorbed crystals or oscillatory zoning in minerals. **c. Assimilation of Crustal Material** - **What happens**: Magma melts and incorporates surrounding crustal rocks as it rises. This is common in thick continental crust where magma has a longer residence time. - **Result**: - Changes magma composition by adding silica and incompatible elements from the crust. - Produces felsic magmas with enriched isotopic signatures (e.g., Sr-87/Sr-86). **d. Partial Melting** - **What happens**: Rocks partially melt, and the melt separates from the solid residue. The melt is enriched in incompatible elements because these elements preferentially enter the liquid phase. - **Result**: - Mantle melts produce basaltic magmas. - Crustal melts can produce rhyolitic or granitic compositions. **2. Tracing Magma Evolution Texturally** Textures in igneous rocks reveal the history of magma crystallization and evolution. **Key Textures:** 1. **Mineral Zoning**: - Reflects changes in magma composition or temperature during crystallization. - Example: Oscillatory zoning in plagioclase, caused by fluctuations in magma chemistry. 2. **Crystal Size Distribution**: - Large crystals (phenocrysts) surrounded by fine-grained matrix indicate slow cooling followed by rapid cooling (e.g., porphyritic texture). 3. **Resorption Features**: - Crystals that have partially dissolved show magma mixing or changes in pressure/temperature. 4. **Intergrowth Textures**: - Examples like ophitic or graphic textures indicate specific cooling histories. 5. **Groundmass Composition**: - Glassy groundmass indicates rapid cooling (e.g., volcanic eruptions). **3. Tracing Magma Evolution Geochemically** Geochemical analyses are crucial for understanding the source of magmas and how they evolve. **Key Geochemical Indicators:** 1. **Major Elements**: - Changes in Mg, Fe, Si, and Al track crystallization or crustal assimilation. - Example: Mg\# (Mg/Mg+Fe) decreases as magma evolves and loses Mg-rich minerals. 2. **Trace Elements**: - **Incompatible elements** (e.g., K, Rb, U): Concentrate in the liquid phase as magma evolves. - **Compatible elements** (e.g., Ni, Cr): Decrease as they are removed into early-formed minerals. 3. **Rare Earth Elements (REEs)**: - REE patterns show the degree of partial melting and fractional crystallization. - Example: Enrichment in light REEs (La, Ce) vs. depletion in heavy REEs (Yb, Lu) indicates source and evolution. 4. **Isotopic Ratios**: - **Sr-87/Sr-86**: High values suggest crustal contamination. - **Nd-143/Nd-144**: High values indicate a depleted mantle source, while low values suggest an enriched source. **4. Geochemical Manifestations and Magma Formation** Geochemical manifestations provide a \"fingerprint\" of how magma forms and evolves. Here\'s how they tie in: **a. Source of the Magma:** - **Mantle-Derived Magma (Basalts)**: - Depleted mantle sources (MORB) produce magmas with low incompatible elements and simple isotopic signatures. - Enriched mantle sources (OIB, LIPs) generate magmas with high incompatible elements (e.g., Nb, U). - **Crustal-Derived Magma (Rhyolites, Granites)**: - Formed by partial melting of the crust; high in silica and incompatible elements. - Isotopic ratios reflect crustal input (e.g., high Sr-87/Sr-86). **b. Processes of Evolution:** - **Fractional Crystallization**: - Results in systematic depletion of Mg and Fe and enrichment of incompatible elements. - REE patterns show progressive enrichment in light REEs. - **Crustal Contamination**: - Introduces silica, potassium, and incompatible elements, producing felsic magmas. - Changes isotopic ratios to reflect crustal material. - **Magma Mixing**: - Produces intermediate compositions with hybrid geochemical signatures. **How This Affects Magma Formation** 1. **Identifying Sources**: - Isotopic ratios (Sr, Nd, Pb) reveal whether magma originates from the mantle, crust, or a mixture of both. 2. **Understanding Processes**: - Trace element concentrations (e.g., Rb/Sr, La/Yb) show the extent of fractional crystallization, partial melting, or crustal contamination. 3. **Relating to Tectonic Settings**: - Geochemical signatures confirm the tectonic environment (e.g., depleted mantle for MORB, enriched mantle for OIB). 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