Themes of the Day:

• Making Magmas: Melting the Mantle
• Crystallization and Compositional Differentiation
• The Big Chart (based on Bowen's Reaction Series)
• Igneous Rock Classification

Making Magmas: Melting the Mantle (Quick Refresher)

• Three ways to cause melting:
  • Increase Temperature (T) - rare on Earth
  • Decrease Pressure (P) - most effective - upwelling mantle at spreading centers (mid-ocean ridges) and hotspots due to mantle convection - partial melts (10-30%) of basaltic composition (dry melting)
  • Add fluids (water) - decreases melting temperature - common at subduction zones where fluids are driven from the subducting oceanic crust into the overlying mantle causing melting (wet melting)
• Important Associations between Plate Tectonics and Magmatism:
  • Depressurization melting - divergent plate boundaries and hotspots - basalts and gabbros are most common rock types - thin crust so little magma differentiation - ophiolite suite (oceanic crust) - Block Diagram of a Mid-Ocean Ridge Magmatic System
  • Melting by Addition of Water - subduction zones (convergent plate boundaries) - granites and andesites are most common rock types - evolved compositions due to magma differentiation by fractional crystallization as magmas rise through the crust - Block Diagram of a Subduction Zone Magmatic System

Crystallization and Compositional Differentiation - Bowen's Reaction Series

• Crystallization of Magmas - not like ice
• Crystallization rate determines texture
• Crystallization causes differentiation - removal of crystals changes the composition of the remaining magma
• Magmas evolve as the result of crystal settling, filter pressing, and magma mixing
• Bowen's Reaction Series describes mineral changes resulting from magma evolution
  • Discontinuous branch - Olivine, Pyroxene (Augite), Amphibole (Hornblende), Biotite
  • Continuous branch - Plagioclase Feldspar - Ca-rich to Na-rich
  • At the Bottom - Muscovite, Orthoclase Feldspar, Quartz
  • Crystallization temperatures
• May result in layered and zoned igneous bodies - e.g., Palisades Sill, NJ/NY

The Big Chart

• The Big Chart
• Types of Igneous Rocks - an excellent companion to "The Big Chart" (see Wednesday's Lecture) from the US Geological Survey

Igneous Rock Classification

• Textures (Detailed discussion in lab)
  • Igneous textures are dependant primarily on cooling rate
  • Intrusive (plutonic) vs. Extrusive (volcanic) igneous rocks
  • Specific textures:
    • coarse-grained - slow cooling, intrusive rocks
    • fine-grained - rapid cooling, extrusive rocks
    • porphyritic - bimodal grain size distribution - slow cooling initially, followed by rapid cooling
    • glassy (e.g., obsidian) - rapid quenching - no minerals
    • vesicular - full of gas bubbles - usually volcanic (pumice is a type of vesicular obsidian)
• Composition (Mineralogy) - Mafic, Intermediate, Felsic - results from differentiation of magmas during crystallization - discussed below
• Igneous Rock Classification - Figure 5.4
• Test yourself - Identify these Igneous Rocks

Lecture #25 - a couple of things for review.

This concludes the material that will be covered on Exam #3.