Essentials of Geology: Bowen's Reaction Series and Magma Evolution

Introduction to Bowen's Reaction Series

• Bowen's Reaction Series: Illustrates the sequence of mineral crystallization from basaltic magma.
• Crystallization process: Begins at high temperatures (~1200°C) and ends at low temperatures (~650°C).
• Minerals:
  • High-temperature minerals: Olivine, Pyroxene, Calcium-rich Plagioclase Feldspar
  • Low-temperature minerals: Potassium Feldspar, Muscovite Mica, Quartz

Magma Evolution

• Observations: Single volcanoes may extrude differing compositions of lava over time.
• Magma Evolution: Concept that magma can change over time, leading to diverse igneous rock compositions.

Composition of Igneous Rocks

• Freezing temperature comparison: Unlike ice, basaltic magma crystallizes over a range of temperatures.
• Crystallization sequence:
  • Begins with the formation of olivine.
  • Continues with minerals like calcium-rich plagioclase and pyroxene.
• Liquid magma composition changes:
  • As minerals crystallize, the remaining magma becomes enriched in sodium, potassium, and silica.

Magmatic Differentiation

• Process: Generates diverse mineralogical compositions from a single parent magma.
• Crystal Settling:
  • Denser minerals settle at the bottom of the magma chamber.
  • Results in remaining melt being more felsic than the initial magma.
• Example: Palisades Sill shows magmatic differentiation through crystal settling.

Assimilation and Magma Mixing

• Assimilation:
  • Rising magma may incorporate surrounding rocks, altering its composition.
  • Changes the chemical composition of the magma.
• Magma Mixing:
  • Occurs when two distinct magma bodies combine.
  • Resulting magma has a blended, intermediate composition.

Partial Melting and Magma Composition

• Partial Melting:
  • Involves melting of minerals with the lowest melting temperatures first.
  • Complete melting is rare; partial melting enriches melt in certain ions.
• Generation of basaltic magma: From partial melting of mantle rock peridotite.

Concept Checks

1. Bowen's Reaction Series: Defines the order of mineral crystallization.
2. Impact of Crystallization: Changes the composition of the remaining magma.
3. Assimilation vs. Magma Mixing:
   • Assimilation involves incorporating external materials.
   • Magma Mixing involves blending of two magma bodies.