Bowen's Reaction Series explains how minerals form as magma cools. It's like a recipe for rocks, showing which ingredients (minerals) come together at different temperatures. This helps geologists understand why certain rocks have specific mineral combinations.

Magmatic differentiation is the process that creates diverse rock types from a single magma source. As the magma cools, early-forming minerals settle out, changing the remaining melt's composition. This leads to a range of rocks, from dark, dense basalts to light, silica-rich granites.

Bowen's Reaction Series

Principles of Bowen's Reaction Series

• Describes sequence of mineral crystallization from cooling magma based on experimental work by Norman L. Bowen in early 20th century
• Two main branches: discontinuous series (olivine → pyroxene → amphibole → biotite) and continuous series (plagioclase feldspar from calcium-rich anorthite → sodium-rich albite)
• Minerals crystallize in specific order based on formation temperature and chemical composition
  • Higher temperature minerals crystallize first (olivine, calcium-rich plagioclase)
  • Lower temperature minerals crystallize later (biotite, sodium-rich plagioclase, quartz)
• Predicts mineral composition of igneous rocks formed at different stages of magma cooling
  • Ultramafic rocks form from early crystallizing minerals (peridotite from olivine, pyroxene)
  • Mafic rocks form from intermediate stages (basalt and gabbro from pyroxene, calcium-rich plagioclase)
  • Felsic rocks form from late-stage minerals (granite and rhyolite from sodium-rich plagioclase, quartz, biotite)

Process of fractional crystallization

• Occurs during cooling and solidification of magma
  • Early-forming minerals crystallize and settle out due to higher density
  • Removal of crystals changes composition of remaining magma
• Remaining magma becomes increasingly enriched in silica, alkalis (sodium and potassium), and volatiles as crystallization progresses
  • These components are incompatible with early-forming minerals and remain in melt
• Leads to magmatic differentiation
  • Magma composition evolves from mafic → intermediate → felsic as cooling and crystallization proceed
  • Different igneous rock types formed at various stages of process
• Sequence of rock types formed follows Bowen's Reaction Series
  1. Early stages produce ultramafic and mafic rocks (peridotite, basalt, gabbro)
  2. Later stages produce intermediate and felsic rocks (andesite, diorite, granite, rhyolite)

Continuous vs discontinuous reaction series

• Discontinuous reaction series involves sequential crystallization of distinct mineral phases
  • Olivine → Pyroxene → Amphibole → Biotite
  • Each mineral has distinct chemical composition and crystallizes over specific temperature range
  • Minerals in series are increasingly silica-rich and water-bearing as temperature decreases
• Continuous reaction series involves gradual change in composition within single mineral group
  • Plagioclase feldspar series: Calcium-rich anorthite → Sodium-rich albite
  • Composition of plagioclase changes continuously with decreasing temperature as calcium is progressively replaced by sodium in crystal structure
• Two series are interconnected at intermediate temperatures
  • Pyroxene in discontinuous series reacts with melt to form amphibole at similar temperature to formation of intermediate plagioclase compositions

Application of Bowen's Series

• Mafic magmas (high temperature, low silica)
  • Early crystallization of olivine and calcium-rich plagioclase followed by pyroxene and intermediate plagioclase
  • Basaltic magma crystallizes olivine, pyroxene, and calcium-rich plagioclase to form basalt or gabbro
• Intermediate magmas (moderate temperature and silica)
  • Early crystallization of pyroxene and intermediate plagioclase followed by amphibole and sodium-rich plagioclase
  • Andesitic magma crystallizes pyroxene, amphibole, and intermediate plagioclase to form andesite or diorite
• Felsic magmas (low temperature, high silica)
  • Early crystallization of sodium-rich plagioclase and amphibole followed by biotite, alkali feldspar, and quartz
  • Rhyolitic magma crystallizes sodium-rich plagioclase, biotite, alkali feldspar, and quartz to form rhyolite or granite

Magmatic Differentiation

Principles of magmatic differentiation

• Process by which single parent magma evolves into range of magma compositions driven by fractional crystallization as described by Bowen's Reaction Series
• As magma cools and minerals crystallize, composition of remaining melt changes
  • Early-forming minerals (olivine, pyroxene, calcium-rich plagioclase) are removed from melt
  • Remaining melt becomes enriched in silica, alkalis, and volatiles
• Leads to formation of spectrum of igneous rock types ranging from ultramafic (peridotite) → mafic (basalt, gabbro) → intermediate (andesite, diorite) → felsic (granite, rhyolite)
• Can occur in various settings
  • Within single magma chamber during cooling and crystallization
  • In series of connected magma chambers at different depths in Earth's crust
  • Through interaction of magma with surrounding rock (assimilation)

Role of fractional crystallization

• Key driver of magmatic differentiation involving separation of early-formed crystals from remaining melt
• As magma cools, minerals crystallize according to Bowen's Reaction Series
  • Higher temperature minerals (olivine, pyroxene, calcium-rich plagioclase) crystallize first
  • Dense crystals settle out of melt, removing certain elements from magma
• Removal of early-formed crystals changes composition of remaining melt
  • Melt becomes progressively enriched in silica, alkalis (sodium and potassium), and volatiles
  • These components are incompatible with early-forming minerals
• As melt composition evolves, types of minerals that crystallize also change following sequence described by Bowen's Reaction Series
• Can occur in multiple stages, each producing distinct magma composition and associated igneous rock type
  • Generates full spectrum of igneous rocks from ultramafic to felsic