3.4 Bowen's Reaction Series and magmatic differentiation
4 min read•july 22, 2024
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
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- Describes sequence of mineral from cooling magma based on experimental work by Norman L. Bowen in early 20th century
- Two main branches: discontinuous series ( → pyroxene → amphibole → biotite) and continuous series (plagioclase 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 ( and gabbro from pyroxene, calcium-rich plagioclase)
- Felsic rocks form from late-stage minerals ( 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
- Early stages produce ultramafic and mafic rocks (peridotite, basalt, gabbro)
- 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 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 ()
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
Key Terms to Review (17)
Assimilation: Assimilation is the geological process where a body of magma incorporates surrounding rock material, leading to changes in its composition and characteristics. This process can significantly affect the evolution of igneous rocks, as the added materials alter the chemical makeup and mineral content of the magma, influencing the resulting intrusive structures and landforms, as well as the differentiation processes within the magma itself.
Basalt: Basalt is a fine-grained, dark-colored igneous rock that forms from the rapid cooling of lava at or near the Earth's surface. It is the most common type of volcanic rock and is primarily composed of plagioclase feldspar and pyroxene, giving it a distinctive texture and mineral composition. Its formation process connects closely to the classification of igneous rocks and can illustrate how different cooling rates and mineral content contribute to the variety seen in these rocks.
Bowen's Reaction Series Diagram: Bowen's Reaction Series Diagram is a scientific representation that illustrates the sequence of mineral crystallization from magma as it cools and solidifies. This diagram helps to understand how different minerals form at various temperatures and compositions, revealing the process of magmatic differentiation where the composition of the melt changes as certain minerals crystallize out first, influencing the characteristics of the resulting igneous rock.
Crystallization: Crystallization is the process by which solid crystals form from a homogeneous solution, melt, or gas, typically as minerals precipitate from magma or fluids. This process is fundamental to the formation and classification of minerals, as well as playing a vital role in the rock cycle, where it leads to the creation of various rock types.
Feldspar: Feldspar is a group of rock-forming minerals that make up about 60% of the Earth's crust, characterized by their aluminum silicate composition. They are crucial in identifying and classifying various rock types due to their abundance and properties, impacting both igneous and sedimentary formations in the Earth's geology.
First minerals: First minerals are the initial crystalline phases that form from molten rock as it begins to cool and solidify. They are essential to understanding the process of magmatic differentiation, as they set the stage for the evolution of different mineral compositions in igneous rocks, reflecting varying temperatures and conditions of crystallization.
Fractional crystallization: Fractional crystallization is a process where different minerals crystallize from magma at different temperatures, leading to the separation of solid minerals from the liquid phase. This method is crucial in understanding how the composition of magma changes over time as it cools and differentiates, ultimately influencing the types of rocks that form. By analyzing this process, one can gain insights into magma properties and the evolutionary paths of magmas as they undergo differentiation.
Granite: Granite is a coarse-grained igneous rock primarily composed of quartz, feldspar, and mica, known for its durability and use in construction and decorative stone. This rock forms from the slow crystallization of magma beneath the Earth's surface, giving it a distinctive texture and mineral composition. Understanding granite's characteristics helps in classifying igneous rocks and provides insight into magmatic processes such as Bowen's Reaction Series and magmatic differentiation.
Last Minerals: Last minerals are the final minerals to crystallize from a cooling magma, typically forming at lower temperatures and higher pressures. They represent the end stage of the crystallization sequence in Bowen's Reaction Series, showcasing how different minerals emerge from magma as it cools and evolves, which also ties into magmatic differentiation processes that influence the composition of igneous rocks.
N.L. Bowen: N.L. Bowen, or Norman L. Bowen, was a Canadian geologist renowned for his groundbreaking work in understanding the processes of magma formation and crystallization, particularly through Bowen's Reaction Series. His studies helped illuminate how different minerals crystallize from magma at varying temperatures and pressures, leading to insights into magmatic differentiation.
Olivine: Olivine is a common silicate mineral composed of magnesium iron silicate, represented by the formula (Mg, Fe)₂SiO₄. It typically occurs in igneous rocks, particularly basalt and peridotite, and plays a crucial role in understanding the formation and classification of these rocks, as well as the processes that occur during magma differentiation.
Partial Melting: Partial melting is the process by which only a portion of a solid rock melts to form magma, leading to the differentiation of minerals within the rock. This phenomenon plays a crucial role in the formation of various types of igneous rocks, as the composition of the resulting magma can vary significantly from the original rock. As different minerals have different melting points, the minerals that melt first will differ from those that remain solid, impacting the overall chemistry and characteristics of the magma produced.
Phase Diagram: A phase diagram is a graphical representation that shows the relationships between the different phases of a substance as a function of temperature, pressure, and composition. This tool is essential for understanding how minerals crystallize and behave under varying conditions, particularly in the context of magmatic differentiation and Bowen's Reaction Series, where it helps illustrate how different minerals form at specific temperatures during cooling.
Plutons: Plutons are large, intrusive igneous rock bodies that form when magma cools and solidifies beneath the Earth's surface. They can vary in size from a few kilometers to hundreds of kilometers across and are typically composed of coarse-grained minerals due to the slow cooling process. The formation of plutons is closely linked to processes like Bowen's Reaction Series, which outlines the crystallization sequence of minerals from magma as it cools.
Silica Content: Silica content refers to the proportion of silicon dioxide (SiO2) present in a rock or magma, which is a key factor in determining its composition and properties. The silica content influences the behavior of magmas during their cooling and crystallization processes, as well as the type of igneous rock that forms. Higher silica content typically leads to more viscous magmas, while lower silica content results in less viscous, more fluid magmas.
Temperature gradient: A temperature gradient is the rate of temperature change in a given direction within a specific environment, often measured as degrees per unit distance. In geology, this concept is crucial for understanding processes like magmatic differentiation and the formation of different mineral types as magma cools. The temperature gradient influences how various minerals crystallize from molten rock, which is essential for recognizing Bowen's Reaction Series.
Volcanic eruptions: Volcanic eruptions are geological events where magma from beneath the Earth's crust is expelled to the surface, often resulting in the release of ash, gases, and lava. These eruptions can significantly shape landscapes and influence climate patterns, highlighting their importance in geology and the interconnectedness of various geological processes.