Bowen's Reaction Principle | Geology Optional Notes for UPSC PDF Download

4.6 BOWEN'S REACTION SERIES

• Geologists use petrographic study to identify different types of igneous rocks on a large scale.
• Petrogenesis is the study of the genetic aspects of igneous rocks.
• Petrologists shifted from a petrographic approach to a petrogenetic approach, focusing on the evolution of igneous rocks.
• Dr. N. L. Bowen's pioneering work in 1922 introduced Bowen's reaction series, outlining the evolution and origin of magmas.
• Bowen's reaction series illustrates how magma solidifies into different types of rocks.

Key Concepts:

• Bowen's reaction series consists of two parallel series: discontinuous and continuous.
• The discontinuous series shows changes in ferromagnesian minerals with decreasing temperatures.
• The continuous series demonstrates the cooling and crystallization of minerals in the plagioclase series.

Example:

• As magma cools, olivine (a ferromagnesian mineral) transforms into pyroxene in the discontinuous series.
• In the continuous series, plagioclase feldspar crystallizes out of the magma as it cools.

Unit 4: Bowen's Reaction Series

• Series and Continuous Series
• Continuous series and discontinuous series converge to form a single series as part of the discontinuous reaction series.
• In the continuous reaction series, minerals react continuously with magma, leading to the formation of the next mineral in the plagioclase series.
• In contrast, the discontinuous reaction series involves new minerals forming when magma reacts with previously formed minerals at specific temperatures.
• Bowen's Reaction Series provides a generalized order in which minerals crystallize from a cooling basaltic magma.
• If early-formed crystals are not removed from the melt, the composition can undergo further changes.
• Once early-formed crystals are removed, the magma reacts with the remaining crystals to produce new minerals.
• The temperature range for Bowen's reaction series is between 1100°C and 573°C.
• The crystallization of silicate minerals starts with spinel group minerals at around 1100°C.

Minerals Order in Bowen's Reaction Series:

• Spinel (MgAl2O4) - Oxide mineral with cubic symmetry, often found as octahedral crystals.
• Ca-rich Plagioclase
• Mg Olivine
• Fe Olivine
• Mg Pyroxene
• Fe Pyroxene
• Hornblende
• Biotite
• K Feldspar
• Muscovite
• Quartz
• Water-rich solution
• Na-rich Plagioclase

Types of Rocks in Bowen's Reaction Series:

• Ultramafics
• Basalts, Hou, and Gabbros (mafic rocks)
• Andesites and Diorites (intermediate rocks)
• Rhyolites, Concept of Magma, and Granites (felsic rocks)

Discontinuous Reaction Series:

Continuing from the previous subsection, the discontinuous reaction series starts with Mg-rich olivine followed by Fe-rich olivine. As olivine forms, it reacts with magma to produce the next mineral in the series. For instance, Mg-rich olivine (fosterite) transforms into Fe-olivine (fayalite) as temperatures drop. This transformation continues until all olivine converts into Mg-pyroxene under ideal conditions. Equilibrium between solid and liquid phases is maintained as magma crystallizes, with early-formed crystals interacting with the liquid to maintain this balance. These minerals are termed 'reaction minerals' in the series.

Igneous Petrology - Bowen's Reaction Series

Discontinuous Reaction Series

• Minerals in magma can change composition as they crystallize.
• For instance, Mg-pyroxene can transform into Fe-pyroxene and then into hornblende and biotite as temperature decreases.

Continuous Reaction Series

• Plagioclase minerals crystallize alongside olivine or slightly later.
• Initial plagioclase crystals are calcium-rich, but as temperature decreases, they become more sodic. This zoning is visible in plagioclase crystals.
• If olivine does not fully react with magma, silica-enriched liquid may result in a mix of olivine, pyroxene, and quartz - known as released minerals.
• Rocks with released minerals are called doliomorphic rocks, indicating incomplete reactions or early mineral relics.

Importance of Bowen's Reaction Series

• Illustrates magma differentiation and diversification.
• Shows the progression from simple silicate structures (like olivine) to complex ones (like quartz).
• States that a single magma type can solidify into various rock types based on fractionation and mineral removal.
• For example, a basaltic magma can solidify as gabbro, diorite, granodiorite, or granite, depending on fractionation and mineral reactions.