Magmatic Differentiation and Assimilation | Geology Optional Notes for UPSC PDF Download

Magmatic Differentiation and Assimilation

4.7: Introduction to Magmatic Differentiation and Assimilation

• The Bowen's reaction series illustrates that early-formed crystals in magmatic processes tend to be denser due to their higher concentrations of Mg and Fe compared to later-formed minerals.
• In 1928, Bowen proposed a model of magmatic evolution centered around the differentiation of primary basaltic magma, which plays a crucial role in the formation of various igneous rocks.

4.7.1: Magmatic Differentiation

• Magmatic differentiation is a process that occurs within magma, involving its solid, liquid, and gaseous phases. It leads to the breakdown of a homogeneous parent magma into fractions with diverse compositions, ultimately giving rise to different types of rocks.
• This process results in the formation of rocks with varying mineral compositions and structures, such as granite, diorite, and gabbro, which are all products of magmatic differentiation.
• For example, when magma undergoes magmatic differentiation, minerals like olivine, pyroxene, and plagioclase crystals may form at different stages due to variations in cooling rates and mineral densities.

Magmatic Differentiation

Introduction

• We must understand that the process of magmatic differentiation deals with variations that occur within a uniform body of magma, not the heterogeneity produced by different magmas' intrusion or assimilation of foreign rock material.

Mechanisms of Magmatic Differentiation

Magma in Fluid State

• When magma is in a fluid state, several processes come into play.
• Liquid Immiscibility
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  • A mixture of components that are homogenous at a certain temperature can separate into immiscible parts as the temperature decreases. This separation explains the presence of sulphides in silicate magma.
• Liquid Fractionation
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  • This process is crucial in magma differentiation, where compounds tend to accumulate in cooler areas due to thermal gradients, leading to stratification under gravity's influence.
• Movement of Volatiles
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  • Volatiles act as selective solvents, transferring material within magma, creating inhomogeneity. Pressure release causes volatiles to move towards lower pressure areas.

Solid Mineral Phases Involved

• Fractional crystallization occurs when solid mineral phases are part of magmatic crystallization.
• Fractional Crystallization
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  • As magma cools, mineral crystallization begins, often concentrated at margins due to faster cooling. This leads to differentiation between felsic (core) and mafic (outer) parts of the body.
• Equilibrium Maintenance
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  • Early-formed crystals react with liquid to maintain equilibrium, as seen in Bowen's reaction series. For example, plagioclase crystals start rich in calcium and become sodic as temperature decreases.

Igneous Petrology-II Summary

Bowen's Reaction Principle

• Bowen's reaction principle explains how a primary basaltic magma can solidify into various rocks like gabbro, dunite, diorite, granodiorite, or granite. This transformation depends on crystal fractionation and the removal of early minerals from the magma.

Fractional Crystallisation Mechanisms

• Gravity Settling
• Filter Pressing
• Flowage of Magma
• Crystal Zoning

Gravity Settling

• Crystal fractionation involves the sinking, floating, or suspension of early-formed crystals within the magma body.
• Monomineralic rocks like dunite, pyroxenite, and anorthosite are created through this process.
• Accumulation of crystals forms cumulates, altering the melt's composition and leading to magma evolution.

Magmatic Differentiation

• Results in evolved magma

Crystal Formation

• Occurs as magma cools and crystals settle to the chamber floor.

Separation of Crystals

• Early-formed crystals separate from the melt under gravity's influence in the magma chamber, leading to crystallization and changes in the remaining melt's composition.

Filter Pressing

• Filter Pressing involves mechanically separating magma from the crystal mesh.
• During crystallization, a loose mesh of crystals with residual liquid is formed.
• If deformation occurs, the residual liquid is squeezed out, leaving behind early formed crystals.
• This process is known as filter pressing.

Flowage of Magma

• Magma's flow behavior differs between the center and contact walls as it moves along a pipe or channel.
• Preferential accumulation of early formed minerals can result from this mechanism.
• The rate of magma flow changes rapidly towards the walls compared to the center of an intrusion.

Crystal Zoning

• Crystal Zoning is common in igneous rocks.
• Disturbance in equilibrium during crystallization leads to zones with different compositions.
• For example, calcic plagioclase can be successively rimmed by more sodic-rich species.

Short Answer Questions

• a) Distinguish between discontinuous and continuous series. - Discontinuous series: Involves minerals that do not have a continuous range of solid solutions. - Continuous series: Involves minerals that have a continuous range of solid solutions.
• b) Define magmatic differentiation. - Magmatic differentiation is the process by which a magma body evolves from a uniform composition to a diverse composition through various processes like fractional crystallization, assimilation, and mixing.
• c) List the mechanisms for fractional crystallization. - Mechanisms include crystal settling, filter pressing, and differential crystallization.
• d) How are the cumulates formed? - Cumulates are formed through the accumulation of crystals settling out of a cooling magma body.

Assimilation

We have learned about mechanisms of magma differentiation. Now, let's explore assimilation.

• Assimilation is the incorporation of foreign rock masses by magmas.
• It includes the mingling of two liquid magmas, known as magma mixing.
• Assimilation contributes to the diversity in igneous rocks.
• It involves processes that lead to heterogeneity and incomplete mixing.

Concept of Magma Assimilation

• Assimilation follows Bowen's reaction principle.
• Magma reacts with host rock during intrusion, leading to contamination.
• Factors affecting assimilation include mechanical incorporation, partial solution, and total dissolution.

Factors Influencing Magma Intrusion

• Temperature of magma at the time of intrusion
• Presence or absence of a significant degree of superheat
• Composition of xenoliths or inclusions
• Concentration of volatiles in the magma
• Conditions that either facilitate or hinder the escape of volatiles into surrounding rocks

Magma Interaction with Country Rocks

Magma invades cracks in country rocks. It breaks off rocks from the country rock and slowly melts them.

Igneous Petrology-II

Reaction between Magmatic Intrusion and Wall Rock

Magma melts the walls of the country rock, leading to the formation of xenoliths.

Host Rock Xenoliths

Fig. 4.4: Illustrates the interaction between magmatic intrusion and wall rock.

Fig. 4.5 a): Shows xenoliths of host rock in magmatic intrusion.

Fig. 4.5 b): Depicts xenoliths of basaltic rock in granite.

Unit 4: Magma Mixing

Definition of Magma Mixing

• Magma mixing occurs when two or more magmas with different chemical compositions meet beneath the Earth's surface.
• This interaction results in the production of magmas with compositions that are intermediate between the original magmas.

Factors Influencing Magma Mixing

• Temperature, density, and viscosity differences between magmas can hinder their mixing.
• When magmas have contrasting compositions like rhyolite and basalt, the likelihood of mixing decreases.
• However, if the chemical compositions of the magmas are more similar, the chances of mixing increase.

Examples of Magma Mixing

• Imagine two magmas, one rich in silica (rhyolite) and the other rich in iron and magnesium (basalt), trying to mix. Due to their stark differences in composition, they may not easily combine.
• In contrast, if two magmas have similar compositions, such as two types of basalt, they are more likely to mix together, forming an intermediate composition.

Real-world Illustration

• In a volcanic region, observations have shown instances where magmas of slightly different compositions have mixed. This blending process can lead to unique volcanic eruptions and the formation of distinct types of lava flows.