Chapter 46 Melting Magma Migration and Differentiation

Year 3 · Igneous and Metamorphic Petrology · G331-L1

Explain partial melting decompression volatile effects crystallization and magma differentiation with a simple compositional calculation.

How cooling, weathering, burial, metamorphism and melting connect Earth materials. Original Geology Hub teaching diagram.

This independent lesson follows topics in the supplied University of Peshawar BS Geology curriculum labelled session 2004–05 onward. It is a learning resource, not an official university course or a statement of current degree requirements.

Learning objectives

Distinguish mechanisms that produce partial melt.
Explain how separation changes magma composition.
Relate intrusive geometry to emplacement history.

Producing a melt

Magma is a mixture that may contain liquid, crystals and dissolved or exsolved gases. Most mantle is solid, so melting requires an explanation. A hot rising mantle parcel can begin to melt as pressure falls faster than temperature, crossing its solidus. Adding water can lower the solidus under suitable conditions. Heat transfer from an intrusion can promote melting of surrounding crust. These mechanisms can operate together.

Partial melting does not normally produce a liquid with exactly the composition of its source. Minerals enter the melt in different proportions, and trace elements partition between solids and liquid. Degree of melting and source composition therefore influence the product. A basaltic melt is not simply “melted basalt”; it can form from a mantle source with a different bulk composition.

Changing magma

As magma cools, minerals crystallize in a sequence influenced by composition, pressure and water. If crystals remain and re-equilibrate, the path differs from one in which they settle, float or are otherwise separated. Fractional crystallization removes selected components and changes the residual liquid. Assimilation incorporates surrounding material, and magma mixing combines distinct magma batches. Similar final compositions can arise through different processes.

A magma series is a compositional association, not proof that all its rocks came from one chamber. Tholeiitic, calc-alkaline and alkaline descriptions require suitable chemical data and classification criteria. Do not infer a complete series from hand-specimen colour alone. Geochemical diagrams must use the correct units, normalization and alteration checks.

Migration and emplacement

Buoyancy, pressure gradients and fractures help magma move through the crust. Dykes cut across host layering, while sills are broadly concordant with it. Stocks and batholiths describe larger intrusive bodies using mapping conventions; their exposed shape is only part of the original three-dimensional system. Chilled margins, cross-cutting relations and contact effects help establish sequence and emplacement conditions.

Crystals record more than a single cooling event. Zoning, reaction rims and resorbed cores can indicate changing conditions or mixing. Describe those features before selecting a mechanism. An internally consistent interpretation links field geometry, petrography and chemistry rather than relying on one familiar rock name.

Explain the solidus and degree of melting

The solidus marks the onset of melting for a specified composition and pressure, while complete melting occurs only at higher conditions appropriate to the system. Between them, solid and liquid coexist. The earliest liquid is controlled by mineral proportions and phase equilibria, so it need not match the bulk source. Increasing the degree of melting changes both the liquid and the residue. A statement that “the mantle melted” should therefore specify partial melting and its context rather than imply that an entire region became a uniform liquid.

Use mass balance to test a differentiation claim

An illustrative 200 kg magma contains 100 kg of a component. Removing 50 kg of crystals containing 15 kg leaves 150 kg melt with 85 kg of that component, or about 56.7%. The removed crystals contain 30%, less than the initial magma’s 50%, so the residual liquid becomes enriched. Reversing the crystal composition reverses the trend. This simple accounting shows why “crystal removal” is not automatically enrichment in every element. A realistic model must satisfy balances for all relevant components and use the actual minerals and conditions.

Distinguish assimilation from mixing

Assimilation incorporates material from surrounding rocks, potentially changing melt composition and consuming thermal energy. Mixing combines magmas or melts with different compositions and temperatures; incomplete mixing may preserve enclaves or contrasting crystal populations. Neither process is established by one unusual grain. A xenocryst may be inherited from wall rock or an earlier magma stage, while a resorbed rim indicates a change in stability that can have several causes. Combine textures with chemical zoning, ages where available and field relationships. The same bulk composition can arise by more than one path.

Relate intrusion form to emplacement sequence

A sill follows a broadly concordant surface, but it may step across layers or connect to feeder dykes. Chilled margins indicate relatively rapid cooling against cooler surroundings; baked host rock demonstrates heat transfer. If dyke B cuts sill A and neither is displaced by an observed later structure, their local relative sequence is A before B, while the absence of displacement constrains only that exposed relationship. Large plutonic bodies may assemble through multiple additions rather than a single instantaneous chamber. Map internal contacts and contrasting crystal populations before treating all rocks within a batholith outline as one age and one magma. The result should explain geometry, textures and composition together.

Worked activity

An invented 100 kg magma contains 50 kg of a chemical component. It loses 20 kg of crystals containing 8 kg of that component. The residual melt is 80 kg with 42 kg of the component: 42/80 × 100 = 52.5%.

Meaning: removing crystals with a lower proportion of the component than the original melt enriches the residual melt in that component. This is a mass-balance exercise, not a full silicate phase-equilibrium calculation. Real fractionation models must account for all components and the minerals actually present.

Self-check

Does decompression melting require the mantle to become hotter?
Answer
Not necessarily. Falling pressure can permit melting during ascent.
Does every crystal remain in equilibrium with the melt?
Answer
No. Separation and changing conditions can prevent it.
Can colour alone identify a magma series?
Answer
No. Appropriate compositional evidence is needed.

Chapter 46 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

A hot mantle parcel rises and crosses its solidus mainly because pressure decreases. Which melting mechanism fits?
1. Heat-transfer melting requiring a hotter intrusion
2. Decompression melting
3. Complete crystallization at constant pressure
4. Metamorphic recrystallization without liquid
Answer and explanation
B. Decompression melting Pressure reduction can allow partial melting during ascent without requiring the parcel to become hotter.
A 100 kg magma contains 40 kg of a component. Removing 25 kg of crystals that contain 5 kg of that component leaves what melt concentration?
1. 35%
2. 40%
3. About 46.7%
4. 60%
Answer and explanation
C. About 46.7% The remaining melt is 75 kg and contains 35 kg of the component. 35/75 × 100 ≈ 46.7%.
Which condition most directly distinguishes fractional crystallization from a system where crystals remain and re-equilibrate?
1. A requirement that the original magma contain no crystals
2. The absence of any compositional change
3. A requirement that the magma reach the surface
4. Separation of crystallized material from the remaining liquid
Answer and explanation
D. Separation of crystallized material from the remaining liquid Removing selected crystals changes the components available to the residual melt and limits continued equilibration.
An intrusion cuts across host-rock layering. Which geometric term is appropriate?
1. Dyke
2. Sill
3. Conformable sedimentary bed
4. Metamorphic isograd
Answer and explanation
A. Dyke Dykes are discordant to host layering; sills are broadly concordant.
A rock series contains resorbed crystal cores and reaction rims. What is the most defensible first step?
1. Assign a unique magma-mixing event from texture alone
2. Describe the features and compare possible changes in crystallization conditions
3. Treat every core and rim as the same growth episode
4. Infer a complete tectonic history from grain colour
Answer and explanation
B. Describe the features and compare possible changes in crystallization conditions Such textures constrain changing conditions but can have several causes. Description precedes a tested genetic model.

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