GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Subject Geology
Paper No and Title Metamorphic and Igneous Petrology
Module No and Title Earth as a Heat Engine; Partial Melting and Crystallization
Module Tag II
Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad
Vice-Chancellor Jamia Millia Islamia Delhi
Prof. Devesh K Sinha Department of Geology University of Delhi Delhi
Prof. P. P. Chakraborty Department of Geology University of Delhi Delhi
Paper Coordinator Content Writer Reviewer Prof. Pulak Sengupta
Department of Geological Sciences,
Jadavpur University Kolkata
Prof. Santosh Kumar Department of Geology Kumaun University Nainital
Prof. Pulak Sengupta Department of Geological Sciences,
Jadavpur University Kolkata
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Table of Content
1. Learning outcomes 2. Earth as a Heat Engine
2.1 Introduction
2.2 Temperature profile of the Earth
2.3 Mode of Heat and Material Transport in the Earth 2.3.1 Conduction and Convection
2.4 Plate Tectonics and Movement of the Plates
3. Melting and Crystallization 3.1 Introduction
3.2 Melting and Magma Generation in the Mantle 3.3 Magma and Magmatic Differentiation
3.3.1 Liquid to liquid fractionation 3.3.2 Liquid-Crystal Fractionation
3.3.3 Quantifying the Fractional Crystallization 6. Summary
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
1. Learning outcomes
After studying this module, you shall be able to know:
The major external and internal heat sources of the Earth, which drive the exogenic and endogenic processes of the Earth respectively.
Conceptual basis of heat and material transport through convection current in the mantle, and movement of the tectonic plates.
Generation of magmas in the mantle, and the processes responsible for magmatic differentiation.
Qualitative and quantitative assessment of fractional crystallization.
2. Earth as a Heat Engine
2.1 IntroductionThe Earth is a dynamic planet that provides a stable platform to leave on it.
The Earth differentiated very early in its history into several layers or shells having different compositions. The internal structure of the Earth was determined based on reflection and refraction of compressed (P) and shear (S) seismic waves. Earth has two chief sources of heat energies: the Sun is the external heat engine and the core of the Earth is its internal heat engine, which are responsible for driving the exogenic and endogenic processes as heat engines of the Earth respectively.
During the process of the evolution of the Earth extra-terrestrial materials such as meteorites impacted on the earth where much of the energy of such collisions converted into heat and retained in the Earth. At present the Earth's internal temperature is comparable to that of the Sun's outer regions, and a central core that developed 4.6 billion years later is still about 20 percent hotter than the Sun's surface. The energy that drives this movement is heat within the Earth, which comes from two main sources. One is the residual heat left over from the formation of our planet 4.6 billion years ago. The second source of energy is naturally occurring radio nuclides in the earth, most notably uranium
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
(U), thorium (Th), and potassium (K) which also release energy in the form of heat through spontaneous decay within the deep earth.
These two sources of heat superheated the Earth’s mantle and cause it to rise and sink. A material heated from below gets hotter and rises, reaches the surface, releases its heat, becomes colder and denser, and sinks again. This central furnace probably melted everything, and the iron then sank, relative to lighter material such as silicates, which rose toward the surface, hardened, and became the crustal and upper mantle rocks (Fig. 1). This intense heat energy continues traversing outward through the 6370-kilometer radius of the Earth. Most geophysicists believe that the greater of the two sources of energy powering the heat engine.
Fig. 1 Differentiation of Earth into a series of concentric physico-chemical layers of differing composition and density. Molten iron and nickel sank to form the core whereas lighter silicates flowed up to form mantle and crust.
2.2 Temperature Profile of the Earth
Much of the radiogenic elements are extracted from the mantle during melting and reside in the continental crust where they are in order of 200 times greater than the mantle’s concentration. Because of the great extent of the mantle its radiogenic heat production is very significant. The Earth's liquid outer core is also a major contributor to heat production at the surface.
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
The exact temperature profile of the Earth is quite difficult to determine and is generally inferred from melting experiments on materials that are analogues to the deep Earth (Fig. 2).
Fig. 2 A typical temperature profile in the Earth which comprises three main parts: 1. The upper part shows a very steep gradient in the crust where heat is transported mostly by conduction. 2. Beneath that the gradient is shallower in the mantle. 3. A steep gradient between the lowermost mantle and outer core.
2.3 Mode of Heat and Material Transport in the Earth
2.3.1 Conduction and Convection: Regardless of source, heat energy is transported in the Earth by two primary mechanisms. First one is conduction in which temperature of an object is raised and heat flows in the direction of cooler regions by diffusion as the molecules in the object vibrate more vigorously. Mass is however not transported by conduction. Crustal materials are rigid and therefore temperature
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
gradient is high because conduction is very efficient in some good conductor materials like metals.
On the other hand, convection is a circulatory motion of heated materials of contrasting temperatures. Melts and other weak materials heated in one area (usually from beneath) experience such motion. In convection the moving materials carry the heat, and generally also conduct heat at the same time. However, the transport of heat by convection in a fluid is usually much more efficient than by conduction. In the Earth's mantle convection is the dominant mechanism of heat transport, although conduction may take place as well. The mantle is considered as a viscous fluid capable of flow albeit very slowly.
2.4 Plate Tectonics and Movements of the Plates
Alfred Wegener proposed a theory of continental drift in 1915 that gave birth of revolutionary idea of plate tectonics in the late 19th century that there are striking similarities in structure, lithology, and flora-fauna distribution on the micro and mega continents which were once parts of a supercontinent named as Pangea, Gondwanaland, Rodinia, and Columbia, etc. The motion of tectonic plates and associated volcanic and earthquake activities is believed to be the consequences of thermal convection occurring in the Earth’s mantle (Fig. 3).
The rigid outer layer of our planet Earth, called the lithosphere, is the cold, occupying top boundary of convection cells in the mantle. Lithosphere is a rigid mass that means they can bend but cannot flow. On the other hand, the asthenosphere behaves as plastic materials, which can flow in response to deformation. Even though it can flow, the asthenosphere is still made of solid (not liquid) rocks. Deep inside the Earth, hot rocks (above about 1300°C) can flow, whereas cold rocks cannot. The lithosphere breaks into the rigid plates which ride on top of the flowing asthenosphere. In terms of chemical composition, there is no difference between the upper part of the
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
asthenosphere and the lower part of the lithosphere. In fact, if the upper part of the asthenosphere cools down it becomes part of the lithosphere.
Fig. 3 The convection current (shown as arrows) occurs within the Earth as hot, less dense portions of the mantle rise and displace the cooler, denser rocks, which then sink into the mantle. Lithosphere moves over the asthenosphere. The underlying "flow" of the materials in the mantle, called mantle convection, drives geological phenomena at the Earth's surface, ranging from earthquakes and volcanoes to the creation of mountains and oceans. Thus the convection plays an important role in the influencing the dynamic nature of the Earth.
Over the millions of years, the dynamics of tectonic plates rearrange the surface configuration of the Earth. Collision between plates has squeezed and accreted the ancient oceans and continents, and produced majestic mountain ranges such as Alps and Himalaya. Thus there are tremendous underlying forces and energies inside the Earth that build and shaped the Earth’s surface manifestation in time and space that operated through synchronous tectonic and rock cycle.
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
3. Melting and Crystallization
3.1 IntroductionMelting point of a solid is the temperature at which the solid starts melting at a given pressure (e.g. melting point of ice at 1 atmosphere is ~0oC). Freezing point of a substance on the other hand, refers to temperature of liquid at which the liquid starts crystallizing at a given pressure (e.g. freezing temperature of water is 0oC at 1 atmosphere). Melting point and freezing point of any substance may or may not be the same. The complete melting of rocks does not occur in nature except in a very unusual situation such as meteorite impacts or shear heating during earthquake. Generally, magma is produced by a process of partial melting where temperature is sufficient to melt a fraction of the source materials (protolith or reservoir) but never melts completely. For example, partial melting of an ultramafic source (e.g.
peridotite and eclogite) will produce a melt (e.g. basalt) less magnesian than the source material leaving behind a refractory solid residue which is more magnesian than the source material before the initiation of melting. The composition of the melt and residual solid depends on the degree of melting that is primarily temperature dependent.
Magma as mixtures of melt plus crystals ascends to a higher level along fractures or as diapirs, because it may crystallize during the course of its accumulation, separation from source, enroute ascend, and at emplacement level (sink regions). However, magma may contain some residual phases derived from the source region. Crystallization of magma may be fractional or equilibrium. In fractional crystallization crystals are continuously removed from the magma from which it crystallizes by some physical processes (e.g. gravity settling) whereas in equilibrium crystallization chemical equilibrium is maintained throughout the crystallization process.
Idealized condition of equilibrium crystallization is never achieved in nature but both fractional and equilibrium crystallization can operate concurrently (Sha, 2012).
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
3.2 Melting and Magma Generation in the Mantle
Mantle is not a static body because interior of the Earth is very hot, and the solid mantle undergoes continuous, although very sluggish, convective movement with mushroom-like plumes of buoyant hotter material which ascends from below (Fig. 4a) whereas dense colder material sinks down (e.g.
cold oceanic lithosphere at subduction zone as shown in Fig. 4b). Solidus temperature of peridotite mantle increases around 10-12°C/Kbar; for an increase of 30 Kbar (~100 km) the solidus temperature increases by 330°C.
To initiate melting the ambient temperature of the mantle at any given pressure (expressed by geothermal gradient) should cross the solidus of the mantle peridotite.
In the mantle there are at least two broad scenario of partial melting by which the geothermal gradient intersects the mantle solidus. (1) Perturbation of lithospheric geotherm by a deep up-welling mantle plume causes shifting of geotherm towards higher temperature which intersects the solidus, and consequently melting begins (Fig 4a), or addition of fluids to mantle peridotite at, for example, subduction zones, thus decreasing its liquidus and solidus which results in melting (Fig. 4b). (2) Another situation is adiabatic decompression melting that takes place in rising mantle diapirs, known as decompression melting which is primary cause of magma generation at mid- ocean ridges (Fig. 4c).
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Fig. 4 Schematic sketch illustrating the initiation of melting in the upper mantle. There are at least two ways by which the geothermal gradient intersects the mantle solidus: 1) either perturbation of lithosphere geotherm by a deep up-welling mantle plume (A), or addition of fluids to mantle peridotite at, for example, subduction zones, thus decreasing its liquidus and solidus (B) and 2) adiabatic decompression melting that takes place in rising mantle diapirs (C). (after Vijay Kumar and Rathna, 2014).
3.3 Magma and Magmatic Differentiation
Magmas are commonly considered as high temperature, high entropy silicate solutions ranging in wide compositions from ultramafic komatiite, through mafic and intermediate (andesite), to silica-saturated and silica- undersaturated felsic igneous rocks. The diversity in mineralogical and geochemical compositions of these rocks suggests their origin and evolution from a few primary magmas by the processes known as magmatic differentiation. It is, however, very difficult to recognize the primary magmas because of uncertainty in the composition of the source region.
Most igneous rocks are members of a rock association in space and time, which may be part of a comagmatic suite, evolved by the processes of fractional differentiation.
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Most viable models of magma production and evolution can be proposed based on textural, mineralogical and chemical criteria. In most cases, two essentially important processes, fractional crystallization (separation of crystals from parental melts) and mixing (hybridization) of magmas, separately or concurrently, have been suggested responsible for the textural and chemical evolution of magma. Many models have been proposed to explain the magmatic differentiation. Magmas may differentiate into crystal- free liquid state (e.g. liquid immiscibility, thermo-gravitational diffusion and melt-melt interaction), crystal-liquid fractionation, and crystal-charged magma mixing (Fig. 5). The later processes are now considered most important in the evolution of magmatic bodies whereas some processes in liquid state are not so significant.
Fig. 5 A summary of the major magmatic processes responsible for magmatic differentiation (modified after Wilson, 1993). Magmatic differentiation can be broadly divided into processes of differentiation in the liquid state and crystal-liquid differentiation or fractional differentiation.
In the classical work of Harker (1909) it was already realized that the great diversity of igneous rocks and compositional variations within many rock bodies could be attributed to processes of fractional differentiation.
Advancement in theories and ideas of magmatic differentiation, postulated that the geochemical variations alone cannot point to the operative processes.
liquid
thermogravitational diffusion
liquid immiscibility magma
mixing
magma
crystal charged
magma mixing
crystal + liquid
fractional crystallization assimilation
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
For example, near-linear Harkers’ variation diagrams (Fig. 6) can be caused by fractional crystallization, mixing of magma end-members, and melt-restite unmixing (Clemens, 1989). Thus the geochemical variation diagrams are unimportant for process diagnosis, provided they are not used in combination with field, textural and mineral-chemical data. Most igneous rocks preserve many textural and chemical characteristics that point to differentiation processes involved in their formation.
Fig. 6 Linear geochemical variation of igneous rocks can be viewed as:
I. Fractional differentiation of a primary magma (A) forming cumulates (B) and subsequently evolved residual or differentiated magma (C). Element ‘X’
can be chosen as “differentiation index”.
II. The same geochemical variation can be generated by the mixing of magma end-members ‘A’ and ‘B’ in various proportions, forming members of hybridized igneous rock suite.
III. The same geochemical variation can be formed by the process of melt- restite unmixing i.e. partial melting of a source region (protolith ‘A’), which will form a small melt fraction at ‘B’ and with the progress of melting degree the resultant melt – will follow the compositional path B-B'-B''. (Kumar, 2014).
3.3.1 Liquid to Liquid Fractionation: An initially homogeneous magma may separate into two or more compositionally distinct magmas by the processes of liquid immiscibility. For example, many tholeiitic basalts contain two co-existing glass phases (quenched melts). There is credible laboratory evidence of liquid immiscibility between alkali silicate liquid and carbonate rich fluids, which forms strong genetic links to understand the evolution of the carbonatite - ijolite -nephelinite
B
C
cummulate primary magma
evolved / residual / differentiated magma
Element 'X'
Element 'Y'
I
A
magma 'A'
Element 'X'
Element 'Y'
magma 'B'
II
A
Element 'X'
Element 'Y'
melt B melt B' melt B''
source (protolith)
increasong degree of melting
III
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
rock association. Such unmixing or magma splitting is restricted to magmas of evolved composition and is not significant to differentiation of more primitive magmas.
Compositional gradients in magmas may cause diffusion and redistribution of elements by what is called as the Soret effect. It may also occur in homogeneous, non-convecting magmas that are subjected to thermal gradients. Unlike the normal trend in fractional crystallization the hotter-end of magma chamber may be silica-rich whereas the cold-end may be iron-rich mafic compositions, which may persist in stagnant magma that usually develop the border zone. It is therefore suggested that while dealing with chilled margin materials care must be taken because gradational compositions of plutonic body may have been produced by the Soret effect.
The diffusion of chemical species in silicate melts governs the kinetics of the most magmatic processes including partial melting, fractional crystallization, magma mixing and crystal growth. Different components of a silicate melt might diffuse in different directions, depending upon diffusion coefficient, in the same temperature gradient.
In a recent contribution (Perugini et al., 2006), it is found that even at a micrometric length-scale small volume of magmas is strongly influenced by coupled action of chemical diffusion and chaotic flow fields because of ‘diffusion fractionation’ process. Some of the co- existing magmas of contrasting temperature and compositions may partially to completely equilibrate in their contents of chemical species by the process of chemical diffusion (Kumar and Rino, 2006). Mixing of two-liquid may define straight line on variation diagrams so long as they are not concurrently fractionating.
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
3.3.2 Crystal-Liquid Fractionation: Crystal fractionation (or fractional crystallization) is considered as the dominant process of magmatic differentiation, where an effective physical separation of phases, normally one liquid and the others crystalline, takes place. Initially gravitational segregation of crystals was thought to as appropriate mechanism of forming the cumulate on the floor or wall of the magma chamber but on closer examination it is clear that other mechanisms such as in-situ crystallization, flowage differentiation, diffusive exchange, compaction (filter-pressing) and convective exchange may also be equally effective. This is because many crystallizing magmas behave as Bhingham liquids, and thus even the dense ferromagnesian minerals may not be able to sink if they are unable to overcome the field strength of the magma. Flowage differentiation calls upon shear stresses in magma to help move the crystals. Convecting magma can transport early crystals in suspension to a distant depositional site. In filter pressing, a mat of crystals compacts under its own weight and expels less (or more) dense interstitial melt. Another mechanism is gas- driven filter pressing that separates melts from crystals (Fig. 7). Melt flows through a dense mat of groundmass crystals in processes driven by the differential pressure between small, recently-nucleated vesicles (higher pressure) and larger, early-formed vesicles (lower pressure).
The small vesicles were formed because crystallization of anhydrous groundmass minerals resulted in exsolution of gases, a process commonly referred to as second boiling.
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Fig. 7: Effective gas-driven filter pressing (not to scale)
A. Gas-saturated residual liquid from nearly solidified magma is driven out a propagating fracture to form an aplitic dike (diagonal line).
B. Mafic recharge magma (cross-hatched) ponds near base of the felsic magma reservoir (unpatterned), above earlier cumulates and mafic sheets (dashes). Rapid crystallization and vesiculation within recharge magma drives evolved liquid (arrows) into overlying felsic magma reservoir (after Sisson and Bacon, 1999).
In-situ crystallization is commonly evident by mineral assemblage that includes zoned crystals as an extended sequence of crystallization, particularly true in the case of small and relatively rapidly cooled small igneous body like Skaergaard intrusion. In fractional crystallization (Rayleigh distillation law) equilibrium is assumed only between the surface of the crystallizing phases and the melt, and crystallized minerals are assumed to get isolated from the residual melt and accumulated on the floor or walls of the magma chamber. The liquid path (LLD: liquid line of descent) for fractional crystallization is almost identical to that for equilibrium crystallization but crystal path (XP) is quite different (Fig. 8).
B A
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Fig. 8 Hypothetical ternary system A-B-C showing a. equilibrium crystallization of a melt with ‘x’ composition, and b. fractional crystallization of melt ‘x’. Dashed curve indicate rock hop. In fractional crystallization crystal path (XP) makes only the bulk composition of crystals presently crystallizing, not the bulk composition of all crystals removed since the beginning of crystallization (after Ragland, 1989). LLD: Liquid line of descend marked by arrow with continuous lines; XP: crystal path; Cotectic boundaries separating A+L, B+L, and C+L primary phase field meet at eutectic in both the ternary diagrams.
Assimilation of crustal-rocks (deeper lithology and/or country-rocks) could be an important process in the compositional diversification of magmas, particularly in deep-crustal magma reservoir. Assimilation coupled with fractional crystallization (AFC) can be an important process in the petrogenesis of several continental-derived magmas.
Crystal-charged magmas may mix together forming mingled and hybrid magma zones. Magma-mixing causes homogenization of the interacting melt phases and conversion of early crystals to partly dissolved (corroded) in hybrid magma, whereas mingling or comingling involves partial mixing interpenetration of contrasting magmas without pervasive changes.
Most magma chambers are episodically replenished by new pulses of magma, periodically tapped and continuously fractionated. In a magmatic system undergoing paired recharge and fractionation, the LLD for major element is similar to that produced by fractional crystallization. For example, in a simple ternary system crystallizing
A + L
B + L C + L
X A
C B
(a) LLD
XP
A
C B
X
(b)
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
olivine-crystallization-plagioclase (Fig. 9), adding a pulse of more primitive magma will push it back into the olivine phase field from where it will evolve back towards the olivine-clinopyroxene cotectic.
In such a delayed long-term situation the amount of olivine + clinopyroxene fractionated from the system will be higher than those in a closed system.
Fig. 9 Magma–mixing viewed in the context of simplified ternary system. Magma ‘a’ in a chamber crystallizing the assemblage ol+pl+cpx would lie at the ternary eutectic ‘c’. Refluxing the chamber with new pulse of magma ‘a’, followed by complete mixing, would generate a new magma composition a' which would evolve back towards the eutectic ‘c’ along the LLD a'-b'-c (after Wilson, 1993).
Abbreviation after Kretz (1983).
We have seen that the Harker variation diagram (Fig. 6) is not much potential to examine the petrological hypothesis precisely, unless combined with field and petrographic evidences. Compositional data of many igneous rock sequences may show good coherence in their variation, which suggest fractional crystallization has played a dominant
Olivine Clinopyroxene
Plagioclase
pl + liq
ol + liq
cpx + liq a
a' b
b' c
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
role but it must be tested against petrographic or other evidence. Often a hypothesis emerges first from a consideration of petrography and is then tested against chemical data-set. In the formulation of crystal fractionation hypothesis, the constructed chemical variations should be capable of showing both the liquid and fractionating minerals, which is possible in a two-element variation diagram, commonly referred to as
‘mixing’ calculation (Cox et al., 1979). The diagrams constructed based on principle of mixing calculation are capable to explain addition or subtraction (or ‘extract’) of phases but do not imply a specific mechanism. Basic principle of mixing calculation lies in the lever rule as commonly used in the phase diagrams. Two chemical parameters X and Y may represent percentages of oxides or parts per million of trace elements or any other weight expression of analytical data (Fig. 10).
Addition of ‘P’ composition to ‘Q’ the resulted mixture M will evolve in a straight line Q-P, depending upon the relative proportions of Q and P in the mixture M.
Fig. 10 X-Y element variation showing the evolution of mixture M in a straight line as a result of addition of P to Q (after Cox et al, 1979).
wt % X w t % Y
Q
P
M
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
At any specific point of mixture such as M the proportion of two end- members P and Q are given by:
Weight of P / Weight of Q = QM / PM and the percentage amounts of end-members are:
Weight % of P = 100 QM / PQ Weight % of Q = 100PM / PQ 3.4 Quantifying the Fractional Crystallization
Fractional crystallization (Rayleigh model) is widely used to constrain geochemical evolution of a crystallizing melt as described by the equation (Gast, 1968; Hanson, 1989)
CLi = Coi F (Di-1)
Where, CLi = concentration of element i in the residual melt, COi = concentration of element i in the initial melt, F = fraction of melt left, and Di
= bulk partition coefficient of the crystallizing mineral assemblage for element i.
The distribution coefficients (Kds) for various phases in basaltic magma are experimentally determined from glass and phenocryst relationship.
Recently, in the modeling of fractional crystallization the damping effect of Mg/Fe during the progress of fractionation that leads to erroneous result, have been checked by the use of multiphase Rayleigh fractionation (Morse, 2006). The modified Rayleigh equation for multiphase fractionation is:
C = Co FL f α (D - 1)
where Co = initial composition, FL = fraction of liquid remaining, fα = fraction of the active crystal phase relative to total crystals, and D = bulk partition coefficient X1S / X1L.
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
4. Summary
The Sun and the core of the Earth act as major sources of heat energies, which are responsible for driving the exogenic and endogenic processes of the Earth respectively. Magma is produced by a process of partial melting when temperature is sufficient to melt a fraction of the source materials but never melts completely.
Magma can be defined as high temperature, high entropy silicate solutions ranging in wide compositions from ultramafic mafic, intermediate, to silica-saturated and silica-under saturated felsic igneous rocks. Magma as mixtures of melt plus crystals ascends to a higher level along fractures in network style or as diapirs, and crystallizes during the course of its accumulation, separation from source, enroute ascend, and emplacement at higher levels (sink regions). The diversity in mineralogical and geochemical of igneous rocks suggests their origin and evolution from a few primary magmas by the processes known as magmatic differentiation.
Magma may differentiate into crystal-free liquid state, crystal-liquid fractionation and crystal-charged magma mixing etc. and therefore magma may differentiate through mixing of crystals and liquids in various proportions.
Frequently Asked Questions-
Q1. What are the major sources of heat within the Earth?
Q2. What are the consequences of thermal convection occurring in the Earth’s mantle?
Q3. Define magma in thermodynamic sense?
Q4. How does the magma in the upper mantle generate?
Q5. What are the major and minor processes responsible for magmatic differentiation?
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Multiple Choice Questions-
1. Magma in the subduction zone is generated because of a. partial melting of oceanic crust
b. dehydration melting of pelitic rocks c. partial melting of Radiolarian chert d. all of the above
Ans: a
2. Addition of water in the melting system
a. depresses the solidus towards lower temperatures b. solidus moves towards higher temperature c. decompression melting starts
d. changes the geothermal structure Ans: a
3. In the fractional crystallization and assimilation (FCA) model, assimilation involves
a. interaction of melts with solids
b. interaction between two or more melts c. interaction between melt and vapours d. all of the above
Ans: a
4. Igneous rocks that are related to a common source are said to be a. contaminated
b. co-magmatic c. differentiated d. hybridized Ans: b
5. Magma-mixing and assimilations are examples of a. closed system magmatic processes b. open system magmatic processes c. equilibrium process
d. none of the above Ans: b
6. Interaction of melt-melt is called a. assimilation
b. crystallization and assimilation c. partial melting
d. magma mixing Ans: d
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
7. Magma mixing leads to
a. accumulation of early crystals b. heterogeneity
c. homogeneity d. both b & c Ans: c
8. Mafic schlierens may be formed by the process of a. fractional crystallization
b. flowage differentiation c. convection current d. both b & c
Ans: b
9. Bimodal tholeiitic and alkaline igneous rock associations are found at a. subduction zone
b. mid-oceanic ridges c. rift environment d. none of the above Ans: c
10. The motion of tectonic plates and associated volcanic and earthquake activities is the consequences of
a. thermal convection in the mantle b. conduction in the mantle
c. mantle adiabate d. mantle up-welling Ans: a
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Paper: Metamorphic and Igneous Petrology Module: Earth as a Heat Engine; Partial Melting and Crystallization
Suggested Readings:
1. Anderson, Don L. (1989). Theory of the Earth. Blackwell Scientific Publications, Oxford. ISBN: 0865423350, 978-0865423350.
2. Clemens, J. D. (1989). The importance of residual source material (restite) in granite petrogenesis: a comment. Journal of Petrology, 30(5), 1313-1316.
3. Cox, K. G., Bell, J. D., & Pankhurst, R. J. (1979). The Interpretation of Igneous Rocks. George Allen & Unwin, London, p. 450. ISBN: 978- 0412534102.
4. Don, R. (2008) Earth as Heat Engine. Department of Geology, San Jose State University, http://oceansjsu.com/105d/exped_commotion/6.html.
5. Fourcade, S., & Allegre, C. J. (1981). Trace elements behavior in granite genesis: A case study The calc-alkaline plutonic association from the Querigut complex (Pyrénées, France). Contributions to Mineralogy and Petrology, 76(2), 177-195.
6. Fowler, C. M. R. (1990). The Solid Earth, p. 472. Cambridge University Press, New York.
7. Ganguly, J. (2005). Adiabatic decompression and melting of mantle rocks:
An irreversible thermodynamic analysis. Geophysical Research Letters, 32(6).
8. Gast, P. W. (1968). Trace element fractionation and the origin of tholeiitic and alkaline magma types. Geochimica et Cosmochimica Acta, 32(10), 1057-1086.
9. Hanson, G. N. (1989). An approach to trace element modeling using a simple igneous system as an example. Reviews in Mineralogy and Geochemistry, 21(1), 79-97.
10. Harker, A. (1909). The natural history of igneous rocks. Methuen, London.
11. Kretz, R. (1983). Symbols for rock-forming minerals. American mineralogist, 68, 277-279.
12. Kumar, S. (2010). Mafic to hybrid microgranular enclaves in the Ladakh batholith, northwest Himalaya: Implications on calc-alkaline magma chamber processes. Journal of the Geological Society of India, 76(1), 5-25.
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