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Igneous Processes
Richard J Arculus, Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia
© 2020 Elsevier Inc. All rights reserved.
Introduction 1
Historical Development 2
Forms of Occurrence 5
Tectonic Associations 6
Magma Sources 8
Sources of Heat and Melting Processes 9
Differentiation Processes 10
Temporal Variations in Composition 13
Trace Element and Isotopic Aspects 13
References 14
Further Reading 15
Glossary
Basalt Fine-grained volcanic rock composed mainly of clinopyroxene and plagioclase, with glassy or finely crystalline
groundmass; olivine, Fe-Ti oxides, and apatite are common additional phases.
Cotectic Conditions of temperature, pressure, and composition under which two or more crystalline phases are in equilibrium
with a melt during crystallization or melting.
Eutectic Minimum temperature at which two or more crystalline phases and melt can coexist.
Fractional crystallization Process involving instantaneous separation of crystals upon formation from a cooling melt with
resultant progressive changes in residual melt composition.
Liquidus Locus in temperature-composition space of the surface or volume representing the maximum solubility of a solid
phase in a melt. A system is completely liquid at temperatures above the liquidus.
Magma Mobile mixture of melt together usually with crystals and dissolved gas.
Parental magma High-temperature magma fromwhich a succession of other magmas is derived by fractional crystallization or
other quantifiable processes.
Peritectic A point (or line) on the liquidus surface for which the composition of the melt cannot be given by positive
quantities of the crystalline phases in equilibrium at that point; an equivalent term is reaction point or curve and is typically
produced by the incongruent melting of one or more of the solid crystalline phases.
Phyric A term describing a rock that contains crystals in a fine-grained groundmass; the crystals may or may not be in
equilibrium with a liquid phase now represented by the groundmass.
Primary magma Magma derived directly from a source rock by partial or complete melting without subsequent differentiation
of any kind.
Solidus Locus in temperature-pressure-composition space of the initial melting point of an individual phase or assemblage of
phases. A system is completely solid at temperatures below the solidus.
Introduction
The study of the formation, migration, crystallization, emplacement, degassing, and solidification of molten rock constitutes
igneous geology (from the Latin ignis, meaning fire). Understanding the processes involved combines field and laboratory studies,
experiments, and modeling, both of terrestrial magma types as well as those of other rocky planets, asteroids, and the Moon. These
processes are fundamental to the ongoing formation of the oceanic and continental crust of Earth, and the consequences of recycling
of these crusts back into the Earth’s interior via tectonic plates. The hugely energetic processes involved in the formation of the
earliest terrestrial bodies, together with heat generation through decay of short-lived isotopes extant in the young solar system,
resulted in massive production of liquid rock comprising so-called magma oceans. These were fundamentally important in the
chemical and physical stratification of the planets (including Earth) and asteroidal bodies. The approach toward and eruption of
magma through the solid crust of the Earth can involve energetic fragmentation of magma and explosive degassing. Generation of a
liquid silicate, oxide, metal, sulfide or carbonate requires the ambient temperature to exceed the appropriate local solidus
temperature. Emplacement into and interaction with the crust, accompanied by the processes of crystallization and volatile
exsolution, can result in the formation of important ore deposits.
yclopedia of Geology, 2nd edition https://doi.org/10.1016/B978-0-08-102908-4.00129-6 1
https://doi.org/10.1016/B978-0-08-102908-4.00129-6
2 Igneous Processes
Historical Development
Recognition of the molten origin of some rocks was in dispute in the late 18th century despite the abundant opportunities for direct
observation of the solidification of liquid ejecta from active volcanoes in the Mediterranean (e.g., Stromboli, Vesuvius, and Etna).
A debate ensued between those who believed that all rocks were precipitated from a globe-encircling sea (“neptunists”) and those
who held that cooling from a high-temperature liquid state was responsible for at least some rock types, especially those traceable to
unequivocal volcanoes (“plutonists”). Nevertheless, proximity to the Mediterranean Sea inspired Lazzaro Spallanzani in 1788 to
hypothesize local involvement of seawater in the triggering of explosive eruptions of these volcanoes. Although this concept is out
of date, interaction of water with magma is an important process, leading to some dangerously explosive eruptions that are hard to
predict, such as occurred at White Island (New Zealand) in December 2019.
Acceptance of an igneous origin for some rock types was coupled with the development by H. C. Sorby in the mid-19th century
of petrographic microscopy; widespread adoption of this instrument produced extensive descriptions of the occurrence, diagnostic
optical properties, and types of minerals and their associations, and the textures or fabrics of igneous rocks. Impressed perhaps by
the success of Linnean classification for the biosphere, and concurrent with the rise of Darwinian theory, petrologists were keen to
find some underlying, natural, and hierarchical classification scheme with which order could be made out of a plethora of igneous
rock types. The notion of a primary magma from which all others might be derived by some process of differentiation or
transformation developed as a particularly strong thread in the web of petrological thought.
Comprehensive tabulations of the compositions of igneous rocks from a wide geographic distribution became available through
the efforts of such silicate analysts as H. S. Washington around the turn of the 19th century, obtained through gravimetric,
volumetric, and colorimetric methods, and various direct and indirect classification methods to handle these data became
commonplace. The development of X-ray fluorescence and then solution nebulization/laser ablation coupled with plasma source
mass spectrometric techniques over the last 50 years have dramatically increased the numbers of samples for which compositional
data are now available, and accessible on web-based data bases. The fundamental aim of this effort has been to document and then
account for the compositional range of igneous rocks. Because oxygen is the most abundant (largest atomic fraction) element in the
Earth, it has been conventional among geochemists to report major elemental abundances for most rocks as oxides. Accordingly, the
classification methods currently in use for igneous rocks still rely heavily on the relative abundances of the major rock-forming
oxides (SiO2, Al2O3, FeO and Fe2O3, MgO, CaO, Na2O, K2O, TiO2, and P2O5—see Fig. 1) and the amount and composition of the
major rock-forming minerals (feldspar, pyroxene, olivine, Fe-Ti- bearing oxides, amphibole, mica, and quartz,: see Table 1).
An early difficulty encountered in the study and classification of igneous rocks was how to handle the presence of glass or
noncrystalline, solidified melt. In the absence of a full chemical analysis, but knowing the regular composition of individual
mineral species, it proved possible for the expert petrographer to estimate the bulk composition of a rock from the relative volume
proportions of the constituent minerals. In order to compare the composition of glassy rock types with only sparse- or complete
absence of crystals with fully crystalline materials, the concept of a normative mineralogy was introduced, whereby a given bulk rock
analysis is recast into a set of minerals that, on theoretical grounds, should have crystallizedgenesis and differentiation. A prescient observation by Gast (1968) of the consistent depletion of the light relative to the
heavy rare earth elements in mid-ocean ridge basalts for example, led to a remarkable conclusion: the source regions of these basalts
had experienced previous melt extraction and therefore the mantle source could not have remained unmodified since its formation,
and the basalts were unlikely to be the sought-after, single primary magma type.
Particular trace element abundance patterns have proved to be diagnostic of specific mineral involvement, such as the
fractionation of Eu with respect to the other lanthanide elements. Selective substitution of Eu2+ into plagioclase (substituting for
Ca2+) results in the depletion of this element in that valence state in a coexisting melt relative to other lanthanides, which are
typically present in a trivalent state, together with remaining Eu3+. The persistence of Eu depletions in lunar mare basalts relative to
the other lanthanides, despite lack of evidence for extensive plagioclase fractional crystallization in the basalts, is strong evidence
that the source regions of these magmas in the lunar mantle had experienced prior Eu depletion, logically by sequestration in the
plagioclase-rich rocks forming the Highlands terrain.
Isotopic systematics are used in several interrelated ways in igneous geology. First, as a record of the “age” of a given rock or time
before present when different parent-daughter element pairs were isolated (“blocked”) in various constituent crystalline or glassy
components. Second, the stable isotopes of heavy elements such as Fe, are used as tracers of particular processes and source
materials, being preferentially partitioned on the basis of valence state into mineral phases and melts, in some cases in minutely
different but measurable proportions. Third, the persistence of fractionation effects of stable isotopes of light elements (e.g., H, C, O,
S) at magmatic temperatures permits their use as geothermometers, as well as tracers of volatile loss from magmatic systems and
contamination processes.
Various radioactive parent—radiogenic daughter pairs are particularly important in covering different periods of age resolution
and showing differing degrees of parent-daughter fractionation in igneous events. In the past 50 years, the development of the
techniques for handling Sm-Nd and Lu-Hf isotopic variation has been particularly significant. The radioactive isotopes 147Sm and
176Lu decay (with half-lives of 106 Ga and 35.5 Ga respectively) to 143Nd and 176Hf respectively. It has been assumed that the
Earth’s mantle, when first formed 4.55 Ga ago, was characterized by 147Sm/144Nd and 176Lu/177Hf ratios equivalent to the solar
system average, which is represented by carbonaceous chondritic meteorites. Note that 144Nd and 177Hf are the nonradiogenic
reference isotopes of Nd andHf respectively. Removal of melt fractions from the terrestrial mantle, either in single or multiple events
results in a slight enrichment of Sm with respect to Nd, and Lu relative to Hf, compared with the starting ratios in the residual solid,
and vice versa in the melt. During the passage of millions of years, the variation in 143Nd/144Nd and 176Hf/177Hf in the residual
mantle tends to increase faster than in the chondritic reference value, whereas variation in 143Nd/144Nd and 176Hf/177Hf of a
solidified melt fraction lags behind that of the reference. In fact, it appears from 146Sm/142Nd systematics that the Earth’s mantle did
not originally have chondritic relative abundances of the lanthanides (Boyet and Carlson, 2006); isotopic systematics of many other
elements have now revealed the Earth’s mantle is likely unique in terms of the components that formed its building blocks during
accretion. It has been shown by application of Sm-Nd and Lu-Hf isotopic analysis, that the present-day upper mantle sources of
ocean ridge basalts have suffered at least one (and probably multiple) previous melt extraction events over some time interval prior
to 1.5 Ga ago, and that the complementary melt fraction now appears to reside in the continental crust, albeit much modified by
subsequent intracrustal processing (Hofmann, 2014).
An accelerating trend in modern igneous geology has been the integration of classic petrologic study with constraints established
by trace element and isotopic systematics. As can be seen from the application of Sm-Nd and Lu-Hf isotopic geochemistry, it is clear
that major progress in understanding the chemical evolution and differentiation of the Earth and other terrestrial planets can be
made by these combined approaches. With the current technological advances in the field of high-pressure experimental petrology,
duplication of melting events over the range of depths appropriate for the mantles of the terrestrial planets is feasible, and direct
study is possible of the phase equilibria that may have been applicable in the past, and others that could be relevant in internal
differentiation at present.
References
Albarede F (1985) Regime and trace-element evolution of open magma chambers. Nature 318: 356–358.
Annen C, Blundy JD, Leuthold J, and Sparks RSJ (2015) Construction and evolution of igneous bodies: Towards an integrated perspective of crustal magmatism. Lithos
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Igneous Processes 15
Arculus RJ (2003) Use and abuse of the terms calcalkaline and calcalkalic. Journal of Petrology 44: 929–935.
Arculus RJ (2004) Evolution of arc magmas and their volatiles. Geophysical Monograph 150: 95–108.
Bowen NL (1928) The Evolution of the Igneous Rocks. Princeton: Princeton University Press.
Boyet M and Carlson RW (2006) A new geochemical model for the Earth’s mantle inferred from 146Sm-142Nd systematics. Earth and Planetary Science Letters 250: 254–268.
Brown GM (1956) The layered ultrabasic rocks of Rhum, Inner Hebrides. Philosophical Transactions of the Royal Society of London B 240: 1–53.
Carmichael ISE (1964) The petrology of Thingmuli, a Tertiary volcano in eastern Iceland. Journal of Petrology 5: 435–460.
Coogan LA (2014) The lower oceanic crust. In: Treatise on Geochemistry, 2nd edn., pp. 497–541. Elsevier.
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(eds.) Physical Geology of Shallow Magmatic Systems, pp. 11–38. Berlin: Springer-Verlag.
Davies GF (1999) Dynamic Earth, Plumes and Mantle Convection. Cambridge: Cambridge University Press.
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12: 774–778.
Eales HV and Cawthorn RG (1996) The Bushveld complex. In: Cawthorn RG (ed.) Layered Intrusions, pp. 181–229. Amsterdam: Elsevier.
Fitton JG and Dunlop HM (1985) The Cameroon line, West Africa and its bearing on the origin of oceanic and continental alkali basalt. Earth and Planetary Science Letters 72: 23–38.
Gast PW (1968) Trace element fractionation and the origin of tholeiitic and alkaline magma types. Geochimica et Cosmochimica Acta 32: 1057–1086.
Gill JB (1981) Orogenic Andesites and Plate Tectonics. Berlin: Springer-Verlag.
Hartmann WK (2014) The giant impact hypothesis: Past, present (and future?). Philosophical Transactions of the Royal Society A 372: 20130249.
Hofmann AW (2014) Sampling mantle heterogeneity through oceanic basalts: Isotopes and trace elements. In: Treatise on Geochemistry, 2nd edn., pp. 67–101. Elsevier.
Ishizuka O, Tani K, and Reagan MK (2014) Izu-Bonin-Mariana forearc crust as a modern ophiolite analogue. Elements 10: 115–120.
Jones AP, Genge M, and Carmody L (2013) Carbonate melts and carbonatites. Reviews in Mineralogy and Geochemistry 75: 289–322.Mazza SE, Gazel E, Bizimis M, Moucha R, Beguelin P, Johnson EA, McAleer RJ, and Sobolev AV (2019) Sampling the volatile-rich transition zone beneath Bermuda. Nature
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O’Hara MJ (1977) Geochemical evolution during fractional crystallisation of a periodically refilled magma chamber. Nature 266: 503–507.
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Schmidt MW and Jagoutz O (2017) The global systematics of primitive arc melts. Geochemistry, Geophysics, Geosystems 18: 2817–2854.
Sillitoe RH (2010) Porphyry copper systems. Economic Geology 105: 3–41.
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Further Reading
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Relevant Websites
https://earthref.org/GERM/#gsc.tab¼0—Geochemical Earth Reference Model.
https://www.esc.cam.ac.uk/research/research-groups/research-projects/tim-hollands-software-pages/thermocalc—Thermocalc.
http://melts.ofm-research.org/—MELTS.
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https://www.esc.cam.ac.uk/research/research-groups/research-projects/tim-hollands-software-pages/thermocalc
http://melts.ofm-research.org/
	Igneous Processes
	Glossary
	Introduction
	Historical Development
	Forms of Occurrence
	Tectonic Associations
	Magma Sources
	Sources of Heat and Melting Processes
	Differentiation Processes
	Temporal Variations in Composition
	Trace Element and Isotopic Aspects
	References
	Further Reading
	Relevant Websitesfrom the given magma. As a result of
Fig. 1 Simple classification (International Union of Geological Sciences) of volcanic igneous rocks on the basis of combined alkalies (Na2O + K2O) versus SiO2, in
wt%. Volcanic rock names are in lowercase and plutonic equivalents in uppercase letters. Where two volcanic names are given, the upper refers to a sodic and the
lower to a potassic variety. The dividing line between rocks of alkaline and subalkaline character is highlighted with a denser weighting, and comprises the
projection into the compositional space represented by this plot of the low-pressure, thermal divide anchored by the cotectic crystallization of olivine-plagioclase-
clinopyroxene. This figure was not intended by the IUGS to be used for plutonic rocks; the compositional range of these specific types is a generalization, because
most plutonic rocks represent cumulative types and are not equivalent in bulk composition (including trace elements) to the nominal volcanic equivalents.
Table 1 Common igneous rock-forming minerals.
General name End-member components Mineral formula
Olivine Forsterite-fayalite Mg2SiO4-Fe2SiO4
Orthopyroxene Enstatite-ferrosilite MgSiO3-FeSiO3
Clinopyroxene Diopside-hedenbergite-jadeite-acmite CaMgSi2O6-CaFeSi2O6
NaAlSi2O6-NaFe
3+ Si2O6
Plagioclase Anorthite-albite CaAl2Si2O8-NaAlSi3O8
Alkali feldspar Orthoclase-albite KAlSi3O8-NaAlSi3O8
Micaa Biotite K2(Mg,Fe
2+)4−6(Fe
3+,TiAl)0–2(Si5–8,Al2–3,O20) (OH,F)4
Muscovite K2Al4(Si6,Al2, O20) (OH,F)4
Amphibolea Hornblende (Na,K)0−1Ca2 (Mg,Fe
2+Fe3+,Al)5(Si6–7,Al1–2,O22) (OH,F)2
Garneta Pyrope-almandine-grossular Mg3Al2Si2O12-Fe3Al2Si3O12-Ca3Al2Si3O12
Quartz SiO2
Spinela Magnetite-chromite-hercynite Fe2+Fe3+2 O4-Fe
2+Cr2O4-Fe
2+Al2O4
Ilmenite FeTiO3
aComplex solid solutions exist with other end-member components.
Table 2 Representative chemical analyses (in weight %) of volcanic igneous rocks.
1 2 3 4 5 6 7 8
SiO2 47.01 46.53 43.52 58.57 72.19 54.55 56.26 45.94
TiO2 3.20 2.28 2.45 0.64 0.33 1.60 0.27 0.34
Al2O3 15.57 14.31 15.76 19.87 12.62 19.9 10.57 2.98
Fe2O3 2.32 3.16 2.82 3.20 3.14 1.75 1.77 6.23
FeO 11.57 9.81 7.14 2.73 1.12 3.78 7.53 4.80
MnO 0.20 0.18 0.16 0.15 0.05 0.16 0.17 0.18
MgO 5.25 9.54 9.57 1.74 0.58 1.76 14.86 33.79
CaO 9.77 10.32 12.28 7.51 2.07 4.07 6.17 4.73
Na2O 3.00 2.85 3.02 4.25 3.45 9.06 1.61 0.15
K2O 0.31 0.84 1.43 0.74 3.70 3.64 0.73 0.03
P2O5 0.32 0.28 0.41 0.10 0.02 0.20 0.06 0.02
1. Subalkaline basalt, Galapagos islands.
2. Alkaline basalt, Hawai’i.
3. Basanite, Kenya.
4. Andesite, St Kitts.
5. Rhyodacite, New Britain.
6. Phonolite, Tenerife.
7. Boninite, Chichijima, Japan.
8. Komatiite, South Africa.
Igneous Processes 3
these efforts, it became clear that a spectrum of rocks of similar major oxide composition are present at or near the Earth’s surface
that owe their origins to complete or only partial crystallization frommolten precursors (Table 2). For example, the rate of cooling is
critical in the development of macroscopically recognizable crystals, and this rate can be slowed dramatically by cooling at deeper
levels within the Earth’s crust. A division of rock types by level of emplacement and coarseness of constituent grains from plutonic
(relatively deep) through hypabassal (shallow) to volcanic (near or above surface) has been accepted. The direct chemical equivalence
of certain rock types is widely asserted, such as gabbro (plutonic)—basalt (volcanic) and granite (plutonic)—rhyolite (volcanic) (see
Fig. 1). In fact however, there is considerable evidence that many plutonic rocks represent accumulations of crystals formed at
temperatures well above the solidus, and from which interstitial melt escapes prior to full crystallization. The bulk composition of a
given gabbro for example, is not in general compositionally equivalent to the fully liquid basalt from which it formed. A rigorous
compositional classification can be unambiguously established in the case of totally glassy rocks, but the situation with variably
crystalline rocks is accordingly complex.
Early attempts at reproducing the high temperatures necessary for the synthesis of igneous rocks were made by Sir James Hall in
the early 19th century, but systematic study under controlled conditions of formation were introduced in the early 20th century by
petrologists at the Geophysical Laboratory of the Carnegie Institution of Washington. Inspired by the thermodynamics of
heterogeneous phase equilibria developed by J. Willard Gibbs, a comprehensive program of investigation of varyingly complex
unary, binary, ternary, and higher order synthetic silicate and oxide systems relevant to the major igneous rock types was initiated,
and the program continues in many laboratories around the world. The fundamental principle underlying these early studies was to
4 Igneous Processes
understand the phase relations (solid-liquid-gas) of the most abundant rock-forming minerals (Table 1), and their combinations.
For example, to a first approximation, the realm of basaltic compositions and their sources in the upper mantle can be understood
through experimentation with the relative stabilities of crystalline phases and their melts within the 7-oxide component system
Na2O-MgO-Al2O3-SiO2-K2O-CaO-FeO. The compositional spectrum of all the common, anhydrous, basalt-gabbro-forming min-
erals is mostly encompassed within this synthetic compositional space. Underlying the typical graphical displays of phase relations
(e.g., Fig. 2) are the fundamental thermodynamic properties of enthalpy, entropy, and volume.
After more than two decades of effort, a summary of progress was published by Bowen (1928), entitled The Evolution of the
Igneous Rocks. This book was a landmark in terms of understanding the genesis of igneous rocks on the basis of sound physico-
chemical principles. Throughout, Bowen emphasized the importance of fractional crystallization to account for the diversity of rock
compositions, and this has had a seminal influence on petrological thought ever since. Upon cooling, instantaneous sequestration
of a given nucleating crystalline phase preventing further equilibration with the liquid comprises fractional crystallization. Its
counterpart, equilibrium crystallization, requires complete compositional re-equilibration of a crystalline phase with no seques-
tration from the remaining liquid during cooling. Accumulations of crystals whose aggregate composition is not equivalent to any
natural melt, are known as cumulates. Both of these processes are recognized to be end-members in terms of the real behavior of a
cooling magma. A particularly important observation by Bowen was that basaltic magmas represent the high-temperature end-
members of a spectrum of liquid compositions; lowest temperature end-members are equivalent to rhyolite or its nominal plutonic
equivalent of granite, and H2O-rich silicate liquids preserved as pegmatites. Intermediate in composition between basalt and
rhyolite are andesite and dacite (Fig. 1). Fractional crystallization of basalt, if followed to the limit, albeit by different sequences and
types of crystalline phase extraction, inevitably results in the production of residual melts in the so-called “petrogeny’s residua
system,” comprising phases crystallizing from residual melt in the system Na2O-K2O-Al2O3-SiO2 in various combinations of
feldspars, feldspathoids, and quartz (Fig. 2). Bowen also pioneered the derivation of thermodynamic properties of crystal-melt
systems from his experimental petrologic studies (see below).
In the 30 years or more following the publication of Bowen’s book. A number of important questions were the foci of
petrological debate, including the following:
1. Is there a single primary basalt magma from which all other igneous rock types are derived by various differentiation processes?
2. Given the varied compositional trends discovered by analysis of rock suites from different regions of the Earth, are systematic
controls, such as the geographic or tectonic environmentof eruption and emplacement, responsible?
3. Some natural occurrences indicate a fractionation trend toward high-Fe with moderate increases in combined alkalies; is the low-
Fe trend of basalt-andesite-dacite-rhyolite produced by contamination of primary basalt with granitic continental crust?
4. Is granite, the single most important rock type on a volumetric basis in the upper continental crust of the Earth, always produced
by fractional crystallization of basalt, or are there other modes of genesis?
In answer to the first question, it became clear from a combination of theoretical, experimental, and direct observational data that
diverse primary basaltic magma compositions are produced in space and time, although certain conditions of generation are
Fig. 2 The system nepheline-kalsilite-silica at atmospheric pressure showing the primary fields of crystallization at the liquidus. Isotherms are dashed lines
labeled in �C. Solid lines are cotectic and peritectic curves. G and S are the granite and syenite minima (lowest melting points in the system), respectively. Tridymite
is a high-temperature crystalline form of quartz (different crystalline symmetry, i.e., a polymorph). L stands for liquid and arrows indicate directions of decreasing
temperature. Thin straight solid lines represent the extent of compositional variation (i.e. “solid solution”) of crystalline phases.
Igneous Processes 5
reproduced more or less systematically, as described in more detail later. Understanding of the critical processes involved in the
generation of diverse compositional trends became focused as a result of the development of the concepts of plate tectonics. For
example, in answer to the second question, certain consistent trends of fractional crystallization and other processes of magmatic
differentiation are associated with particular types of plate boundary, whether at divergent (e.g., rifting and spreading such as ocean
ridges) or convergent (island and continental volcanic arcs) régimes, and to a large extent reflect the nature of the source materials
involved in melt generation, the amount and relative proportions of dissolved volatile compounds (e.g., H2O and CO2), and the
pathways traveled to the Earth’s surface. The development of plate tectonic theory was primarily the result of major geophysical and
bathymetric surveys of the ocean floor. While geologists Arthur Holmes and Harry Hess were prominent in the development of
ideas concerning mantle convection, sea-floor spreading, and the origins of mid-ocean ridges and deep-sea trenches, their expertise
in igneous petrology was not prominent in development of plate tectonics.
The pioneering experiments conducted by Bowen and his colleagues initially controlled neither the redox state of the system nor
investigated the role of important but potentially volatile compounds, such as H2O and CO2. In the first instance, the redox state of
Fe (as Fe0, Fe2+ or Fe3+) is critical in terms of understanding this element’s behavior in igneous systems. Clarence Fenner, a
contemporary of Bowen, emphasized the high-Fe fractionation trend highlighted by the third question. Similar conclusions were
also reached by workers such as Wager and Deer (1939) through studies of the plutonic rocks of the Skaergaard Intrusion, an
example in Greenland of what became to be known as layered igneous intrusions, characterized by cumulus rocks. A similar trend
was demonstrated in the case of volcanic rocks of Thingmuli volcano in Iceland (Carmichael, 1964). In island and continental arcs
however, the “Bowen trend” of basalt-andesite-dacite-rhyolite is commonly documented. Experiments conducted by Elburt F.
Osborn, C. Wayne Burnham and their colleagues introduced respectively, controlled redox, and both singly and in combination,
H2O-CO2-bearing experiments. More oxidized conditions promote the early appearance (saturation) of an Fe3+-rich spinel
(magnetite; Table 1) in the crystallization sequence of a basalt magma relative to more reduced conditions. High water contents
delay the appearance of plagioclase in this sequence, and upon saturation, its composition is shifted toward the anorthite-rich (i.e.,
Na-Si-poor) component of the plagioclase feldspar solid solution. Both of these effects account for the different fractionation trends
of high-Fe (so-called tholeiitic) and low-Fe (calc-alkaline) characteristic of mid-ocean ridges and island arcs respectively (Arculus,
2003, 2004).
Partial resolution of the fourth question followed an acrimonious debate between those who argued in favor of a “transformist”
or “metasomatic” (i.e., solid-state) origin for granite, and those who were convinced of a magmatic origin. Although local evidence
in favor of granitization of pre-existing sedimentary and igneous rock can be found, in general, evidence for high-temperature
(�800 �C) metamorphic aureoles in country rock around granite plutons, magmatic textures, and the coincidence of granite
compositions with the lowest temperature thermal troughs and eutectics in petrogeny’s residua system (Fig. 2) are conclusively
in favor of an igneous origin (Tuttle and Bowen, 1958).
Subsequent study of this important problem has shown that rather than a genesis by fractional crystallization alone of basaltic
parent magmas, some granites are the molten products of ultra-metamorphism of preexisting continental crustal material including
shales and greywackes, possibly combined with mixing of these types of crustally-derived melts with mantle-derived basalt (and
fractionally crystallized, derivative melts). Broadly speaking, the compositions of granite represent the lowest melting temperature
product of a wide variety and mixture of rock types, and whenever the temperature of the continental crust is raised on a regional
scale to appropriately high levels, partial fusion of the crust followed by migration of granitic/rhyolitic melt and further fractional
crystallization at varied crustal levels, is the consistent result.
More recent developments in the study of igneous processes have been significant. First, there have been sustained efforts to use
fundamental physical and chemical properties in thermobarometric estimation, melting and fractional crystallization modeling
(see “relevant websites”). Data required comprise relevant enthalpies, entropies of formation, and molar volumes of end-member
crystalline phases, enthalpies, entropies, and volume changes during fusion, heat capacities, thermal expansivities and compress-
ibilities, activity-composition relationships in liquid and solid solutions, and detailed structural and other physical property
measurements of crystalline, glassy, and liquid states. Second, the comparative study of meteorites (especially those known to be
derived from Mars and the asteroid belt), lunar and terrestrial igneous rock types, has resulted in a common perception of the
overall significance of the molten state for the evolution of the rocky planetary bodies. Finally, improved technology for sampling
the submerged portion of our planet, together with the increased resolution of equipment designed to gather isotopic and trace
element data has led to a dramatic improvement in the understanding of the temporal and spatial variation in the composition and
evolution of the Earth.
Our understanding of different igneous processes is summarized in the sections that follow. Continued technological improve-
ments enable reproduction of temperatures and pressures equivalent to those likely prevailing at almost any stage in the
development of the Earth, following accretion from the solar nebula. And measurements of element abundances at ultratrace
levels combined with microresolution of isotopic characteristics are in progress, and seem certain to profoundly alter our concepts
of terrestrial evolution over the next few decades.
Forms of Occurrence
Igneous rocks occur as fragmental and coherent units of varied form. Disruption of liquid rock near and at the surface by the
explosive expansionof dissolved gases (predominantly H2O, CO2, and SO2), results in the production of variously sized materials
6 Igneous Processes
from dust and sand particles through to boulder and house-sized blocks, which assembled in differing proportions constitute
pyroclastic rocks. Direct study of the most violently explosive volcanic eruptions is of course hazardous and difficult, but direct
observations of comparatively mild eruptions such as those of Mount Saint Helens in 1980 and Mount Pinatubo in 1991 (about 1
and 10 km3 of new material erupted, respectively), in combination with detailed mapping, theoretical analysis, and simulation of
fluidized gas-particle systems, have resulted in some understanding of the eruption behavior of volcanoes.
Critical factors involved in the form of occurrence and emplacement characteristics are the composition of the magma and
interrelated properties, such as viscosity and density, the volatile content, the rate of emission, and the density contrast between
magma and conduits to the surface. Total disruption of a magma column with high discharge rate can result in the injection of rock
particles into the stratosphere (e.g., the eruption column fromMount Pinatubo rose to�40 km) with resulting wide dispersal of the
ejecta and long-term effects (days to years) on Earth’s climate. Typically, the rock types involved are of andesite, dacite, and rhyolite
(see Fig. 1 and Table 2) in composition. Collapse of the eruption column results in a ground-hugging but buoyant flow of magma,
rock particles, and gas that can traverse considerable distances away from the vent (up to 500 km or more for the largest eruptions
consisting of up to 103 km3 of rock) as a pyroclastic density current or ignimbrite. Accumulation of interlayered fragmental deposits
and lava flows of relatively viscous (�106 Pa-s; a function of silica content and temperature) magma results in the typical form of
stratovolcanoes such as Mayon (Philippines), Mt. Fuji (Japan) and Klyuchevskoy (Russia); somewhat unusually, the latter two are
composed dominantly of basaltic lavas and pyroclastic rocks. Higher magmatic production rates and more explosive activity are
characteristic of topographic depressions, such as those occupied by Lake Toba in Sumatra or Lake Taupo in the North Island of New
Zealand.
In contrast, low viscosity (103 km3) typical of granite plutons with
boundaries that cross-cut preexisting rock formations, and apparently crystallized at depths within the continental crust of
0.5–10 km or more. There is debate whether these plutons are composites of multiple sill injections or ensembles of fewer but
larger pulses (e.g., Cruden et al., 2018). Without doubt some of the world’s largest plutons are gabbroic, forming the lower oceanic
crust and portions of ophiolites, representing at one extreme, continuous layers 1000 km2 in lateral extent, generally �5 km-thick,
or as screens in peridotite (Coogan, 2014).
Some of the world’s most significant economic mineral resources are associated with these plutons including the Cu, Mo, and
Au, deposits of Chile and western United States (Sillitoe, 2010), and the Cr and Pt-group metal deposits of South Africa, Zimbabwe,
and Montana (United States). In general, the Cu-Mo-Au associations are developed as dispersed sulfides in rock types ranging from
diorite to granite in composition, whereas the Pt-Cr deposits tend to be concentrated as metallic alloys, sulfides, and oxides in thin
layers within layered igneous rocks such as the huge Bushveld intrusion and Stillwater complex, predominantly composed of
gabbroic to noritic (plagioclase-orthopyroxene) rocks with basal harzburgite (olivine-orthopyroxene) and orthopyroxenite layers.
Historically, considerable petrologic effort has been expended in understanding the origin and sequences of crystallization of
these gabbroic plutons. This effort follows from the hypothesis that mineral sequences preserved in gabbroic plutons are significant
guides to the important processes of crystallization and magmatic differentiation, which have to be otherwise inferred from rapidly
erupted, and essentially homogeneously cooled, volcanic rocks. The stimulus of economic significance has also resulted in dramatic
recent progress in understanding crystallization in pluton-sized bodies from a combination of theoretical and practical applications
of fluid dynamics. The processes that are particularly significant are boundary layer effects resulting from forced cooling proximal to
wall rocks, mixing of separate pulses of compositionally varied magma, and more controversially, the creation of stratified,
separately convecting cells (double-diffusive-convection) in a magma chamber consequent to contrasts in the diffusivities of heat
and composition in molten silicates. The contrast between the olivine-plagioclase-clinopyroxene-dominated gabbros of the oceanic
crust are in fact unlike the largest terrestrial layered igneous intrusions where orthopyroxene is a relatively early-crystallizing phase.
Some have invoked boninite parental magmas to account for the dominance of enstatite in these intrusions; others point to the
isotopic indicators of extensive contamination of the parental magmas by continental crust and suggest siliceous high-Mg basalt
formed through crustal contamination of komatiite was involved (Eales and Cawthorn, 1996).
Tectonic Associations
Specific igneous rock types are consistently associated with divergent and convergent plate boundaries, respectively. Indeed the
presence of these distinctive rock types in ancient strata has been used to infer the former existence of plate boundaries where these
are no longer preserved. Some form of plate tectonics may also occur on Venus, but other processes of melting and magmatic
Igneous Processes 7
migration independent of plate tectonics such as mantle plumes and small volume hotspots manifest on planets and small rocky
bodies, such as Mars, the Moon and asteroids, may also be significant on Earth.
The generation of new terrestrial oceanic crust takes place mostly at submerged mid-ocean and back-arc ridges, and this crust is
characterized essentially at the surface by extruded basalt. Two thirds of Earth’s surface (i.e., the majority of the submerged portion)
is composed at near-surface of basalt, with a thin veneer of sediment (up to 1–2 km thick, depending on the age of the ocean crust)
typically younger than 200 Ma old. At the mid-ocean ridges, crystallization of basaltic melts in near-surface (0–6 km depth) magma
chambers gives rise to plutons and an array of erupted basalts, feeder dikes, and plutonic materials overlying mantle rocks
(predominantly harzburgite) residual from the extraction of basalt. This newly formed lithosphere moves away from the ridge,
cooling by conductionand circulation of hydrothermal fluids, thickening with time, and ultimately encountering a convergent plate
margin where it is generally subducted back into the Earth’s interior. However, exposed slices of oceanic crust are occasionally
trapped above sea level (called ophiolites) on overriding plates at convergent margins, such as in Cyprus, Oman, Papua New Guinea,
and Newfoundland. Studies of these in combination with direct sampling of the ocean floor has helped to formulate the model of
the generation of new oceanic crust outlined above, but in detail, most ophiolites likely represent oceanic crust formed during
initiation of new subduction zones (Ishizuka et al., 2014). One of the few ophiolites that represents oceanic crust formed at an intra-
oceanic spreading center is exposed on Macquarie Island, south of New Zealand.
Magma generation at convergent plate boundaries (island and continental arcs like the Izu-Bonin-Mariana and Andean chains,
respectively) is second only in volume to that occurring at ridges (1–5 km3 a−1 in arcs cf. >20 km3 a−1 at ridges). In general terms,
the presence of this activity is surprising given the expected localized cooling of the Earth’s upper mantle following insertion of cold
subducted lithosphere at the adjacent trench. Release of H2O-rich fluids from dehydrating silicates (including amphibole, mica
(Table 1) and others such as epidote and lawsonite, and serpentine in the sub-crustal mantle) is assumed to trigger partial melting of
the upper mantle wedge overlying the subducted lithospheric plate. The hydration occurs during exposure to hydrothermally
circulated seawater and metamorphism in the sea floor. The result of partial melting of the wedge is production of a range of basalt
and high-Mg andesite (Schmidt and Jagoutz, 2017) plus boninite magma types.
Within each of these major magma types, considerable detailed compositional variability exists reflecting the variety of source
materials juxtaposed in plate convergence and subduction. To some extent, given the effect of H2O-rich fluids in lowering the
solidus of various peridotite and pyroxenite types in the wedge, subduction zones are one of the most useful “probes” of the variety
of lithologies present in the mantle.
The least voluminous and understood igneous activity on the Earth in terms of association with plate tectonics is that developed
at isolated “hotspots” like Hawai’i, La Réunion, the Galápagos Islands, Heard Island and Mt. Erebus. Although dominated by a
spectrum of extruded basalt compositions, a variety of other rock types, typically alkaline in character (Fig. 1), are also present in
these hotspots, in considerably less volume but also consistent with origins by fractional crystallization of basaltic parent magmas.
The hotspots were originally thought to retain a near-constant angular separation from each other, and were used as an independent
and absolute frame of reference for plate motions. But scientific ocean drilling of the chain of volcanically extinct seamounts
traceable to the Hawai’i hotspot, for example, has demonstrated through the paleomagnetic orientation of the constituent lavas that
the hot-spot has moved southwards with respect to the north magnetic pole (e.g., Tarduno and Koppers, 2019). Ultimate rooting of
hotspot sources at a thermal boundary layer, such as the Transition Zone (�400–670-km depth) between upper and lower mantle
or the core-mantle boundary (�2900-km depth), has been suggested. In fact, Torsvik et al. (2006) have shown that many large
igneous provinces and clusters of hotspots overlie the margins of two “large low shear velocity provinces” on the surface of the outer
core of the Earth. These appear to represent the ultimate source regions of mantle plumes that manifest at the Earth’s surface as the
largest hotspots.
Despite the low volumetric significance on Earth of hot-spot igneous output (�1 km3 a−1) relative to ridges and arcs, the
processes driving this type of igneous activity must be considered the most common form of magmatic surface expression on a
solar-system-wide basis. Understanding this type of activity is essential for unraveling the nature of combined thermal and material
transport in the inner planets. At least on Earth it has been suggested, based on distinctive isotopic characteristics, that a
considerable portion of the magmatic flux incorporated in hot-spot activity represents the recycling (and partial remelting) of
subducted tectonic plates after a residence time in the mantle on the order of 1 Ga (and possible sojourn near the core-mantle
boundary). A general view is that the cycle of lithospheric plate (upper thermal and mechanical boundary layers of the mantle)
creation and destruction is the prime mode by which the mantle sheds heat (�90% of total terrestrial heat flow), whereas hot spot
activity is ultimately the mechanism by which the core sheds heat (�10% of terrestrial heat flow; Davies, 1999).
The greatest diversity of rock types within a single geographic province of particular tectonic style occurs at intracontinental rifts,
such as the one extending from Ethiopia to Zambia in eastern Africa. Intracontinental rifts are precursors to the creation of new
ocean basins. The magmatism associated with these rifts has thus been characteristic of many continent fragmentations in the past.
In the initial stages of rifting, highly alkaline lavas (high Na2O plus K2O relative to SiO2 (Fig. 1) plus strong variations of Na/K) and
magmas composed of carbonates (e.g., Na2CO3 and CaCO3) are associated with extreme enrichments of elements usually found in
trace quantities, such as the lanthanides, Nb, Ta, Zr, and Hf (Jones et al., 2013). Contrasts in terms of trace element and isotopic
geochemistry between intracontinental rift magmas and those of midocean ridges have been taken as evidence for distinct
compositional heterogeneity of the source regions in the upper mantle beneath continents versus oceans. This is a controversial
issue given the constraints that long-lived isolation of subcontinental from sub-oceanic mantle must impose in terms of convective
re-homogenization within the Earth. For example, the geochemical and isotopic consistency of alkali volcanic rocks of the
Cameroon line (West Africa), that straddles the ocean-continent boundary, does not reveal any contrast in the mantle source
8 Igneous Processes
regions of the respective magmas erupted (Fitton and Dunlop, 1985). Other volumetrically trivial but profoundly important,
typically intracontinental magma types, that sample both the subcontinental lithosphere and deeper regions of the mantle through
to the Transition Zone and beyond, are kimberlites with their entrained suites of varied peridotite types, eclogite, and diamonds
(Sparks, 2013).
Magma Sources
In the early part of the 20th century, two major sources of basaltic magmas were proposed: some kind of basaltic composition
(including glass) that would require total melting to yield a basalt magma, and alternatively a source more magnesian and less
siliceous than basalt that on partial melting, would yield a basaltic melt. The latter is generally favored for the genesis of the most
abundant igneous rock type on Earth: mid-ocean ridge basalt. This is based on a combination of (1) experimental study in the
laboratory; (2) examination of materials exhumed from high pressures and temperatures within the Earth by rapid eruptive
transport; (3) fragments of upper mantle preserved as slices faulted within the crust, such as ophiolites; and (4), geophysical studies
of the travel times (and hence diagnostic velocities) of earthquake waves within the Earth’s mantle. The dominant source material is
a rock type called peridotite, which is essentially composed at pressures >1.5 and�10 km-thick upper layers of basaltic rocks, back
into the mantle at subduction zones, raises the possibility that these lithologies may also subsequently serve as magma sources.
In fact there is growing evidence, both petrologic and isotopic, that partial melting in mantle plumes involves some olivine-free
(garnet-pyroxenite or eclogite) lithologies (Sobolev et al., 2005).
Over the past 50 years, the methods of experimental petrology have been used to duplicate the conditions of formation of
various basaltic magma types from peridotite and pyroxenite source materials, and current technology is extending the range of
study to pressures in excess of 25 GPa, equivalent to depths on the order of 1000 km and 2000 �C. The major experimental
apparatus used have been gas- and solid-media (piston-in-cylinder) presses, and multianvil presses; and for investigation of the
phase relationships within the mantle to pressures equivalent to the core-mantle boundary, diamond anvil cells have been used;
pressures of �400 GPa have been attained with these instruments.
A number of significant points have emerged from these studies:
1. At low pressures ( 10 wt% at 45–50 wt% SiO2;
Fig. 1) to komatiite (MgO > 20 wt% at 35–45 wt% SiO2; Viljoen and Viljoen, 1969), discussed in a later section.
3. It has been demonstrated in the relatively low-pressure range (2.5 Ga) when both the
ambient mantle temperatures and those of subducted lithosphere were higher, assuming that some form of subduction zone existed
at this time. The characteristic tonalite-trondhjemite-granodiorite component of Archean crust may have formed in this way (Deng
et al., 2019).
In view of the opposing effects of CO2 and H2O on melt compositions generated from peridotite (see Fig. 3), it is worth noting
that the pressure ranges in which subsolidus storage of these volatiles takes place in the upper mantle can result in some relative
fractionation. For example, H2O can be stored in amphibole, mica, and hydrated Ca-Al-bearing silicates at pressures 1.5 GPa. Selective entrapment of H2O at relatively shallow and CO2 at
relatively deeper levels within the upper mantle prior to any convective recycling is a possible consequence of these phase
relationships. However, research on the solubility of H in nominally anhydrous silicates and the stability of hydrous phases at
high pressures (>5 GPa), has shown that numerous hydrated silicates and oxides may be stable at least to the Transition Zone
(Mazza et al., 2019), but the question of relative fractionation of volatiles at high pressures is a rich research field.
Sources of Heat and Melting Processes
Measurements of heat flow from within the Earth as well as experimental duplication and direct study of magmatic eruption
temperatures show that, in general, the conductive thermal gradient within the nonconvecting part of the crust and upper mantle
varies from about 10–30 �C km−1 and that temperatures of 100 �C or more may be reached at depths as shallow as 2 km at ocean
ridge crests to 1–5 km beneath rifted regions like the Basin and Range Province in Nevada and Utah.
The major source of heat within the Earth at present (and for most of its history) is the energy released from radioactive decay of
232Th, 238U, 235U, and 40K. Although radioactive heat generation decays exponentially with time, the efficiency of heat transport
from within the Earth to the surface is the critical factor determining the variation in temperature of the mantle, and the possibility
of localized melting. Full understanding of the thermal evolution of the Earth also depends on knowing the contributions of the
mode and duration of initial accretion of the Earth, segregation of the core from the mantle (gravitational energy release), and the
energy pulse released from the decay of short-lived radionuclides such as 26Al (half-life of 717 ka), if trapped during terrestrial
accretion. This heat source is known to be important in the melting of some planetesimals (e.g., asteroids) as sampled within the
meteorite population. Progressive solidification of Earth’s inner core (latent heat of crystallization of liquid Fe-Ni alloy) is also
significant, and sustained through time.
Fig. 3 Representation of the compositions (open and solid dots) of initialmelts generated at a variety of pressures in the system forsterite-nepheline-silica (a
model peridotite system). The open arrow indicates the increasingly alkaline character of dry melt products from atmospheric pressure (100 kPa) through to 3 GPa.
Note the opposing trends of CO2 and H2O saturation on the compositions of melts generated at high pressures (2 GPa shown as an example with solid arrows). The
line forsterite-albite is a portion of the critical plane (thermal divide) that divides alkaline from subalkaline rocks at low pressures (and Fe2SiO4 in crystalline olivine
solid solutions, as well as in natural magmas, remains nearly constant over a wide range of natural compositions so that (1) the
degree of equilibrium between a given olivine crystal and adjacent melt can be determined from the deviation of the pair from the
value of the ratio (FeO/MgO)crystal/(FeO/MgO)melt �0.3; and (2) the range of basalt magmas can be screened to examine in detail
those that could have been in equilibrium with olivine (forsterite 90 mol%, fayalite 10 mol%) predominant in upper mantle
peridotite source materials, and thus represent primary magmas.
Solid solution in another of the major rock-forming silicate phases between the components anorthite (CaAl2Si2O8) and albite
(NaAlSi3O8) is also continuous and close to ideality at magmatic temperatures. The continuous and variable nature of modal (the
actual phase composition present) and normative plagioclase compositions in igneous rocks, in combination with the relative ease
with which these compositions may be determined with a petrographic microscope, were significant factors in the selection of
plagioclase compositions in general as a rock classification index.
Discontinuous reactions between rock-forming silicates and oxides have also proved significant in terms of differentiation and
classification. For example, in the system MgO-SiO2 at low pressures (0.7 GPa), the
compositional join olivine-plagioclase becomes unstable in favor of aluminous pyroxenes plus spinel, and compositional trends
resulting from fractional crystallization can be very different from the lower-pressure situation. The appropriate phase relationships
are known only in preliminary fashion, but rock suites that reflect the processes of fractional crystallization at these higher pressures
are known from a number of settings. For example, the delay in plagioclase saturation in hydrous basaltic magmas in arcs, especially
deep in the crust (>�0.3 GPa), removes one of the phase anchors of the thermal divide. Accordingly, fractionally crystallizing
magmas can cross from subalkaline to alkaline compositions or even straddle the thermal divide. A similar effect is produced by the
stabilization of amphibole as one of the crystallizing phases. The aH2O during melting and differentiation is also important in terms
of stabilizing the incongruent melting relationship of enstatite [2MgSiO3 ¼Mg2SiO4 + SiO2 (component in melt)] seen in Fig. 5, to
2 rather than 0.4 GPa. This phenomenon is important for understanding the process of boninite generation. The experimental effort
required to map out the phase relationships is immense and has stimulated the effort toward understanding the thermodynamics of
silicate melts and crystal-melt equilibria, with at least one of the aims being to interpolate within and extrapolate from the available
experimental data (see for example, the web-based software MELTS and Thermocalc).
In addition to fractional crystallization, at least two other processes are known to be significant in terms of the development of
the diversity of igneous rocks. The first of these is the phenomenon of silicate-silicate and silicate-carbonate liquid immiscibility.
The former results particularly in systems rich in alkalies and iron, and is observed to form coexisting melts enriched respectively in
FeO, CaO, and P2O5 and the other in SiO2 and Al2O3. Although ubiquitous as a late-stage event in many types of lunar and
terrestrial igneous rock suites, the overall effect does not seem to be major in terms of generation of large volumes of rock.
In contrast, there is clear evidence that silicate-carbonate liquid immiscibility is particularly significant in the genesis of carbonatite
magmas, such as those associated with strongly SiO2-poor, alkaline magmas in continental rifts.
The second major process that can operate is the combination of assimilation with fractional crystallization. This process has
proved particularly difficult to characterize because a number of possible events can take place, depending on the nature of the
magma and assimilant involved. Bulk contamination of magma will move the resultant composition directly toward the contam-
inant, but if melt-crystal equilibrium occurs, then the resultant composition may not necessarily trend toward the contaminant,
depending on the nature of the phases crystallizing at the instant of assimilation. Furthermore, it is probable that selective
assimilation of the lowest-melting components of the particular contaminant occurs, rather than bulk fusion, and in terms of
interaction of basalt magma with continental crustal materials that are not directly accessible to study (e.g., the lower crust), these
components are not always well known or characterized.
Studies of the crystallization processes of intra-crustal, magmatic plumbing systems and the consequences for the compositions
of erupted lavas, have revealedmany complexities in the past 70 years.The Skaergaard Intrusion represents, to a first approximation,
the consequences of progressive crystallization of a single pulse of magma as a closed system. On the other hand, cumulates
Igneous Processes 13
comprising the “rhythmic units” of Hallival and Askival on the island of Rum in the Inner Hebrides of Scotland, were shown by
Brown (1956) to be the products of an open system: magma tapped from a crustal magma chamber as lava was replenished by a
new magmatic pulse from deeper in the crust or mantle, with concurrent crystallization of various combinations of spinel-olivine-
plagioclase-clinopyroxene. O’Hara (1977) quantified and asserted the generality of this type of open system behavior in magmatic
systems. Albarede (1985) subsequently showed the specific sequence of processes involving tapping (T), recharge (R), mixing (M),
and crystallizing (X) is important in the generation of geochemical characteristics of mid-ocean ridge basalts. The global compo-
sitional variability of ocean floor basalts has since been successfully reproduced by O’Neill and Jenner (2012), with application of a
RMTX sequence of processes.
Temporal Variations in Composition
Within the past 50 years, important discoveries of new rock types (and rediscovery of older literature) have resulted in the
recognition that major secular differences in melts erupted from Earth’s interior exist. In Archean terrains (i.e., rock units older
than 2.5 Ga), a characteristic rock type called “komatiite” (after the first-described locality in the Barberton Mountain Land of
southern Africa; Viljoen and Viljoen, 1969) is present, distinguished by very high MgO contents and relatively low Al2O3, CaO,
TiO2, Na2O, and K2O compared with other basaltic rocks, and approaching peridotite in composition (Table 2) Anhydrous liquidus
temperatures at surface pressure for these melts are on the order of 1500–1600 �C, or at least 300–400 �C higher than the most
magnesian lavas being erupted at the present time. These rocks are believed to form from large degrees of partial melting,
approaching 40% or more of the original solid peridotite parent, and are the best direct evidence for the existence of elevated
thermal conditions within the Earth during the Archean. Only one occurrence of komatiite younger than Archean is well known at
present (Gorgona Island, Colombia), and so there does appear to be clear evidence for a change in the conditions (primarily
pressure or depth) under which the upper mantle has undergone melting during geological time. Komatiites represent a magma
type that was not countenanced by early experimental petrologists such as Bowen.
In more general terms, the lack of preservation of igneous rocks dating from the interval between the formation of the Earth at
4.567 Ga and the oldest extensive portions of continental crust at about 4 Ga focuses attention on our neighbors, the Moon, Mars
and the asteroids. It is believed that the lack of a comprehensive terrestrial record is the result of the intensity of crustal recycling
events following the creation and solidification of a magma ocean that accompanied the Moon’s formation and a succeeding period
of major bombardment by large asteroid-like objects. Our understanding of igneous processes has been directly applied to study of
the evolution and genesis of the Moon, and conversely, improvements in instrumentation, techniques and skills that were
developed before and during the NASA Apollo program have been turned to good effect in terrestrial research. The process
continues with the realization that some meteorites are sourced from Mars, and the current flow of direct sampling of that planet’s
surface (as well as comets and asteroids) achieved by robotic landers.
The origin and general evolutionary history of the Moon is well known in broad outline, from the formation through the “Giant
Splash” (Hartmann 2014) to formation of a magma ocean in which fractional crystallization was significant. Cumulus processes
and differential sinking/flotation were clearly critical in the development of the lunar structure. The feldspar-rich Highlands of the
Moon are the bombarded remnants of early cumulates. Subsequent build-up of heat within the Moon by radioactive decay and
remelting of the mantle took place in the interval between about 4.2 and 3.0 Ga; the basaltic melts generated were erupted, in part,
into the topographic depressions created by major impact events, forming the so-called maria. The spectrum of mare basalt
compositions, particularly in terms of TiO2 content, results in part from the variety of source compositions that were formed
during the differentiation of the lunar mantle.
It is unlikely that the Earth’s mantle, if ever subjected to melting on the scale apparently developed on the Moon, would have
remained solidified with comparable compositional stratification, given the scale of top-down plate-driven convective cycling that
continues in the Earth. In addition, the greater pressure range over which melting takes place in the Earth results in some
significantly different melt-residual solid-dynamic effects. In general, the lower density of melts of peridotite compared with the
residual solid fraction provides a powerful buoyant force promoting upward migration of the liquid. At pressures greater than about
8–10 GPa, however, a density inversion is thought to take place in peridotite systems, and melts may be denser than the residual
solid, analogous to the ice-water system at atmospheric pressure. Consequently, rather than experiencing eruption, melts may be
trapped within the Earth’s mantle and fail to separate from solid residues, leading to substantial re-equilibration.
Trace Element and Isotopic Aspects
The field of igneous geology has been revolutionized within the past 20 years by the advent of automated, routine analytical
equipment capable of measuring concentrations of elements in the parts-per-million to trillion range (ultra-trace elements) as well
as mass spectrometers equipped with multi-collectors and various input sources. The concentration ranges of many trace elements
extend over several orders of magnitude within the spectrum of igneous rock composition, and can serve as sensitive monitors when
calibrated for processes such as partial melting, fractional crystallization, contamination, assimilation, and liquid immiscibility.
In general, trace elements can substitute more or less favorably in the crystallographic sites of the major rock-forming minerals
14 Igneous Processes
Similarity of ionic size and charge are particularly important with respect to compatibility (minimization of crystallographic site
strain) in these structures (Wood and Blundy, 2014), but crystal-field effects are also critical in the case of the trace transition metals.
In the dilute range, the substitution of a trace element in a given crystalline silicate or oxide can be characterized by a Henry’s law
constant, such that the concentration of the element in a crystal (Xcrystal) is a direct function of the concentration of the same element
in a coexisting melt (Xmelt). The partition coefficients (D ¼ Xcrystal/Xmelt) for most trace elements are now known over a range of
crystalline phase and melt composition, pressure, temperature, and redox state. Given the hypothesis that a particular suite of rocks
is related by a process such as fractional crystallization, it is possible to test for overall consistency by application of specific laws of
trace element variation such as Rayleigh fractionation. In this case Cl/Co ¼F(D−1) where Cl is the concentration of the trace element in
the melt after some fraction of crystallization, Co is the original concentration of the trace element in the melt, and F is the fraction of
melt remaining. Equations describing the variation in concentration of a trace element during different types of melting and
combined assimilation-fractional crystallization are also available and can be used to test for the operation of these processes during
magma