9 781292 021539

ISBN 978-1-29202-153-9

Principles ofIgneous and Metamorphic PetrologyJohn D. WinterSecond Edition

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Magma Diversity

Ab Or

Qtz

1 atm

1 GPa

3 GPa

Ptotal = PH2O

FIGURE 3 Position of the H2O-saturated ternary eutectic(minimum melt composition) in the albite–orthoclase–silicasystem at various pressures. The shaded portion represents thecomposition of most granites. Included are the compositions of the Tuolumne Intrusive Series, with the arrow showing thedirection of the trend from early to late magma batches.Experimental data from Wyllie et al. (1976).

The previous analysis indicates that only mafic intru-sions are susceptible to the effects of differentiation by grav-ity settling of crystals. There is evidence, however, thatmany silicic bodies have evolved along a liquid line of de-scent. Several investigators have used the approach to theternary eutectic in the systems albite–orthoclase–silica(Figure 3) to indicate the evolution of late granitic liquidstoward the thermal minimum, or eutectic, composition. Oth-ers have used the same clustering of analyses around theminimum to indicate eutectic partial melts of sialic materialin the continental crust. Certainly, a eutectic magma couldresult from either process. Included in Figure 3 are thecation norm components of the successive intrusive phasesof the Tuolumne Intrusive Series. Note the progressive ap-proach of these magmas to the eutectic composition along atrend that follows the decompression eutectic path. What-ever the origin of the parental magma in this case, this seriesappears to have evolved toward the low-pressure thermalminimum. Harker-type bivariate variation diagrams of intru-sive sequences also indicate evolutionary trends. Batemanand Chappell (1979) interpreted the trends for the TuolumneIntrusive Series to be the result of fractional crystallization.Similar fractional crystallization-based interpretations forSierran zoned granitoids have been proposed by Batemanand Nokelberg (1978) and Noyes et al. (1983). Although thecompositional trends may be compatible with fractionalcrystallization, crystal settling has been considered veryproblematic in such viscous silicic magmas (Brandeis andJaupart, 1986; Sparks et al., 1984). Harper et al. (2005),however, cited viscosity and field criteria supporting crystalsettling in hydrous granites. Later in this chapter, we shallexplore methods by which fractional crystallization can

occur without crystal settling. As we shall see in Section 4,mixing of silicic (crustal melts) and mafic (mantle melts) isa popular alternative interpretation to the evolutionarytrends in some of these systems.

In addition to gravity settling, three other mechanismsmay facilitate the separation of crystals and liquid. Filterpressing (compaction), mentioned earlier in reference topartial melting, is also possible in crystal mushes that formas cumulates or crystal suspensions. The amount of trappedintercumulus liquid between cumulate minerals may be ashigh as % (Irvine, 1980b). With the added weight offurther accumulation, the crystal mush may be compacted(McKenzie, 1984), squeezing much of the liquid out into themain magma body. Another method of filter pressing in-volves the movement of a phenocryst-laden crystal mush.Any constriction in the conduit causes the crystals to inter-fere and slow with respect to the liquid.

Another similar mechanism by which crystals maybe segregated from the liquid occurs when crystal-rich mag-mas flow in a laminar fashion near the walls of the magmabody. The process is known as flow segregation (orflow[age] separation, or flow[age] differentiation). The mo-tion of the magma past the stationary walls of country rock(Figure 4) creates shear in the viscous liquid as a result ofthe velocity gradient near the walls. The resulting differen-tial motion forces the magma to flow around phenocrysts,

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Grain dispersivepressure

CountryRock

FIGURE 4 Flow of magma adjacent to a wall of country rock results in differential motion and shear in the magma.Where such shear is constricted, as between adjacentphenocrysts or between phenocrysts and the contact, a force (called grain-dispersive pressure) is generated and pushes the phenocrysts apart and away from the contact.

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thereby exerting pressure on them at constrictions wherephenocrysts are near one another or near the contact itself.The pressure, called grain dispersive pressure (Komar,1972), forces the grains apart and away from the contact.This effect is greatest near the walls, and it drops off quicklytoward the magma interior, where the flow becomes uni-form. Phenocrysts thus concentrate away from the walls tomitigate the pressure buildup. This concentration is most apparent in dikes and sills, where the volume affected bythe contact comprises a substantial proportion of the body, resulting in a distinct concentration of coarse phenocryststoward the center (Figure 5). Flow segregation is an inter-esting, though localized, phenomenon and cannot be respon-sible for the evolution of more than a small proportion of ig-neous rocks.

A third mechanism involves the separation and rise ofbuoyant liquids from boundary layers in which crystals formwithout themselves moving. This relatively new model has be-come popular recently and will be introduced in Section 5.

The majority of fractional crystallization models as-sume that fractionation has taken place in a stationarymagma chamber at constant pressure. The rise of basalticmagmas, as pointed out by O’Hara (1968b), may involvefairly continuous fractional crystallization as it rises, whichmust obviously be a polybaric fractionation process. One result is that the fractionating minerals vary as their stabilityfields are crossed (e.g., garnet to spinel to plagioclase). Another is that the shift in the eutectic point with pressurealso causes the quantity of the liquidus phases that crystal-lize to vary. In particular, the increase in the size of the field

for olivine with decreasing pressure requires that a lot ofolivine must form as the melt composition follows the liq-uidus away from the olivine side of the diagram in a risingbasaltic melt (see Problem 1). Thus, the relative amount ofolivine that crystallizes with a rising basaltic magma will be far greater than the amount that forms during isobaric crystallization.

The broad acceptance of Bowen’s (1928) contentionthat fractional crystallization is the predominant mechanismof magmatic differentiation is now being questioned. Thisone process cannot account for all of the diversity in thebroad spectrum of natural igneous rocks, even if we allowfor variations due to the influence of changes in pressure orassociated fluids. Some observed chemical trends simplycannot be accomplished by fractional crystallization. Otherclassical examples of fractional crystallization have notwithstood more critical analysis and the test of time. For ex-ample, the 300-m thick Triassic diabase Palisades Sill, onthe eastern banks of the Hudson River, is commonly cited asan example of a vertically differentiated sill with layersformed by gravity settling. The overall composition of thesill is tholeiitic basalt, as demonstrated by the upper andlower chill zones. The 10- to thick olivine-rich layer atthe base is commonly attributed to differentiation by settlingand accumulation of early-forming dense olivine crystals.Although vertical chemical trends in the sill are compatiblewith fractional crystallization of pyroxene and pyroxene ac-cumulation zones occur near the bottom of the sill, the strik-ing olivine layer is not compatible with the trends, andolivine is far too rare elsewhere in the sill to be consistentwith the concentration in the layer. The layer has recentlybeen reinterpreted as one of several late intrusions of magmainto the crystallizing tholeiitic liquid of the sill. This injectedpulse was olivine rich and dense, so it accumulated near thebase (Husch, 1990). Some magma series, such as the calc-alkaline series associated with subduction zones, may involvemixing of components to a greater extent than fractionalcrystallization trends.

Other cases against fractional crystallization werebased on proportionality arguments. The great granitebatholith belts, for example, are thought to be too extensiveto have been created by fractional crystallization from abasaltic parent. It would require approximately 20 parts oforiginal basalt to create 1 part late granitic liquid by frac-tional crystallization. We need not walk long in places likethe Sierra Nevada, with so many square kilometers ofgranitic rocks, to wonder where all of the basalt went! Mod-ern theories that consider granite batholiths to be much thin-ner than originally thought (Hamilton and Myers, 1967, seeChapter 4) reduce the magnitude of the problem, so wecould still follow Bowen (1948) and postulate that the lowerlevels of the crust are composed of the denser fractionatedgabbros. Seismic and gravity surveys, however, argue againstthis possibility, making the origin of granite batholiths viafractionation from a basaltic parent untenable (Presnall,1979). These arguments serve only to demonstrate that frac-tional crystallization cannot lead to all of the magmaticrocks now exposed at the surface of the Earth. It is still a

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FIGURE 5 Increase in size and concentration of olivinephenocrysts toward the center of small dikes by flowdifferentiation. Isle of Skye, Scotland. After Drever andJohnston (1958). Reproduced by permission of the Royal Society of Edinburgh.

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common and important process, particularly in the earlycrystallization of mafic liquids, but there are other importantdifferentiation processes and other primary magmas.

2.2 Volatile Transport

Chemical differentiation can also be accomplished when aseparate vapor phase coexists with a magma and liquid–vaporfractionation takes place. A vapor phase may be introducedin any of three principal ways. First, a fluid may be releasedby heating of hydrated or carbonated wall rocks. We shalldiscuss some ramifications of this process in later sectionsof this chapter.

Second, as a volatile-bearing but undersaturated magmarises and pressure is reduced, the magma may eventually be-come saturated in the vapor, and a free vapor phase is released.Because the vapor phase has a lower density than the melt, itrises, diffusing through the magma, and concentrates near thetop of the magma chamber. Such concentrated fluid may evenpermeate into the roof rocks. This process usually involves anH2O-rich fluid, and it produces a variety of hydrothermal alter-ation effects. For example, the alkali metasomatism known asfenitization above nephelinite– carbonatite bodies has been at-tributed to alkali-rich fluids derived from the highly alkalineintrusives.

A third mechanism for generating a separate fluidphase is a result of late-stage fractional crystallization. Mostearly-formed igneous minerals are anhydrous (even hydrousminerals are less so than associated melts), so their segrega-tion from a hydrous melt enriches the melt in H2O and othervolatile phases. Eventually the magma reaches the satura-tion point, and a hydrous vapor phase is produced. Thissomewhat paradoxical “boiling off” of water as a magmacools has been called retrograde (or resurgent) boiling.

Of course, the three processes by which a vapor can beproduced need not be entirely separate, and all three maycontribute to saturation and volatile release from a magma,depending upon the composition of the original magma, therates of cooling and rise, the initial volatile content, the ex-tent of fractional crystallization, the temperature, the natureof the wall rocks, etc. As a separate vapor is produced, thechemical constituents in the system partition themselves be-tween the liquid and vapor phases in appropriate equilib-rium proportions, some remaining preferentially in the meltand others becoming enriched in the vapor phase. The resultis a silicate-saturated vapor phase in association with avapor-saturated silicate liquid phase.

The cation sites in minerals are much more con-strained and selective than in melts, so the chemical con-stituents in minerals are generally much simpler. As a result,the process of fractional crystallization tends to remove onlya few elements from the liquid in significant quantities, anda number of incompatible, LIL, and non-lithophile elementsbecome concentrated in the latest liquid fraction. Many ofthese will further concentrate in the vapor, once formed.This is particularly true in the case of resurgent boiling because the melt already is evolved by the time the vapor

phase is released. The vapor phase may contain unusuallyhigh concentrations of volatile constituents such as H2O,CO2, S, Cl, F, B, and P, as well as a wide range of incompatibleand chalcophile elements.

The volatile release and concentration associated withpluton rise or resurgent boiling may momentarily increasethe pressure at the top of the intrusion and fracture the roofrocks in some shallow intrusions (it may also initiate vol-canic eruptions). Both the vapor phase and some of the latesilicate melt are likely to escape along a network of thesefractures as dikes of various sizes. The silicate melt com-monly crystallizes to a mixture of quartz and feldspar. It istypically found in small dikes with a sugar-like texture,which is informally called aplite. The vapor phase is typi-cally concentrated as dikes or pods in, or adjacent to, theparental granitic pluton, where it crystallizes to form a char-acteristically magmagenic form of pegmatite.

Although pegmatite is used as a textural classificationterm for very coarse grain size, and there are other methods ofcreating large crystals, the type of pegmatite described aboveis the most common. The large grain size in magmagenic peg-matites is not due to a slow cooling rate but is a result of poornucleation and very high diffusivity in the H2O-rich phase,which permits chemical species to migrate readily and add torapidly growing minerals. The size of crystals in pegmatitescan occasionally be impressive, such as spodumene, micro-cline, or mica crystals across. Most pegmatites are“simple,” essentially very coarse granites. Others are morecomplex, with a tremendous con centration of incompatibleelements and a highly varied mineral ogy, commonly display-ing a concentric zonation (Jahns and Burnham, 1969; Cerny,1991; Simmons et al., 2003), as shown in Figure 6. Becausethe late fluid segregation concentrates several unusual ele-ments, pegmatites are important economic resources and aremined for Li, Be, the rare earths, W, Zr, and a host of otherselements that are rarely concentrated in other environments.They are also a major source of gems.

Vapors that completely escape the magma and move tohigher levels may cool further and precipitate low-temperatureminerals, such as sulfides in a hydrothermal system (commonlymixed in part with meteoric water).

Miarolitic pods, or cavities, are smaller fluid segre-gations trapped in the plutonic host. When finally exposed atthe surface, they are coarse mineral clusters (usually a fewcentimeters across), the centers of which are typically hol-low voids from which the fluid subsequently escaped. Thehollow cavities have euhedral crystals (of the same mineralscomprising the pluton) that extend inward, where they grewinto the fluid, unimpeded by other minerals. Like complexpegmatites, some miarolitic cavities or pods have a concen-tric structure consisting of layers of different mineralogy(Jahns and Burnham, 1969; McMillan, 1986.

Because the addition of H2O lowers the melting pointof magmas, the release of hydrous fluid into the countryrocks causes the liquidus temperature in the main magmabody to rise suddenly, resulting in rapid crystallization ofmuch of the liquid remaining with the previously formed

6 to 10 m

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minerals. This is an alternative way of generating porphyritictexture and is common in many silicic plutons.

2.3 Liquid Immiscibility

Two liquids that don’t mix seems an unlikely occurrence,and it is. Yet most of us are familiar with salad oil and oilslicks, so we have some concept of the phenomenon. Manyoils do not mix with water, and, because they are less dense,the oil floats to the top of the water, forming a distinct layer.Most immiscible phases, whether liquids or solids, homoge-nize at elevated temperatures due to the increased entropyand molecular vibrational energy, although for oil–water atatmospheric pressure, the homogenizing temperature isabove the boiling point of water. The solvus, representingliquid or solid immiscibility on a phase diagram, is thereforeconvex upward on a temperature–composition diagram. Wehave already encountered immiscible liquids in theforsterite–silica system, where, on the high-silica side of thediagram, a highly silica-rich liquid separates from a less sil-ica-rich one.

Throughout the 20th century, geologists appealed to liq-uid immiscibility as a mechanism for magmatic differentia-tion, thinking that it might be responsible for the separation ofa granitic liquid from an evolving system (presumably from aninitial basaltic parent). Such a separation into contrasting liq-uid systems was also used to explain enigmatic cases of bi-modal volcanism, such as the basalt–rhyolite occurrences ofthe Snake River–Yellowstone area, or the Basin-and-Range ofthe southwestern United States.

There are two problems with applying the forsterite–silica liquid immiscibility gap to natural magmas. First, thetemperature of liquid immiscibility is far too high (over

) to represent a reasonable crustal process. Of course,the Mg-Si-O system is rather restricted, leading one to askwhether the addition of other components, required to createmore natural magmas, would lower the temperature of thesolvus. The effect, however, of adding alkalis, alumina, etc. isto eliminate the solvus completely. When this was experi-mentally demonstrated, liquid immiscibility was relegated tothe compost pile of magmatic processes.

Interest was renewed when Roedder (1951) discovered alow-temperature immiscibility gap in the central portion of thefayalite–leucite–silica system (Figure 7) at temperatures andcompositions that are conceivable for some Fe-rich naturalmagmas. Roedder (1979) provided a review of liquid immis-cibility in silicate magmas, citing dozens of references inwhich natural occurrences of immiscible liquids were de-scribed, including a significant proportion of the lunar sam-ples returned by the Apollo program.

Three natural magmatic systems are widely recognizedas having immiscible liquids in some portion of their compo-sitional range. The first is the system mentioned above,which most commonly translates to natural Fe-rich tholeiiticbasalts, which experience an initial trend toward iron enrich-ment. In the later stages of fractionation, a “granitic” melt( SiO2) separates from a basaltic melt ( ).Once separated, the silicic liquid must have a much lower density than the Fe-rich mafic liquid, and we wouldexpect it to rise and collect near the top of the magma

775% '40% SiO2

1700°C

FIGURE 6 Schematic sections of three zoned fluid-phase deposits (not at the same scale). (a) Miarolitic pod in granite (severalcentimeters across). (b) Asymmetric zoned pegmatite dike with aplitic base (several tens of centimeters across). (c) Asymmetric zonedpegmatite with granitoid outer portion (several meters across). From Jahns and Burnham (1969).

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chamber. Crystallization of the magma must be advanced bythe time liquid separation occurs, however, and both liquidsare likely to become trapped in the already-formed crystalnetwork. Philpotts (1982) described the textures of some Fe-rich Hawaiian basalts in which small droplets of the two im-miscible liquids are mingled in the interstitial glass trappedbetween plagioclase and augite crystals. The separate dropletcompositions may be determined by microprobe and are pro-jected into the Fa-Lc-silica system in Figure 7, along with the liquid immiscibility gap of Roedder (1951). The actualliquid compositions plot slightly outside the experimentalgap, probably because of the effects of Fe2O3, TiO2, andP2O5, which expand the immiscible field. The low oxygenfugacity of the lunar basalts is the probable reason that im-miscible liquids are so common in them.

Observing immiscible droplets is clear evidence of theprocess, but evidence is far less obvious that immisciblegranitic liquids have separated and formed substantial segrega-tions from Fe-rich tholeiites that are over 70% crystallized. Per-haps filter pressing may aid the process, and the granophyriclayers and lenses at the top of many mafic intrusions, includingthe Palisades Sill and the Skaergård intrusion (McBirney, 1975)may be the products of immiscible liquids. In such cases, liquidimmiscibility is a late-stage addition to a more extensiveprocess of fractional crystallization in these mafic intrusions.Granitic bodies and other large-scale evolved liquids, however,are unlikely products of immiscible liquids.

A second system displaying immiscible liquid behav-ior is the separation of a sulfide-rich liquid from a sulfide-saturated silicate magma. Less than one-tenth of a percent ofsulfur is sufficient to saturate a silicate magma and release

an iron–sulfide melt that is also rich in Cu, Ni, and otherchalcophile elements. Small, round, immiscible sulfidedroplets in a silicate glass matrix, similar to Philpotts’(1982) granitic–tholeiitic examples above, have been ob-served in a number of quenched ocean basalt glasses. Eco-nomically important massive sulfide segregations in large,layered mafic complexes have formed by separation and ac-cumulation of immiscible sulfide melts.

A third liquid immiscibility gap occurs in highly alka-line magmas that are rich in CO2. These liquids separate intotwo fractions, one enriched in silica and alkalis and the otherin carbonate. These give rise to the nephelinite–carbonatiteassociation.

Although these are the three generally recognized oc-currences of immiscible liquids, other magmas might sepa-rate into two liquid phases under certain circumstances.These possibilities include lamprophyres (Philpotts, 1976;Eby, 1980), komatiites, lunar mare, and various other vol-canics (see Roedder, 1979, for a summary).

The close spatial and temporal association of contrastingliquids may result from a number of processes in addition to liquidimmiscibility. We can apply three tests to juxtaposed rocks to eval-uate them as products of immiscible liquids. First, the magmasmust be immiscible when heated experimentally, or they mustplot on the boundaries of a known immiscibility gap, as in Figure7. Second, immiscible liquids are in equilibrium with each other, and thus they must also be in equilibrium with the sameminerals. If the two associated liquids crystallized different miner-als or the same mineral with different compositions, they cannotbe an immiscible pair. Finally, we may be able to use the pattern oftrace element fractionation between the two liquids to evaluatethem as immiscible. Partitioning of minor and trace elements be-tween Fe-rich mafic liquids and granitic liquids, for example, canbe distinctive when compared to the more common mafic mag-mas with less Fe. Some incompatible elements (P, for example)are preferentially incorporated into an Fe-rich mafic liquid overthe complimentary silicic one. A granitic rock relatively depletedin these incompatible trace elements may be a product of liquidimmiscibility. Of course, a low concentration in a particular traceelement can also result if the liquid was derived from a similarlydepleted source. It is far more reliable, then, if rocks representingboth of the immiscible liquids can be evaluated. This has been ac-complished for some mixed dike rocks (Vogel and Wilband,1978), but no one has yet succeeded in identifying a medium-sized or larger granite as derived from an immiscible liquid.

Although liquid immiscibility is now widely acceptedas a phenomenon in natural magmas, the extent of theprocess is still in question, and its importance in generatinglarge bodies or a significant proportion of evolved magmaticrocks is doubtful.

3 MAGMA MIXING

Magma mixing is a bit like liquid immiscibility in reverse,and so was some of the reasoning behind its historical origins.The reigning paradigm of fractional crystallization implies

Lc SiO2

FaSiO2

SiO2Kfs

1000

1100

1200 Fa

Two liquids

FIGURE 7 Two immiscibility gaps in the systemfayalite–leucite–silica. The central one is of a composition, andat a low enough temperature (see the section in the upper left)to be attainable in some Fe-rich natural magmas (after Roedder,1979, copyright © the Mineralogical Society of America).Projected into the simplified system are the compositions of natural immiscible silicate pair droplets from interstitial Fe-rich tholeiitic glasses (Philpotts, 1982).

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