Chapter 11: Diversification of Chapter 11: Diversification of MagmasMagmas

Magmatic DifferentiationMagmatic Differentiation Any process by which a magma is able to Any process by which a magma is able to

diversify and produce a magma or rock of diversify and produce a magma or rock of different compositiondifferent composition

Magmatic DifferentiationMagmatic Differentiation Two essential processesTwo essential processes

1. Creates a compositional difference1. Creates a compositional difference in one or more in one or more phasesphases

2. Preserves2. Preserves the chemical difference by the chemical difference by segregatingsegregating (or (or fractionatingfractionating) the chemically distinct portions) the chemically distinct portions

Partial MeltingPartial Melting

Separation of a partially melted liquid from Separation of a partially melted liquid from the solid residuethe solid residue

Effects of removing liquid at various stages of meltingEffects of removing liquid at various stages of melting Eutectic systemsEutectic systems

First melt First melt alwaysalways = eutectic composition = eutectic composition MajorMajor element composition of eutectic melt is element composition of eutectic melt is

constant until one of the source mineral phases is constant until one of the source mineral phases is consumed consumed (trace elements differ)(trace elements differ)

Once a phase is consumed, the next increment of melt Once a phase is consumed, the next increment of melt will be different X and Twill be different X and T

Separation of a partially melted liquid from Separation of a partially melted liquid from the solid residue requires a critical melt %the solid residue requires a critical melt %

Sufficient melt must be produced for it to Sufficient melt must be produced for it to Form a continuous, interconnected film Form a continuous, interconnected film Have enough interior volume that it is not Have enough interior volume that it is not

all of it is adsorbed to the crystal surfacesall of it is adsorbed to the crystal surfaces

The ability to form an interconnected film is dependent The ability to form an interconnected film is dependent upon the upon the dihedral angle (dihedral angle ()) a property of the melt a property of the melt

Figure 11-1Figure 11-1 Illustration of the dihedral Illustration of the dihedral

angle (angle () of melt droplets that typically ) of melt droplets that typically form at multiple grain junctions.form at multiple grain junctions. After After Hunter (1987)Hunter (1987) In I. Parsons (ed.), Origins In I. Parsons (ed.), Origins of Igneous Layering. Reidel, Dordrecht, of Igneous Layering. Reidel, Dordrecht, pp. 473-504.pp. 473-504.

Gravitational effects (Gravitational effects (buoyantbuoyant liquid) liquid) Filter pressingFilter pressing, or , or compaction,compaction, of of crystal mushcrystal mush Shear -Shear - the RCMP drops considerably the RCMP drops considerably RCMP varies with RCMP varies with

TT viscosityviscosity XX

Dominant mechanism by which most Dominant mechanism by which most magmas, once formed, differentiate?magmas, once formed, differentiate?

Crystal FractionationCrystal Fractionation

Gravity settlingGravity settling The differential motion of crystals and liquid The differential motion of crystals and liquid

under the influence of gravity due to their under the influence of gravity due to their differences in densitydifferences in density

Gravity settlingGravity settling

Cool point Cool point aa olivine layer at base of pluton if olivine layer at base of pluton if first olivine sinksfirst olivine sinks

Next get ol+cpx layerNext get ol+cpx layer

finally get ol+cpx+plagfinally get ol+cpx+plag

Cumulate texture:Cumulate texture:Mutually touching Mutually touching phenocrysts with phenocrysts with interstitial crystallized interstitial crystallized residual meltresidual melt

Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams.

Krieger Publishers.

Figure 11-2Figure 11-2 Variation diagram using MgO as the abscissa for lavas associated with the 1959 Kilauea eruption in Hawaii. Variation diagram using MgO as the abscissa for lavas associated with the 1959 Kilauea eruption in Hawaii. AAfter Murata and Richter, 1966 (as modified by Best, 1982)fter Murata and Richter, 1966 (as modified by Best, 1982)

Stoke’s LawStoke’s Law

VV = the settling velocity (cm/sec)= the settling velocity (cm/sec)

gg = the acceleration due to gravity (980 cm/sec= the acceleration due to gravity (980 cm/sec22) )

rr = the = the radiusradius of a spherical particle (cm) of a spherical particle (cm)

ss = the density of the solid spherical particle (g/cm= the density of the solid spherical particle (g/cm33))

ll = the density of the liquid (g/cm= the density of the liquid (g/cm33))

= the viscosity of the liquid (1 c/cm sec = 1 poise)= the viscosity of the liquid (1 c/cm sec = 1 poise)

V2gr ( )

9

2

s l

Olivine in basaltOlivine in basalt

Olivine (Olivine (ss = 3.3 g/cm = 3.3 g/cm33, , r = 0.1 cmr = 0.1 cm) )

Basaltic liquid (Basaltic liquid (ll = 2.65 g/cm = 2.65 g/cm33, , = 1000 poise) = 1000 poise)

V = 2·980·0.1V = 2·980·0.12 2 (3.3-2.65)/9·1000 = (3.3-2.65)/9·1000 = 0.0013 cm/sec0.0013 cm/sec

RhyoliticRhyolitic melt melt = 10= 1077 poise and poise and ll = 2.3 g/cm = 2.3 g/cm33

hornblendehornblende crystal ( crystal (ss = 3.2 g/cm = 3.2 g/cm33, , r = 0.1 cmr = 0.1 cm) ) V = 2 x 10V = 2 x 10-7-7 cm/sec, or cm/sec, or 6 cm/year6 cm/year

feldsparsfeldspars ( (ll = 2.7 g/cm = 2.7 g/cm33) ) V = 2 cm/yearV = 2 cm/year = = 200 m in the 10200 m in the 1044 years years that a stock might cool that a stock might cool If 0.5 cm in radius (If 0.5 cm in radius (1 cm diameter1 cm diameter) settle at ) settle at 0.65 0.65

meters/yearmeters/year, or 6.5 km in 10, or 6.5 km in 1044 year cooling of stock year cooling of stock

Stokes’ Law is overly simplifiedStokes’ Law is overly simplified

1. Crystals are not spherical1. Crystals are not spherical

2.2. Only basaltic magmas very near their liquidus Only basaltic magmas very near their liquidus temperatures behave as Newtonian fluidstemperatures behave as Newtonian fluids

Many silicic magmas approach the ternary eutecticMany silicic magmas approach the ternary eutecticEither Either fractional crystallizationfractional crystallization does take place or they does take place or they

are are minimum (eutectic) meltsminimum (eutectic) melts

Figure 11-3Figure 11-3 Position of the H2O-saturated Position of the H2O-saturated ternary eutectic in the albite-orthoclase-ternary eutectic in the albite-orthoclase-silica system at various pressures. The silica system at various pressures. The shaded portion represents the shaded portion represents the composition of most granites. Included composition of most granites. Included are the compositions of the Tuolumne are the compositions of the Tuolumne Intrusive Series (Figure 4-32), with the Intrusive Series (Figure 4-32), with the arrow showing the direction of the trend arrow showing the direction of the trend from early to late magma batches. from early to late magma batches. Experimental data from Wyllie Experimental data from Wyllie et alet al. (1976). . (1976). From Winter (2001) An Introduction to From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Igneous and Metamorphic Petrology. Prentice HallPrentice Hall

PolybaricPolybaric Fractional Crystallization Fractional Crystallization

1. Stability of phases changes (hi-P garnet...)1. Stability of phases changes (hi-P garnet...)

2. Shift of the eutectic point with pressure will 2. Shift of the eutectic point with pressure will cause the quantity of the liquidus phases to varycause the quantity of the liquidus phases to vary

Ol

Low-P

Pyx

Hi-PHigh-P (red tie-line) High-P (red tie-line) has liq > olhas liq > ol

Low-P (yellow tie-Low-P (yellow tie-line) has ol > liquidline) has ol > liquid

Expansion of olivine field at low pressure causes Expansion of olivine field at low pressure causes an increase in the quantity of crystallized olivinean increase in the quantity of crystallized olivine

Two other mechanisms that facilitate the Two other mechanisms that facilitate the separation of crystals and liquidseparation of crystals and liquid

1. Compaction1. Compaction

Two other mechanisms that facilitate the Two other mechanisms that facilitate the separation of crystals and liquidseparation of crystals and liquid

2. Flow segregation2. Flow segregation

Figures 11-4 and 11-5Figures 11-4 and 11-5 Drever and Johnston (1958). Royal Drever and Johnston (1958). Royal Soc. Edinburgh Trans., 63, 459-499.Soc. Edinburgh Trans., 63, 459-499.

Volatile TransportVolatile Transport1. Vapor released by heating of hydrated or carbonated 1. Vapor released by heating of hydrated or carbonated

wall rockswall rocks

Volatile TransportVolatile Transport2. As a volatile-bearing 2. As a volatile-bearing

(but undersaturated) (but undersaturated) magma rises and magma rises and pressure is reduced, the pressure is reduced, the magma may eventually magma may eventually become saturated in the become saturated in the vapor, and a free vapor vapor, and a free vapor phase will be releasedphase will be released

Figure 7-22. From Burnham and Davis (1974). A J Sci., 274, 902-940.

3. Late-stage fractional crystallization3. Late-stage fractional crystallization

Fractional crystallization enriches late melt in Fractional crystallization enriches late melt in incompatible, LIL, and non-lithophile elementsincompatible, LIL, and non-lithophile elements

Many concentrate further in the vaporMany concentrate further in the vapor Particularly enriched with resurgent boiling (melt Particularly enriched with resurgent boiling (melt

already evolved when vapor phase released)already evolved when vapor phase released) Get a silicate-saturated vapor + a vapor-saturated Get a silicate-saturated vapor + a vapor-saturated

late derivative silicate liquidlate derivative silicate liquid

Volatile release raises liquidus temperature Volatile release raises liquidus temperature porphyritic textureporphyritic texture

May increase P - fracture the roof rocksMay increase P - fracture the roof rocks Vapor and melt escape along fractures as dikesVapor and melt escape along fractures as dikes

Silicate meltSilicate melt quartz and feldspar quartz and feldspar

small dikes of small dikes of apliteaplite Vapor phaseVapor phase dikes or pods of dikes or pods of pegmatitepegmatite

Concentrate incompatible elementsConcentrate incompatible elements Complex: varied mineralogyComplex: varied mineralogy

May display May display concentric zonationconcentric zonation

Figure 11-6Figure 11-6 Sections of three zoned fluid-phase deposits (not at the same scale). Sections of three zoned fluid-phase deposits (not at the same scale). a.a. Miarolitic pod in granite (several cm Miarolitic pod in granite (several cm across). across). b.b. Asymmetric zoned pegmatite dike with aplitic base (several tens of cm across). Asymmetric zoned pegmatite dike with aplitic base (several tens of cm across). c. c. Asymmetric zoned pegmatite Asymmetric zoned pegmatite with granitoid outer portion (several meters across). with granitoid outer portion (several meters across). From Jahns and Burnham (1969). From Jahns and Burnham (1969). Econ. Geol.Econ. Geol., 64, 843-864. , 64, 843-864.

8 cm tourmaline crystals 8 cm tourmaline crystals from pegmatitefrom pegmatite

5 mm gold from a 5 mm gold from a hydrothermal deposithydrothermal deposit

Liquid immiscibility in the Fo-SiOLiquid immiscibility in the Fo-SiO22 system systemLiquid ImmiscibilityLiquid Immiscibility

Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.

The effect of adding The effect of adding alkalis, alumina, etc. is alkalis, alumina, etc. is to to eliminate the solvus eliminate the solvus completelycompletely

Figure 7-4. Isobaric diagram illustrating the cotectic and peritectic curves in the system forsterite-anorthite-silica at 0.1 MPa. After Anderson (1915) A. J. Sci., and Irvine (1975) CIW Yearb. 74.

Renewed interest when Roedder (1951) discovered a Renewed interest when Roedder (1951) discovered a second immiscibility gap in the iron-rich Fa-Lc-SiOsecond immiscibility gap in the iron-rich Fa-Lc-SiO22

systemsystem

Figure 11-7. Two immiscibility gaps in the system fayalite-leucite-silica (after Roedder, 1979). Yoder (ed.), The Evolution of the Igneous Rocks. Princeton University Press. pp. 15-58. Projected into the simplified system are the compositions of natural immiscible silicate pair droplets from interstitial Fe-rich tholeiitic glasses (Philpotts, 1982). Contrib. Mineral. Petrol., 80, 201-218.

Some ExamplesSome Examples Late silica-rich immiscible droplets in Fe-rich Late silica-rich immiscible droplets in Fe-rich

tholeiitic basalts (as in Roedder)tholeiitic basalts (as in Roedder) Sulfide-silicate immiscibility (massive sulfide Sulfide-silicate immiscibility (massive sulfide

deposits)deposits) Carbonatite-nephelinite systems (Chapter 19)Carbonatite-nephelinite systems (Chapter 19)

Tests for immiscible origin of Tests for immiscible origin of associated rock pairsassociated rock pairs

1. The magmas must be immiscible when 1. The magmas must be immiscible when heated experimentally, or they must plot on heated experimentally, or they must plot on the boundaries of a known immiscibility the boundaries of a known immiscibility gap, as in Fig. 11-7gap, as in Fig. 11-7

2. Immiscible liquids are in equilibrium with 2. Immiscible liquids are in equilibrium with each other, and thus they must be in each other, and thus they must be in equilibrium with the same minerals equilibrium with the same minerals

Tests for immiscible origin of Tests for immiscible origin of associated rock pairsassociated rock pairs

Compositional Convection and Compositional Convection and In In SituSitu Differentiation Processes Differentiation Processes

In-situ: crystals don’t sink/move Typically involves

Diffusion Convective separation of liquid and crystals

The Soret Effect and The Soret Effect and Thermogravitational DiffusionThermogravitational Diffusion

Thermal diffusionThermal diffusion, or the , or the Soret effectSoret effect HeavyHeavy elements/molecules migrate toward the elements/molecules migrate toward the

coldercolder end and end and lighterlighter ones to the ones to the hotterhotter end of end of the gradientthe gradient

Walker and DeLong (1982) subjected two basalts to Walker and DeLong (1982) subjected two basalts to thermal gradients of nearly 50thermal gradients of nearly 50ooC/mm (!) C/mm (!)

Found that:Found that: Samples reached a steady Samples reached a steady

state in a few daysstate in a few days Heavier elements Heavier elements cooler cooler

end and the lighter end and the lighter hot hot endend

The chemical concentration The chemical concentration is similar to that expected is similar to that expected from fractional from fractional crystallization crystallization

Figure 7-4. After Walker, D. C. and S. E. DeLong (1982). Contrib. Mineral.

Petrol., 79, 231-240.

Thermogravitational diffusionThermogravitational diffusion

Stable and persistent stagnant boundary layers Stable and persistent stagnant boundary layers have been shown to occur near the top and have been shown to occur near the top and sides of magma chamberssides of magma chambers

Hildreth (1979) 0.7 Ma Hildreth (1979) 0.7 Ma Bishop TuffBishop Tuff at Long at Long Valley, CaliforniaValley, California

Vertical Vertical compositionalcompositional variation in the stratified variation in the stratified tufftuff

Thermal gradient in chamberThermal gradient in chamber

ModelModel

Figure 11-11. Schematic section through a rhyolitic magma chamber undergoing convection-aided in-situ differentiation. After Hildreth (1979). Geol. Soc. Amer. Special Paper, 180, 43-75.

Langmuir ModelLangmuir Model Thermal gradient at Thermal gradient at

wall and cap wall and cap variation in % variation in % crystallizedcrystallized

Compositional Compositional convection convection evolved magmas evolved magmas from boundary from boundary layer to cap (or mix layer to cap (or mix into interior)into interior)

Figure 11-12Figure 11-12 Formation of boundary layers Formation of boundary layers along the walls and top of a magma along the walls and top of a magma chamber. chamber. From Winter (2001) An From Winter (2001) An Introduction to Igneous and Metamorphic Introduction to Igneous and Metamorphic Petrology. Prentice HallPetrology. Prentice Hall

Magma MixingMagma Mixing End member mixingEnd member mixing for a suite of rocks for a suite of rocks Variation on Harker-type diagrams Variation on Harker-type diagrams

should lie on a straight lineshould lie on a straight line between the between the two most extreme compositionstwo most extreme compositions

Figure 11-2Figure 11-2 Variation diagram using MgO as the abscissa for lavas associated with the 1959 Kilauea eruption in Hawaii. Variation diagram using MgO as the abscissa for lavas associated with the 1959 Kilauea eruption in Hawaii. AAfter Murata and Richter, 1966 (as modified by Best, 1982)fter Murata and Richter, 1966 (as modified by Best, 1982)

Basalt pillows Basalt pillows accumulating at the bottom accumulating at the bottom

of a in granitic magma of a in granitic magma chamber, Vinalhaven chamber, Vinalhaven

Island, MaineIsland, Maine

Comingled basalt-Rhyolite Comingled basalt-Rhyolite Mt. McLoughlin, OregonMt. McLoughlin, Oregon

Figure 11-8Figure 11-8 From Winter (2001) An From Winter (2001) An Introduction to Igneous and Introduction to Igneous and Metamorphic Petrology. Prentice HallMetamorphic Petrology. Prentice Hall

AssimilationAssimilation Incorporation of wall rocks (diffusion, Incorporation of wall rocks (diffusion,

xenoliths)xenoliths) Assimilation by melting is limited by Assimilation by melting is limited by

the heat available in the magmathe heat available in the magma

Zone meltingZone melting

Crystallizing igneous material at the Crystallizing igneous material at the base equivalent to the amount melted base equivalent to the amount melted at the topat the top

Transfer heat by convection Transfer heat by convection

Detecting and assessing assimilationDetecting and assessing assimilationIsotopesIsotopes are generally the best are generally the best

Continental crust becomes progressively enriched Continental crust becomes progressively enriched in in 8787Sr/Sr/8686Sr and depleted in Sr and depleted in 143143Nd/Nd/144144NdNd

Figure 9-13.Figure 9-13. Estimated Rb and Sr Estimated Rb and Sr isotopic evolution of isotopic evolution of the Earth’s upper the Earth’s upper mantle, assuming a mantle, assuming a large-scale melting large-scale melting event producing event producing granitic-type granitic-type continental rocks at continental rocks at 3.0 Ga b.p After 3.0 Ga b.p After Wilson (1989). Wilson (1989). Igneous Igneous Petrogenesis. Unwin Petrogenesis. Unwin Hyman/Kluwer.Hyman/Kluwer.

9-21 238U 234U 206Pb ( = 1.5512 x 10-10 a-1)

9-22 235U 207Pb ( = 9.8485 x 10-10 a-1)

9-23 232Th 208Pb ( = 4.9475 x 10-11 a-1)

U-Th-Pb system as an indicator of continental U-Th-Pb system as an indicator of continental contamination is particularly usefulcontamination is particularly useful

All are incompatible LIL elements, so they All are incompatible LIL elements, so they concentrate strongly into the continental crustconcentrate strongly into the continental crust

Detecting and assessing assimilationDetecting and assessing assimilation

Mixed ProcessesMixed Processes May be more than coincidence: two May be more than coincidence: two

processes may operate in conjunction processes may operate in conjunction (cooperation?)(cooperation?) AFC: FX supplies the necessary heat AFC: FX supplies the necessary heat

for assimilationfor assimilation Fractional crystallization + recharge of Fractional crystallization + recharge of

more primitive magmamore primitive magma

Tectonic-Igneous AssociationsTectonic-Igneous Associations

Associations on a larger scale than the Associations on a larger scale than the petrogenetic provincespetrogenetic provinces

An attempt to address global patterns An attempt to address global patterns of igneous activity by grouping of igneous activity by grouping provinces based upon similarities in provinces based upon similarities in occurrence and genesisoccurrence and genesis

Mid-Ocean Ridge VolcanismMid-Ocean Ridge Volcanism Ocean Intra-plate (Island) volcanismOcean Intra-plate (Island) volcanism Continental Plateau BasaltsContinental Plateau Basalts Subduction-related volcanism and plutonismSubduction-related volcanism and plutonism

Island ArcsIsland Arcs Continental ArcsContinental Arcs

Granites (not a true T-I Association)Granites (not a true T-I Association) Mostly alkaline igneous processes of stable Mostly alkaline igneous processes of stable

craton interiorscraton interiors Anorthosite Massifs Anorthosite Massifs

Tectonic-Igneous AssociationsTectonic-Igneous Associations