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Your text, p. 185, suggests the Your text, p. 185, suggests the following reactions for transitions in following reactions for transitions in the upper mantle:the upper mantle:

From Plagioclase to Spinel PeridotiteFrom Plagioclase to Spinel PeridotiteCaAl2Si2O8 +Mg2SiO4 =2 MgSiO3 CaAl2Si2O8 +Mg2SiO4 =2 MgSiO3

+CaMgSi2O6+MgAl2O4+CaMgSi2O6+MgAl2O4

An + Fo = 2 En + Di + SpinelAn + Fo = 2 En + Di + Spinel

BTW not all of the Olivine is consumed, and we saw earlier BTW not all of the Olivine is consumed, and we saw earlier that Mg2SiO4 has a Spinel STRUCTURE at depth.that Mg2SiO4 has a Spinel STRUCTURE at depth.

From Spinel to Garnet PeridotitesFrom Spinel to Garnet PeridotitesMgSiO3 + MgAl2O4 = Mg2SiO4 +Mg3Al2Si3O12MgSiO3 + MgAl2O4 = Mg2SiO4 +Mg3Al2Si3O12

En + Spinel = Fo + GarnetEn + Spinel = Fo + Garnet

We discussed mantle composition and found some differences in magmas may depend on the depth of the source.

Ch 11. Magmatic Ch 11. Magmatic DifferentiationDifferentiation

In Chapter 10 we created a primary magma In Chapter 10 we created a primary magma by partial melting of the mantleby partial melting of the mantle

• It is a It is a basaltbasalt Can we get the diversity of igneous rocks that Can we get the diversity of igneous rocks that

we find at the surface from this parent?we find at the surface from this parent?

Magmatic Differentiation: any process by Magmatic Differentiation: any process by which a magma is able to diversify and which a magma is able to diversify and produce a magma or rock of different produce a magma or rock of different compositioncomposition

Magmatic DifferentiationMagmatic Differentiation Two essential processesTwo essential processes

1. Creates a compositional difference1. Creates a compositional difference in one in one or more phasesor more phases

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

SegregationSegregation

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

Incongruent meltingIncongruent melting

Many minerals do not melt uniformly. Instead they Many minerals do not melt uniformly. Instead they decompose as they melt, becoming melt plus a new decompose as they melt, becoming melt plus a new solid mineral species. One example is solid Forsterite solid mineral species. One example is solid Forsterite (Mg(Mg22SiOSiO44), which decomposes to solid Enstatite ), which decomposes to solid Enstatite (MgSiO(MgSiO33) plus liquid silica (SiO) plus liquid silica (SiO22) in the melt.) in the melt.

We say Forsterite is chemically incompatible with We say Forsterite is chemically incompatible with quartz, because the reaction ensures Enstatite forms quartz, because the reaction ensures Enstatite forms from Olivine and silica. Forsterite reacts with Quartz as from Olivine and silica. Forsterite reacts with Quartz as follows:follows:

Forsterite (MgForsterite (Mg22SiOSiO44) (s) + Quartz (SiO) (s) + Quartz (SiO22) (l) = 2 Enstatite (MgSiO) (l) = 2 Enstatite (MgSiO33) ) (s)(s)

Incongruent Solidification of a Mantle partial melt: Incongruent Solidification of a Mantle partial melt: considering only components Mgconsidering only components Mg++++ and (SiO and (SiO44))-4-4

• We start with a mantle melt between Forsterite Olivine Mg2SiO4 and Enstatite MgSiO3 in composition.

• At a, the melt begins cooling.

Diagram courtesy of Steve Dutch

Incongruent Solidification of a Mantle partial melt:Incongruent Solidification of a Mantle partial melt: considering just components Mg considering just components Mg++++ and (SiO and (SiO44))-4-4

•At T= b, the melt has reached the liquidus temperature and solid Forsterite begins to form

Incongruent Solidification of a Mantle partial melt:Incongruent Solidification of a Mantle partial melt: considering just components Mg considering just components Mg++++ and (SiO and (SiO44))-4-4

•At c, a bit more than half the melt has solidified as Forsterite. The melt has passed the composition of Enstatite, but is still too hot for it to crystallize out.

Incongruent Solidification of a Mantle partial melt: Incongruent Solidification of a Mantle partial melt: considering just components Mgconsidering just components Mg++++ and (SiO and (SiO44))-4-4

•At d, we have reached the freezing/melting point of Enstatite. We are on the boundaries of fields containing both Forsterite and Enstatite. Therefore we must have both solid phases present, and Enstatite begins to form. When Enstatite cools, some Enstatite forms directly from the melt, but some forms at the expense of Forsterite.

Incongruent Solidification of a Mantle partial melt: Incongruent Solidification of a Mantle partial melt: considering just components Mgconsidering just components Mg++++ and (SiO and (SiO44))-4-4

•Once solid Enstatite begins to form at d, the Temperature remains constant for the phase change, and the solidus moves horizontally as the proportion of En increases in the En + Fo mush. •For example at e, Enstatite is forming and the solid composition moves toward Enstatite. When it reaches the original system composition, the system is completely solidified.

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

Sufficient melt must be produced Sufficient melt must be produced for it to for it to – Form a continuous, interconnected Form a continuous, interconnected

film film – Have enough interior volume that not Have enough interior volume that not

all of it is adsorbed to the crystal all of it is adsorbed to the crystal surfacessurfaces

The ability to form an interconnected The ability to form an interconnected film is dependent upon the film is dependent upon the dihedral dihedral angle (angle ()) a property of the melt: easier a property of the melt: easier with smaller anglewith smaller angle

Liquid separation motivated by density effects (more buoyant liquid rises and escapes)

Filter pressing, or compaction, in which a crystal mush is squeezed like a sponge by weight of crystals above.

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

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

liquid under the influence of gravity liquid under the influence of gravity due to their differences in densitydue to their differences in density

Crystal FractionationCrystal Fractionation

Gravity settlingGravity settling

Cool point Cool point aa olivine layer at base of pluton if first olivine layer at base of pluton if first olivine sinksolivine 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.

Stoke’s LawStoke’s Law

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

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

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

ss = the density of the solid spherical = the density of the solid spherical particle (g/cmparticle (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 = the viscosity of the liquid (1 c/cm sec = 1 poise)= 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)

– Use Stoke’s Law:Use Stoke’s Law:

– 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 = r = 0.1 cm0.1 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 that a stock

might coolmight cool If 0.5 cm in radius (If 0.5 cm in radius (1 cm diameter1 cm diameter) )

settle at settle at 0.65 meters/year0.65 meters/year, or 6.5 km in , or 6.5 km in 101044 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 Only basaltic magmas very near their liquidus temperatures their liquidus temperatures behave as Newtonian fluidsbehave as Newtonian fluids

Ol

Low-P

Pyx

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

Low-P (lower tie-line) Low-P (lower tie-line) has ol > liquidhas ol > liquid

Expansion Expansion of oof olivine field at low pressure causes livine field at low pressure causes an increase in the quantity of crystallized olivinean increase in the quantity of crystallized olivineThus, the amount of olivine that crystallizes with a Thus, the amount of olivine that crystallizes with a rising basaltic magma will be greater that the rising basaltic magma will be greater that the amount that forms during isobaric crystallizationamount that forms during isobaric crystallization

bulk

bc adff ee

See Lever Principle, Figs. 6-8 and 6-9 For example, the lower tie line hasamount liquid = ef ~ 1/2 there is about twice as much solid Olivine as meltamount solid de

liquid

all solids

all solids

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

1. Flow segregation1. Flow segregation

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

Idea: The motion of the magma past the Idea: The motion of the magma past the stationary walls of the country rock stationary walls of the country rock creates shear in the viscous liquidcreates shear in the viscous liquid

Magma must flow around phenocrysts, Magma must flow around phenocrysts, thereby exerting pressure on them at thereby exerting pressure on them at constrictions where phenocrysts are constrictions where phenocrysts are near one another or the contactnear one another or the contact

grain dispersive pressuregrain dispersive pressure, forcing the , forcing the grains apart and away from the contactgrains apart and away from the contact

This is probably a relatively minor This is probably a relatively minor effecteffect

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

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

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

3. Late-stage Fractional 3. Late-stage Fractional CrystallizationCrystallization Fractional crystallization Fractional crystallization

enriches late melt in non-rock-enriches late melt in non-rock-forming (non-lithophile) forming (non-lithophile) elementselements

Particularly enriched with Particularly enriched with resurgent boiling (melt already resurgent boiling (melt already evolved when vapor phase evolved when vapor phase released)released)

Get a silicate-saturated vapor + Get a silicate-saturated vapor + a vapor-saturated late a vapor-saturated late derivative silicate liquidderivative silicate liquid

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-En-SiOLiquid immiscibility in the Fo-En-SiO22 systemsystem

Liquid 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.

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

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

steady state in a steady state in a few daysfew days

Heavier elements Heavier elements cooler end and the cooler end and the lighter lighter hot end hot end

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

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

Petrol., 79, 231-240.

Si at top, Fe Mg Ti Ca on bottom

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

of a granitic magma of a 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

Magma Mixing

AssimilationAssimilation Incorporation of wall rocks Incorporation of wall rocks

(diffusion, xenoliths)(diffusion, xenoliths) Assimilation by melting is limited Assimilation by melting is limited

by the heat available in the by the heat available in the magma magma

Xenolith melts if the melting point Xenolith melts if the melting point of the country rock is (much) less of the country rock is (much) less than the temperature of the than the temperature of the magmamagma

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

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

• Some trace elements are much more abundant in the continental crust than in mantle-derived magmas.• The assimilation of a modest amount of crustal material rich in that element may have a considerable effect on a magma that initially contained very little of it.

•During the fractional crystallization of magma, and magma generation by the partial melting of the Earth's mantle and crust, elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase). An incompatible element is an element that is unsuitable in size and/or charge to the cation sites of the minerals

Detecting and Assessing AssimilationDetecting and Assessing Assimilation

9-22 238U 234U 206Pb ( = 1.5512 x 10-10 a-1)9-23 235U 207Pb ( = 9.8485 x 10-10 a-1)9-24 232Th 208Pb ( = 4.9475 x 10-11 a-1)

Detecting and assessing assimilationDetecting and assessing assimilation

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

All incompatibles similar to Zr+4, All incompatibles similar to Zr+4, so they concentrate strongly into so they concentrate strongly into the continental crust because they the continental crust because they are not removed during early are not removed during early fractionation.fractionation.

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

two processes may operate in two processes may operate in conjunction.conjunction.

E.g. fractional crystallization + E.g. fractional crystallization + recharge of more primitive recharge of more primitive magmamagma

As we shall see next time.As we shall see next time.