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

reactions for transitions in the upper mantle:

mantle:

 From Plagioclase to Spinel PeridotiteFrom Plagioclase to Spinel Peridotite

CaAl2Si2O8 +Mg2SiO4 =2 MgSiO3 +CaMgSi2O6+MgAl2O4 CaAl2Si2O8 +Mg2SiO4 =2 MgSiO3 +CaMgSi2O6+MgAl2O4 An + Fo = 2 En + Di + Spinel An + Fo = 2 En + Di + Spinel

BTW not all of the Olivine is consumed, and we saw earlier that BTW not all of the Olivine is consumed, and we saw earlier that

Mg2SiO4 has a Spinel STRUCTURE at depth.

 From Spinel to Garnet PeridotitesFrom Spinel to Garnet Peridotites

MgSiO3 + MgAl2O4 = Mg2SiO4 +Mg3Al2Si3O12 MgSiO3 + MgAl2O4 = Mg2SiO4 +Mg3Al2Si3O12 En + Spinel = Fo + Garnet

En + Spinel = Fo + Garnet



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

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Ch 11. Magmatic Differentiation Ch 11. Magmatic Differentiation

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

partial melting of the mantle

• It is a It is a

basalt basalt

 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 which a Magmatic Differentiation: any process by which a magma is able to diversify and produce a

magma is able to diversify and produce a magma or rock of different composition magma or rock of different composition

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Magmatic Differentiation Magmatic Differentiation

 Two essential processesTwo essential processes

1. Creates a compositional difference

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

more phases 2. Preserves

2. Preserves the chemical difference by the chemical difference by segregating

segregating (or (or fractionatingfractionating) the chemically ) the chemically distinct portions

distinct portions

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Segregation Segregation

Separation of a partially melted liquid from Separation of a partially melted liquid from

the solid residue

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Incongruent melting Incongruent 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 solid decompose as they melt, becoming melt plus a new solid

mineral species. One example is solid Forsterite mineral species. One example is solid Forsterite

(Mg(Mg₂₂SiOSiO₄₄), which decomposes to solid Enstatite (MgSiO), which decomposes to solid Enstatite (MgSiO₃₃) ) plus liquid silica (SiO

plus liquid silica (SiO₂₂) in the melt.) in the melt.

 We say Forsterite is chemically incompatible with quartz, We say Forsterite is chemically incompatible with quartz, because the reaction ensures Enstatite forms from Olivine because the reaction ensures Enstatite forms from Olivine

and silica. Forsterite reacts with Quartz as follows:

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

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Incongruent Solidification of a Mantle partial melt:

considering only components Mg

considering 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

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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

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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.

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Incongruent Solidification of a Mantle partial melt: considering just Incongruent Solidification of a Mantle partial melt: considering just

components Mg

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.

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Incongruent Solidification of a Mantle partial melt: considering just Incongruent Solidification of a Mantle partial melt: considering just

components Mg

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.

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

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

melt % melt %



Sufficient melt must be produced for it Sufficient melt must be produced for it to to

– Form a continuous, interconnected film Form a continuous, interconnected film

– Have enough interior volume that not all of Have enough interior volume that not all of it is adsorbed to the crystal surfaces

it is adsorbed to the crystal surfaces

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The ability to form an interconnected film is The ability to form an interconnected film is

dependent upon the

dependent upon the dihedral angle ( dihedral angle (   ) ) a a property of the melt: easier with smaller property of the melt: easier with smaller

angle

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

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Filter pressing, or compaction, in which a

crystal mush is squeezed like a sponge by

weight of

crystals above.

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

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

magmas, once formed, differentiate?

Gravity settling Gravity settling

– The differential motion of crystals and The differential motion of crystals and

liquid under the influence of gravity due to liquid under the influence of gravity due to

their differences in density their differences in density

Crystal Fractionation

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Gravity settling Gravity settling

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

 Next get ol+cpx layerNext get ol+cpx layer

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

Cumulate texture:

Mutually touching Mutually touching phenocrysts with phenocrysts with

interstitial crystallized interstitial crystallized residual melt

residual melt

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

Krieger Publishers.

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Stoke’s Law Stoke’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)

_s_s= the density of the solid spherical particle = the density of the solid spherical particle (g/cm

(g/cm³³))

_l_l = the density of the liquid (g/cm= the density of the liquid (g/cm³³))

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

poise)

V 2gr ( )

9

 2   

^s ^l

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Olivine in basalt Olivine in basalt

– Olivine (Olivine (_s_s = 3.3 g/cm = 3.3 g/cm³³, , r = 0.1 cmr = 0.1 cm) )

– Basaltic liquid (Basaltic liquid (_l_l = 2.65 g/cm = 2.65 g/cm³³, ,  = 1000 = 1000 poise)

poise)

– Use Stoke’s Law:Use Stoke’s Law:

– V = 2·980·0.1V = 2·980·0.1²²(3.3-2.65)/9·1000 = (3.3-2.65)/9·1000 = 0.0013 0.0013 cm/sec

cm/sec

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Rhyolitic

Rhyolitic melt melt

  = 10= 10⁷⁷ poise and poise and _l_l = 2.3 g/cm = 2.3 g/cm³³

– hornblendehornblende crystal ( crystal (_s_s = 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 ( (_l_l = 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 that a stock might coolcool

 If 0.5 cm in radius (If 0.5 cm in radius (1 cm diameter1 cm diameter) settle at ) settle at 0.65 meters/year

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

cooling of stock

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Stokes’ Law is overly simplified Stokes’ Law is overly simplified

1. Crystals are not spherical 1. Crystals are not spherical

2. 2. Only basaltic magmas very near their Only basaltic magmas very near their liquidus temperatures behave as

liquidus temperatures behave as Newtonian fluids

Newtonian fluids

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Ol

Low-P

Pyx

High-P (upper tie-line) Hi-P

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

has liq > ol

Low-P (lower tie-line) Low-P (lower tie-line)

has ol > liquid has 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 olivine an increase in the quantity of crystallized olivine Thus, 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 crystallization amount that forms during isobaric crystallization

bulk

b

c a

f d

f ee

See Lever Principle, Figs. 6-8 and 6-9 For example, the lower tie line has

amount liquid = ef ~ 1/2 there is about twice as much solid Olivine as melt amount solid de

liquid

all solids

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Two other mechanisms that facilitate the Two other mechanisms that facilitate the

separation of crystals and liquid separation of crystals and liquid

1. Flow segregation 1. 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.

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

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 near one constrictions where phenocrysts are near one another or the contact

another or the contact

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

This is probably a relatively minor effect This is probably a relatively minor effect

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Volatile Transport Volatile Transport

2. As a volatile-bearing 2. As a volatile-bearing (but undersaturated) (but undersaturated)

magma rises and magma rises and

pressure is reduced, pressure is reduced,

the magma may the magma may

eventually become eventually become

saturated in the saturated in the

vapor, and a free vapor, and a free

vapor phase will be vapor phase will be

released released

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

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3. Late-stage Fractional Crystallization 3. Late-stage Fractional Crystallization



Fractional crystallization enriches Fractional crystallization enriches late melt in non-rock-forming (non- late melt in non-rock-forming (non-

lithophile) elements lithophile) elements



Particularly enriched with resurgent Particularly enriched with resurgent boiling (melt already evolved when boiling (melt already evolved when

vapor phase released) vapor phase released)



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

vapor-saturated late derivative silicate liquid

silicate liquid

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8 cm tourmaline crystals 8 cm tourmaline crystals

from pegmatite from pegmatite

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

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 Liquid immiscibility in the Fo-En-SiOLiquid immiscibility in the Fo-En-SiO

Liquid Immiscibility Liquid Immiscibility

₂₂ system system

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.

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Walker and DeLong (1982) subjected two basalts Walker and DeLong (1982) subjected two basalts

to thermal gradients of nearly 50

^o^o

C/mm! C/mm!

Found that:

 Samples reached a Samples reached a steady state in a few steady state in a few days 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

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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, Maine Island, Maine

Comingled basalt-Rhyolite Comingled basalt-Rhyolite

Mt. McLoughlin, Oregon Mt. McLoughlin, Oregon

Figure 11-8

Figure 11-8 From Winter (2001) An From Winter (2001) An Introduction to Igneous and

Introduction to Igneous and

Metamorphic Petrology. Prentice Hall Metamorphic Petrology. Prentice Hall

Magma Mixing

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Assimilation Assimilation



Incorporation of wall rocks Incorporation of wall rocks (diffusion, xenoliths)

(diffusion, xenoliths)



Assimilation by melting is limited by Assimilation by melting is limited by the heat available in the magma

the heat available in the 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 magma

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Detecting and assessing assimilation Detecting and assessing assimilation

Isotopes

Isotopes are generally the best are generally the best

– Continental crust becomes progressively Continental crust becomes progressively enriched in

enriched in ⁸⁷⁸⁷Sr/Sr/⁸⁶⁸⁶Sr and depleted in Sr and 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

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Detecting and Assessing Assimilation

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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 assimilation Detecting and assessing assimilation

 U-Th-Pb system as an indicator of U-Th-Pb system as an indicator of

continental contamination is particularly continental contamination is particularly

useful useful

 All incompatibles similar to Zr+4, so All incompatibles similar to Zr+4, so they concentrate strongly into the they concentrate strongly into the

continental crust because they are not continental crust because they are not

removed during early fractionation.

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Mixed Processes Mixed Processes



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