Chemistry of Igneous Rocks

• Characterization of different types (having different chemistries):– Ultramafic Mafic Intermediate Felsic

• Composition commonly presented in weight % of the oxides– 40-78% SiO2

– 12-18% Al2O3

Melts• Liquid composed of predominantly silica and

oxygen. Like water, other ions impart greater conductivity to the solution

• Si and O is polymerized in the liquid to differing degrees – how ‘rigid’ this network may be is uncertain…

• Viscosity of the liquid increases with increased silica content, i.e. it has less resistance to flow with more SiO2… related to polymerization??

• There is H2O is magma 2-6% typically – H2O decreases the overall melting T of a magma, what does that mean for mineral crystallization?

Thermodynamic definitions

• Gi(solid) = Gi(melt)

• Ultimately the relationships between these is related to the entropy of fusion (S0

fus), which is the entropy change associated with the change in state from liquid to crystal

• These entropies are the basis for the order associated with Bowen’s reaction series greater bonding changes in networks, greater entropy change lower T equilibrium

0fus

fus

i S

RT

dX

dT

• Minerals which form are thus a function of melt composition and how fast it cools (re-equilibration?) governed by the stability of those minerals and how quickly they may or may not react with the melt during crystallization

Liquid hotMAGMA

Ca2+ Na+Mg2+

Fe2+

Si4+

Si4+Si4+ O2-

O2-

O2-

O2-

O2- O2-

O2-

O2-

O2-

O2-

rockcooling

Mg2+ Fe2+

Mg2+

Processes of chemical differentiation

• Partial Melting: Melting of a different solid material into a hotter liquid

• Fractional Crystallization: Separation of initial precipitates which selectively differentiate certain elements…

• Equilibrium is KEY --? Hotter temperatures mean kinetics is fast…

Melting• First bit to melt from a solid rock is generally

more silica-rich• At depth in the crust or mantle,

melting/precipitation is a P-T process, governed by the Clausius-Clapeyron Equation – Slope is a function of entropy and volume changes!

• But with water… when minerals precipitate they typicaly do not pull in the water, melt left is ‘diluted’ develop a negative P-T slope

Melt-crystal equilibrium 1• Magma at composition

X (30% Ca, 70% Na) cools first crystal bytownite (73% Ca, 27% Na)

• This shifts the composition of the remaining melt such that it is more Na-rich (Y)

• What would be the next crystal to precipitate?

• Finally, the last bit would crystallize from Z

XX

YY

ZZ

Melt-crystal equilibrium 1b

• Precipitated crystals react with cooling liquid, eventually will re-equilibrate back, totally cooled magma xstals show same composition

• UNLESS it cools so quickly the xstal becomes zoned or the early precipitates are segregated and removed from contact with the bulk of the melt

Why aren’t all feldspars zoned?

• Kinetics, segregation

• IF there is sufficient time, the crystals will re-equilibrate with the magma they are in – and reflect the total Na-Ca content of the magma

• IF not, then different minerals of different composition will be present in zoned plagioclase or segregated from each other physically

• What about minerals that do not coexist well – do not form a solid solution – are immiscible??

• More than 1 crystal can precipitate from a melt – different crystals, different stabilities…– 2+ minerals that do not share equilibrium in a melt are

immiscible (opposite of a solid solution)– Liquidus Line describing equilibrium between melt and

one mineral at equilibrium– Solidus Line describing equilibrium with melt and solid– Eutectic point of composition where melt and solid can

coexist at equilibrium

Diopside is a pyroxeneAnorthite is a feldspar

Solidus

Liquidus

Eutectic

• Melt at composition X cools to point Y where anorthite (NOT diopside at all) crystallizes, the melt becomes more diopside rich to point C, precipitating more anorthite with the melt becoming more diopside-rich

• This continues and the melt continues to cool and shift composition until it reaches the eutectic when diopside can start forming

A

B S1

Z

C S2

At eutectic, diopside AND anorhtite crystals precipitateLever Rule diopside/anorthite (42%/58%) crystallize until last of melt precipitates and the rock composition is Z

• Melting when heated to eutectic, the rock would melt such that all the heat goes towards heat of fusion of diopside and anorthite, melts so that 42% diopside / 58% anorthite…

• When diopside gone, temperature can increase and rest of anorthite can melt (along liquidus)

Melt-crystal equilibrium 2 - miscibility

• 2 component mixing and separation chicken soup analogy, cools and separates

• Fat and liquid can crystallize separately if cooled slowly

• Miscibility Gap – no single phase is stable

• SOUP of X composition cooled in fridge Y vs freezer Z

Miscibility Gap

FatWater

% fat in soup

Tem

pera

ture

(T

empe

ratu

re ( º

C)

ºC)

-20-20

5050

00

100100

1010 9090707050503030

SOUPSOUP

XX

fatsfatsiceice

YY

ZZ

Melt-crystal equilibrium 2 - miscibility

• 2 component mixing and separation chicken soup analogy, cools and separates

• Fat and liquid can crystallize separately if cooled slowly

• Miscibility Gap – no single mineral is stable in a composition range for x temperature

Miscibility Gap

microcline

orthoclase

sanidine

anorthoclasemonalbite

high albite

low albite

intermediate albite

OrthoclaseKAlSi3O8

AlbiteNaAlSi3O8

% NaAlSi3O8

Tem

pera

ture

(T

empe

ratu

re ( º

C)

ºC)

300300

900900

700700

500500

11001100

1010 9090707050503030

Combining phase and composition diagrams for mineral groups

Mica ternaryMica ternary

Biotite seriesBiotite series

AnniteAnniteKFeKFe33(AlSi(AlSi33OO1010)(OH))(OH)22

PhlogopitePhlogopiteKMgKMg33(AlSi(AlSi33OO1010)(OH))(OH)22

MuscoviteMuscoviteKAlKAl22(AlSi(AlSi33OO1010)(OH))(OH)22

No micasNo micasMiscibility GapMiscibility Gap

SOLID SOLUTION• Occurs when, in a crystalline solid, one

element substitutes for another.

• For example, a garnet may have the composition: (Mg1.7Fe0.9Mn0.2Ca0.2)Al2Si3O12.

• The garnet is a solid solution of the following end member components:

Pyrope - Mg3Al2Si3O12; Spessartine - Mn3Al2Si3O12;

Almandine - Fe3Al2Si3O12; and Grossular - Ca3Al2Si3O12.

GOLDSCHMIDT’S RULES

1. The ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than approximately 15%.

2. Ions whose charges differ by one unit substitute readily for one another provided electrical neutrality of the crystal is maintained. If the charges differ by more than one unit, substitution is generally slight.

3. When two different ions can occupy a particular position in a crystal lattice, the ion with the higher ionic potential forms a stronger bond with the anions surrounding the site.

RINGWOOD’S MODIFICATION OFGOLDSCHMIDT’S RULES

4. Substitutions may be limited, even when the size and charge criteria are satisfied, when the competing ions have different electronegativities and form bonds of different ionic character.

This rule was proposed in 1955 to explain discrepancies with respect to the first three Goldschmidt rules.

For example, Na+ and Cu+ have the same radius and charge, but do not substitute for one another.

INCOMPATIBLE VS. COMPATIBLE TRACE ELEMENTS

Incompatible elements: Elements that are too large and/or too highly charged to fit easily into common rock-forming minerals that crystallize from melts. These elements become concentrated in melts.

Large-ion lithophile elements (LIL’s): Incompatible owing to large size, e.g., Rb+, Cs+, Sr2+, Ba2+, (K+).

High-field strength elements (HFSE’s): Incompatible owing to high charge, e.g., Zr4+, Hf 4+, Ta4+, Nb5+, Th4+, U4+, Mo6+, W6+, etc.

Compatible elements: Elements that fit easily into rock-forming minerals, and may in fact be preferred, e.g., Cr, V, Ni, Co, Ti, etc.

Changes in element concentration in the magma during crystal fractionation of the Skaergaard

intrusion: Divalent cations

Changes in element concentration in the magma during crystal fractionation of the Skaergaard

intrusion: Trivalent cations

THREE TYPES OF TRACE-ELEMENT SUBSTITUTION

1) CAMOUFLAGE

2) CAPTURE

3) ADMISSION

CAMOUFLAGE

• Occurs when the minor element has the same charge and similar ionic radius as the major element (same ionic potential; no preference.

• Zr4+ (0.80 Å); Hf4+ (0.79 Å)

• Hf usually does not form its own mineral; it is camouflaged in zircon (ZrSiO4)

CAPTURE

• Occurs when a minor element enters a crystal preferentially to the major element because it has a higher ionic potential than the major element.

• For example, K-feldspar captures Ba2+ (1.44 Å; Z/r = 1.39) or Sr2+ (1.21 Å; Z/r = 1.65) in place of K+ (1.46 Å, Z/r = 0.68).

• Requires coupled substitution to balance charge: K+ + Si4+ Sr2+ (Ba2+) + Al3+

ADMISSION

• Involves entry of a foreign ion with an ionic potential less than that of the major ion.

• Example Rb+ (1.57 Å; Z/r = 0.637) for K+ (1.46 Å, Z/r = 0.68) in K-feldspar.

• The major ion is preferred.

Partition Coefficients• How can we quantify the distribution of trace

elements into minerals/rocks?• Henry’s Law describes equilibrium distribution of

a component (we usedit for thinking about gases dissolved in water recently): – ai

min = kiminXi

min

– aimelt = ki

meltXimelt

– All simplifies to:

• Often termed KD, values tabulated…http://www.earthref.org/databases/index.html?main.htm

Dimelt

i

imelti

i KX

X

ppm

ppm

minmin

Limitations of KD

• What factors affect how well any element gets into a particular rock???

Melting and Crystallization

• Considering how trace elements incorporate the melt or solid:

• Where KD(rock)=KD(j minerals)Xj

• For consideration of trace elements into a solid, use Rayleigh fractionation equation:

• Where F is the fraction of melt remaining

FFKrockC

CDii

melti

)1(

1

)(0

1)(

0 DiK

rockimelti FCC

• However, most KD values reported close to equilibrium T-P values commonly encountered (?) and are reasonable, at least in terms of relative values between different elements…

Homework

• Chapter 8

• Problems 2, 6