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Petrology Lecture 5. Reaction Series and Melting Behavior GLY 4310 - Spring, 2012. Norman Levi Bowen. Canadian geologist who was one of the most important pioneers in the field of experimental petrology

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petrology lecture 5

Petrology Lecture 5

Reaction Series and Melting Behavior

GLY 4310 - Spring, 2012

norman levi bowen
Norman Levi Bowen
  • Canadian geologist who was one of the most important pioneers in the field of experimental petrology
  • Widely recognized for his phase-equilibrium studies of silicate systems as they relate to the origin of igneous rocks
  • 1887 - 1956
reaction principle
Reaction Principle
  • Continuous
  • Discontinuous
discontinuous reaction
Discontinuous Reaction
  • The second reaction was seen before in the phase diagrams shown in mineralogy
  • What was that type of reaction called?
name of reaction
Name of reaction?
  • This was the reaction
gibbs free energy definition
Gibbs Free Energy Definition
  • We can formulate a differential equation to represent changing geologic conditions:
  • In igneous petrology, we are most often interested in the conditions involved at the liquid-solid phase boundary
solid liquid reaction
Solid-Liquid Reaction
  • Considering a reaction between a solid and a liquid (S = L) we can rewrite the previous equation as
  • Δ represents a change as the result of a reaction - here, going from solid to liquid or vice versa
slide10
ΔV
  • Since most solids are denser than their liquids at the melting point, ΔV is positive on going from solid to liquid
  • Water is a notable exception
melting reaction
Melting Reaction
  • Schematic P-T diagram of a melting reaction
  • This figure shows the behavior of an arbitrary phase
  • In the region labeled “Solid” the solid phase is stable, because GS < GL
  • In the region labeled “Liquid” the liquid phase is stable, because GS > GL
isobaric system
Isobaric System
  • Because Sliquid > Ssolid, the slope of G vs. T is greater for the liquid than the solid
  • At low temperatures the solid phase is more stable, but as temperature increases, the liquid phase becomes stable
equilibrium temperature
Equilibrium Temperature
  • Relationship between Gibbs Free Energy and temperature for the solid and liquid forms of a substance at constant pressure.
  • Teq is the equlibrium temperature
isothermal system
Isothermal System
  • Because Vliquid > Vsolid, the slope of G vs. P is greater for a liquid than a solid
  • The liquid phase has lower G, and is thus more stable, at low pressure, but the solid phase is more stable at higher pressure
  • This is why the inner core is solid
equilibrium presssure
Equilibrium Presssure
  • V is positive, and therefore the slope of (δG/δP) is positive.
equilibrium curve
Equilibrium Curve
  • Any two points on the equilibrium curve for a solid-liquid interface must have ΔG =0, and therefore dΔG = 0
  • Substituting gives
clapeyron equation
Clapeyron Equation
  • Rearranging the previous equation gives:
diopside anorthite system
Diopside – Anorthite System

Figure 6-11.Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), American Journal of Science, 40, 161-185.

fluid saturation
Fluid Saturation
  • A fluid-saturated melt contains the maximum amount of dissolved volatile species possible at a given set of P-T-X conditions
  • Any increase in volatile content will produce one or more additional phases
fluid pressure
Fluid Pressure
  • The fluid pressure (Pf) is used to define the state of volatiles in a melt
  • If Pf = Ptotal, the melt is saturated with volatiles
  • If Pf = 0, the system does not contain volatiles, and is often called “dry”
le ch tlier s principle
Le Châtlier’s Principle
  • Any change imposed on a system at equilibrium will drive the system in the direction that reduces the imposed change
melting of hydrous minerals
Melting of Hydrous Minerals
  • Adding water to the system should cause melting, according to Le Châtlier’s Principle
  • Adding water drives the reaction from left to right
  • Removing water, such as by loss of volatiles near the surface, should cause crystallization
h 2 o solubility
H2O Solubility
  • Solubility of H2O at 1100°C in three natural rock samples and albite
  • After Burnham (1979)
albite h 2 o
Albite – H2O
  • Effect of H2O saturation on the melting of albite
  • After Burnham and Davis, 1974
  • Dry melting curve from Boyd and England, 1963
melting of albite
Melting of Albite
  • This reaction has a large negative ΔV on going from left to right, thus stabilizing the liquid phase and lowering the melting point
  • At higher pressures, ΔV is less negative, and the slope of the line is less
application of clapeyron equation
Application of Clapeyron Equation
  • For the dry case, ΔV is positive, and the slope of the melting curve is positive
  • For the wet case, ΔV is negative, and the slope of the melting curve is negative (melting point is depressed with increasing pressure)
melting of gabbro
Melting of Gabbro
  • Effect of H2O saturation on the melting of albite (Burnham and David, 1974)
  • Dry melting curve from Boyd and England (1963)
melting curves
Melting Curves
  • H2O saturated curves are solid
  • H2O free curves are dashed
  • Mafic rocks have higher melting points than felsic rocks
albite h 2 o system
Albite – H2O System
  • Pressure-temperature projection of the melting relationships in the system albite – H2O
  • After Burnham and Davis, 1974
  • Red curves = melting for a fixed mol % water in the melt (Xw)
  • Blue curves tell the water content of a water-saturated melt
albite melting percentage
Albite Melting Percentage
  • Percentage of melting for albite with 10 mol % H2O at 0.6 GPa as a function of temperature along traverse e-i
albite h 2 o system1
Albite – H2O System
  • Pressure-temperature projection of the melting relationships in the system albite – H2O
  • After Burnham and Davis, 1974
melting relationships
Melting Relationships
  • Pressure-temperature projection of the melting relationships in the system albite – H2O with curves representing constant activity of H2O
  • After Burnham and Davis, 1974
diopside anorthite liquidus
Diopside-Anorthite Liquidus
  • The affect of H2O on the diopside-anorthite liquidus
albite melting with fluids
Albite Melting with Fluids
  • Experimentally determined melting of albite
    • Dry
    • H2O saturated
    • In presence of fluid containing 50% each of H2O and CO2
ternary eutectic

Ne

P = 2 GPa

CO2

H2O

dry

Ab

Highly undesaturated

(nepheline-bearing)

alkali olivine

basalts

Oversaturated

(quartz-bearing)

Undersaturated

tholeiitic basalts

tholeiitic basalts

Fo

En

SiO2

Ternary Eutectic
  • Effect of volatiles on ternary eutectic in the system Forsterite – Nepheline – Silica at 2 Gpa
  • Water moves the (2 GPa) eutectic toward higher silica, while CO2 moves it to more alkaline types
ternary eutectic1

Ne

Volatile-free

3GPa

2GPa

1GPa

Ab

Highly undesaturated

(nepheline-bearing)

1atm

alkali olivine

basalts

Oversaturated

(quartz-bearing)

Undersaturated

tholeiitic basalts

tholeiitic basalts

Fo

En

SiO2

Ternary Eutectic
  • Effect of Pressure on the position of the eutectic in the basalt system
  • Increased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersat.alkaline basalts