Petrology Lecture 5
Reaction Series and Melting Behavior
GLY 4310 - Spring, 2016
1

Norman Levi Bowen
• Canadian geologist who was one of the most important pioneers in the field of experimental petrology
• Widely recognized for his phaseequilibrium studies of silicate systems as they relate to the origin of igneous rocks
•
Reaction principle . He recognized two types of reaction, continuous and discontinuous . (1922)
• 1887 - 1956
2

Continuous Reaction
Mineral
A
 Melt
X
 Mineral
B
 Melt
Y
Melt
X
 Mineral  Melt
Y
3

Discontinuous Reaction

2
Mineral Melt  Mineral
2
• The second reaction was seen before in the phase diagrams shown in mineralogy
• What was that type of reaction called?
4

Name of reaction?


• This was the reaction
5

Bowen’s Reaction Series
6

Gibbs Free Energy Definition
G

H

TS
• We can formulate a differential equation to represent changing geologic conditions: dG

VdP

SdT
• In igneous petrology, we are most often interested in the conditions involved at the liquid-solid phase boundary
7

Solid-Liquid Reaction
• Considering a reaction between a solid and a liquid (S ↔ L) we can rewrite the previous equation as d G
 
VdP
 
SdT
• Δ represents a change as the result of a reaction here, going from solid to liquid or vice versa
8


V

V
L

V
S
• 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
9

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 G
S
< G
L
• In the region labeled
“Liquid” the liquid phase is stable, because G
S
> G
L
10



Isobaric System

G

T


P
 
S
• Because S liquid
> S solid
, 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
11

Equilibrium Temperature
• Relationship between
Gibbs Free Energy and temperature for the solid and liquid forms of a substance at constant pressure.
• T eq is the equlibrium temperature
12

Isothermal System

 
G

P

 
T
V
• Because V liquid
> V solid
, 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
13

Equilibrium Presssure
• V is positive, and therefore the slope of
(δG/δP) is positive.
14

Equilibrium Curve
• Any two points on the equilibrium curve for a solid-liquid interface must have ΔG = 0, and therefore dΔG = 0
• Substituting gives

0
 
Vdp
 
Sdt
15

Clapeyron Equation
• Rearranging the previous equation gives: dP dT


S

V
16

Diopside – Anorthite System
Figure 6-11.Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), American Journal of Science,
40, 161-185.
17

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
18

Fluid Pressure
• The fluid pressure (P f
) is used to define the state of volatiles in a melt
• If P f
= P volatiles total
, the melt is saturated with
• If P f
= 0, the system does not contain volatiles, and is often called “dry”
19

Le Châtlier’s Principle
• Any change imposed on a system at equilibrium will drive the system in the direction that reduces the imposed change
20

Melting of Hydrous Minerals


( aq )
• 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
21

H
2
O Solubility
• Solubility of H
2
O at 1100°C in three natural rock samples and albite
• After Burnham
(1979)
22

Albite – H
2
O
• Effect of H
2
O saturation on the melting of albite
• After Burnham and Davis, 1974
• Dry melting curve from Boyd and England,
1963
23

Melting of Albite
( vapor )

( aq )
• 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
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Application of Clapeyron Equation dP


S dT 
V
• 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)
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Melting of
Gabbro
• Effect of H
2
O saturation on the melting of gabbro
(Burnham and
David, 1974)
• Dry melting curve from
Boyd and
England
(1963)
26

Melting Curves
• H
2
O saturated curves are solid
• H
2
O free curves are dashed
• Mafic rocks have higher melting points than felsic rocks
27

Albite –
H
2
O System
• Pressure-temperature projection of the melting relationships in the system albite –
H
2
O
• After Burnham and
Davis, 1974
•
Red curves = melting for a fixed mol % water in the melt (X w
•
Blue curves tell the water content of a water-saturated melt
)
28

Albite Melting
Percentage
• Percentage of melting for albite with 10 mol % H
2
O at
0.6 GPa as a function of temperature along traverse e-i
29

Albite – H
2
O
System
• Pressure-temperature projection of the melting relationships in the system albite –
H
2
O
• After Burnham and
Davis, 1974
30

Melting
Relationships
• Pressure-temperature projection of the melting relationships in the system albite –
H
2
O with curves representing constant activity of H
2
O
• After Burnham and
Davis, 1974
31

Diopside-Anorthite Liquidus
• The affect of H
2
O on the diopsideanorthite liquidus
32

Albite Melting with Fluids
• Experimentally determined melting of albite
 Dry
 H
2
O saturated

In presence of fluid containing 50% each of
H
2
O and CO
2
33

System
CO
2
Solubility
Pressure CO
2
Solubility
5-6% Albite-H
2
O-CO
2
2 GPa
Enstatite-H
2
O-CO
2
2 GPa
Diopside-H
2
O-CO
2
2 GPa
18%
35%
34

Ternary Eutectic
Fo
P = 2 GPa
CO
2 dry
Highly undesaturated
(nepheline-bearing) alkali olivine basalts
H
2
O
Ab
En
Ne
Oversaturated
(quartz-bearing) tholeiitic basalts
SiO
2
• 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
CO
2 moves it to more alkaline types
35

Fo
Ternary Eutectic
Ne
Volatile-free
3GPa
Highly undesaturated
(nepheline-bearing) alkali olivine basalts
2GPa
1GPa
Ab
1atm
En
Oversaturated
(quartz-bearing) tholeiitic basalts
• Effect of Pressure on the position of the eutectic in the basalt system
SiO
2
• Increased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersat.alkaline basalts
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