Transcript Introduction to Environmental Geochemistry

Petrology Lecture 5
Reaction Series and Melting Behavior
GLY 4310 - Spring, 2015
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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
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Continuous Reaction
Mineral  Melt
A
Melt
X

X

Mineral  Melt
B
Mineral  Melt
Y
Y
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Discontinuous Reaction
Melt  Mineral  Mineral
1
Mineral  Melt  Mineral
1
2
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?
Leucite  Melt  K  spar
• This was the reaction
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Bowen’s Reaction Series
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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
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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
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ΔV
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
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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
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Isobaric System
 G 


 T 
 S
P
• 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
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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
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Isothermal System
 G


 P 
V
T
• 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
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Equilibrium Presssure
• V is positive, and
therefore the slope of
(δG/δP) is positive.
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Equilibrium Curve
• Any two points on the equilibrium curve for a
solid-liquid interface must have ΔG = 0, and
therefore dΔG = 0
• Substituting gives
d G  0   Vdp   Sdt
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Clapeyron Equation
• Rearranging the previous equation gives:
dP  S
dT V
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Diopside – Anorthite System
Figure 6-11.Isobaric T-X phase diagram at atmospheric
pressure. After Bowen (1915), American Journal of Science,
40, 161-185.
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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
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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”
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Le Châtlier’s Principle
• Any change imposed on a system at
equilibrium will drive the system in the
direction that reduces the imposed change
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Melting of Hydrous Minerals
Solid  H 2O  Liq ( 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
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H2O Solubility
• Solubility of H2O
at 1100°C in three
natural rock
samples and albite
• After Burnham
(1979)
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Albite – H2O
• Effect of H2O
saturation on the
melting of albite
• After Burnham
and Davis, 1974
• Dry melting
curve from Boyd
and England,
1963
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Melting of Albite
HO
2
( vapor )
 albite 
liquid
( 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 H2O
saturation on
the melting of
gabbro
(Burnham and
David, 1974)
• Dry melting
curve from
Boyd and
England
(1963)
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Melting Curves
• H2O
saturated
curves are
solid
• H2O free
curves are
dashed
• Mafic rocks
have higher
melting
points than
felsic rocks
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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 28
Albite Melting
Percentage
• Percentage of melting for albite with 10 mol % H2O at
0.6 GPa as a function of temperature along traverse 29e-i
Albite – H2O
System
• Pressure-temperature
projection of the
melting relationships
in the system albite –
H2O
• After Burnham and
Davis, 1974
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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
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Diopside-Anorthite Liquidus
• The affect of H2O
on the diopsideanorthite liquidus
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Albite Melting with Fluids
• Experimentally
determined melting of
albite
 Dry
 H2O saturated
 In presence of fluid
containing 50% each of
H2O and CO2
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CO2 Solubility
System
Pressure
CO2 Solubility
Albite-H2O-CO2
2 GPa
5-6%
Enstatite-H2O-CO2
2 GPa
18%
Diopside-H2O-CO2
2 GPa
35%
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Ternary Eutectic
Ne
P = 2 GPa
CO2
dry
Highly undesaturated
(nepheline-bearing)
alkali olivine
basalts
H2O
Ab
Oversaturated
(quartz-bearing)
tholeiitic basalts
Fo
En
SiO2
• 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
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Ternary Eutectic
Ne
Volatile-free
3GPa
2GPa
1GPa
Highly undesaturated
(nepheline-bearing)
alkali olivine
basalts
Ab
1atm
Oversaturated
(quartz-bearing)
tholeiitic basalts
Fo
En
SiO2
• 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
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