Chapter 8: Major Elements

1
Figure 27-1. Temperature-pressure phase diagram
for the reaction Albite Jadeite Quartz
calculated using the program TWQ of Berman (1988,
1990, 1991). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
2
3. Solid-Solid Net-Transfer Reactions

• If minerals contain volatiles, the volatiles must
  be conserved in the reaction so that no fluid
  phase is generated or consumed
• For example, the reaction
• Mg3Si4O10(OH)2 4 MgSiO3 Mg7Si8O22(OH)2
• Tlc En Ath
• involves hydrous phases, but conserves H2O
• It may therefore be treated as a solid-solid
  net-transfer reaction

3
3. Solid-Solid Net-Transfer Reactions

• When solid-solution is limited, solid-solid
  net-transfer reactions are discontinuous
  reactions
• Discontinuous reactions tend to run to completion
  at a single temperature (at a particular
  pressure)
• There is thus an abrupt (discontinuous) change
  from the reactant assemblage to the product
  assemblage at the reaction isograd

4
4. Devolatilization Reactions

• Among the most common metamorphic reactions
• H2O-CO2 systems are most common, but the
  principles same for any reaction involving
  volatiles
• Reactions dependent not only upon temperature and
  pressure, but also upon the partial pressure of
  the volatile species

5
4. Devolatilization Reactions

• For example the location on a P-T phase diagram
  of the dehydration reaction
• KAl2Si3AlO10(OH)2 SiO2 KAlSi3O8 Al2SiO5
  H2O
• Ms Qtz Kfs Sill
  W
• depends upon the partial pressure of H2O
  (pH2O)
• This dependence is easily demonstrated by
  applying Le Châteliers principle to the reaction
  at equilibrium

6
4. Devolatilization Reactions

• The equilibrium curve represents equilibrium
  between the reactants and products under
  water-saturated conditions (pH2O PLithostatic)

P-T phase diagram for the reaction Ms Qtz Kfs
Al2SiO5 H2O showing the shift in equilibrium
conditions as pH2O varies (assuming ideal H2O-CO2
mixing). Calculated using the program TWQ by
Berman (1988, 1990, 1991). After Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
7
KAl2Si3AlO10(OH)2 SiO2 KAlSi3O8 Al2SiO5
H2O Ms Qtz Kfs Sill W

• Suppose H2O is withdrawn from the system at some
  point on the water-saturated equilibrium curve
  pH2O lt Plithostatic
• According to Le Châteliers Principle, removing
  water at equilibrium will be compensated by the
  reaction running to the right, thereby producing
  more water
• This has the effect of stabilizing the right side
  of the reaction at the expense of the left side
• So as water is withdrawn the Kfs Sill H2O
  field expands slightly at the expense of the Mu
  Qtz field, and the reaction curve shifts toward
  lower temperature

8
Figure 26-2. P-T phase diagram for the reaction
Ms Qtz Kfs Al2SiO5 H2O showing the shift
in equilibrium conditions as pH2O varies
(assuming ideal H2O-CO2 mixing). Calculated using
the program TWQ by Berman (1988, 1990, 1991).
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
9
4. Devolatilization Reactions

• pH2O can become less than PLith by either of two
  ways
• Pfluid lt PLith by drying out the rock and
  reducing the fluid content
• Pfluid PLith, but the water in the fluid can
  become diluted by adding another fluid component,
  such as CO2 or some other volatile phase
• In Fig. 26-2 I calculated the curves for the
  latter case on the basis of ideal H2O-CO2 mixing

10
4. Devolatilization Reactions

• An important point arising from Fig. 26-2 is
• The temperature of an isograd based on a
  devolatilization reaction is sensitive to the
  partial pressure of the volatile species involved
• An alternative T-Xfluid phase diagram
• Because H2O and CO2 are by far the most common
  metamorphic volatiles, the X in T-X diagrams is
  usually the mole fraction of CO2 (or H2O) in
  H2O-CO2 mixtures
• Because pressure is also a common variable, a
  T-Xfluid diagram must be created for a specified
  pressure

11
4. Devolatilization Reactions
Figure 26-4. T-XH2O phase diagram for the
reaction Ms Qtz Kfs Sil H2O at 0.5 GPa
assuming ideal H2O-CO2 mixing, calculated using
the program TWQ by Berman (1988, 1990, 1991).
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
12
4. Devolatilization Reactions
Figure 26-4. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
Figure 26-2. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
13
4. Devolatilization Reactions

• Shape of all dehydration curves on T-Xfluid
  diagrams is similar to the curve in Fig. 26-2
• Maximum temperature at the pure H2O end, and
  slope gently at high XH2O, but steeper toward low
  XH2O, becoming near vertical at very low XH2O
• Reaction temperature can thus be practically any
  temperature below the maximum at pH2O Plith
• Must constrain the fluid composition (if
  possible) before using a dehydration reaction to
  indicate metamorphic grade

14
A rare exception
Figure 26-3. Calculated P-T equilibrium reaction
curve for a dehydration reaction illustrating the
full loop that is theoretically possible. From
Winter (2001). An Introduction to Igneous and
Metamorphic Petrology, Prentice Hall.
15
4. Devolatilization Reactions

• Decarbonation reactions may be treated in an
  identical fashion
• For example, the reaction
• CaCO3 SiO2 CaSiO3 CO2 (26-6)
• Cal Qtz Wo
• Can also be shown on a T-XCO2 diagram
• Has the same form as reaction (26-5), only the
  maximum thermal stability of the carbonate
  mineral assemblage occurs at pure XCO2

16
4. Devolatilization Reactions
Figure 26-5. T-XCO2 phase diagram for the
reaction Cal Qtz Wo CO2 at 0.5 GPa assuming
ideal H2O-CO2 mixing, calculated using the
program TWQ by Berman (1988, 1990, 1991). Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Figure 26-1. A portion of the equilibrium
boundary for the calcite-aragonite phase
transformation in the CaCO3 system. After
Johannes and Puhan (1971), Contrib. Mineral.
Petrol., 31, 28-38. Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
17

• 5 types of devolatilization reactions, each with
  a unique general shape on a T-X diagram
• Type 3 Tmax at XCO2 determined by the
  stoichiometric ratio of CO2/H2O produced

Ca2Mg5Si8O22(OH)2 3 CaCO3 2 SiO2
Tr Cal Qtz 5
CaMgSi2O6 3 CO2 H2O Di
Figure 26-6. Schematic T-XCO2 phase diagram
illustrating the general shapes of the five types
of reactions involving CO2 and H2O fluids. After
Greenwood (1967). In P. H. Abelson (ed.),
Researches in Geochemistry. John Wiley. New York.
V. 2, 542-567. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
18
5. Continuous Reactions
Figure 26-8. Geologic map of a hypothetical field
area in which metamorphosed pelitic sediments
strike directly up metamorphic grade. From Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
19
5. Continuous Reactions

• Two possible reasons
• 1. Such contrasting composition that the garnet
  reaction is different
• Example garnet in some pelites may be created
  by the (unbalanced) reaction
• Chl Ms Qtz ? Grt Bt H2O (26-11)
• Whereas in more Fe-rich and K-poor pelites,
  garnet might be generated by an (unbalanced)
  reaction involving chloritoid
• Chl Cld Qtz ? Grt H2O (26-12)

20
5. Continuous Reactions

• 2. The reaction on which the isograd is based is
  the same in each unit, but it is a continuous
  reaction, and its location is sensitive to the
  composition of the solutions (either solid of
  fluid) involved
• The offsets this creates in an isograd are
  usually more subtle than for reason 1, but in
  some cases they can be substantial
• We will concentrate on this second reason here

21
5. Continuous Reactions
Fig. 6-10. Isobaric T-X phase diagram at
atmospheric pressure After Bowen and Shairer
(1932), Amer. J. Sci. 5th Ser., 24, 177-213.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Melt-in isograd?
22
5. Continuous Reactions

• Discontinuous reactions occur at a constant grade
• Chl Ms Qtz ? Grt Bt H2O (26-11)
• in KFASH F C f 2 5 4 2 1

23
5. Continuous Reactions

• Chl Ms Qtz ? Grt Bt H2O (26-11) in
  KFMASH
• were a continuous reaction, then we would find
  chlorite, muscovite, quartz, biotite, and garnet
  all together in the same rock over an interval of
  metamorphic grade above the garnet-in isograd
• The composition of solid solution phases vary
  across the interval, and the proportions of the
  minerals changes until one of the reactants
  disappears with increasing grade

24

• Continuous reactions occur when F ? 1, and the
  reactants and products coexist over a temperature
  (or grade) interval

Fig. 26-9. Schematic isobaric T-XMg diagram
representing the simplified metamorphic reaction
Chl Qtz ? Grt H2O. From Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
25
6. Ion Exchange Reactions

• Reciprocal exchange of components between 2 or
  more minerals
• MgSiO3 CaFeSi2O6 FeSiO3 CaMgSi2O6
• Annite Pyrope Phlogopite Almandine
• Expressed as pure end-members, but really
  involves Mg-Fe (or other) exchange between
  intermediate solutions
• Basis for many geothermobarometers
• Causes rotation of tie-lines on compatibility
  diagrams

26
Figure 27-6. AFM projections showing the relative
distribution of Fe and Mg in garnet vs. biotite
at approximately 500oC (a) and 800oC (b). From
Spear (1993) Metamorphic Phase Equilibria and
Pressure-Temperature-Time Paths. Mineral. Soc.
Amer. Monograph 1. MSA. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
27
6. Redox Reactions

• Involves a change in oxidation state of an
  element
• 6 Fe2O3 4 Fe3O4 O2
• 2 Fe3O4 3 SiO2 3 Fe2SiO4 O2

• At any particular pressure these become oxygen
  buffers

Fig. 26-10. Isobaric T-fO2 diagram showing the
location of reactions (26-13) - (26-15) used to
buffer oxygen in experimental systems. After
Frost (1991), Rev. in Mineralogy, 25, MSA, pp.
469-488. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
28
7. Reactions Involving Dissolved Species

• Minerals plus ions neutral molecules dissolved in
  a fluid
• One example is hydrolysis
• 2 KAlSi3O8 2 H H2O Al2Si2O5 (OH)4 SiO2
  2 K
• Kfs aq. species kaolinite aq. species

• Can treat such reactions in terms of the phase
  rule and the intensive variables P, T, and
  concentrations of the reactant species
• T-P diagrams for fixed or contoured Ci
• Isobaric T-Ci diagrams
• Isobaric and isothermal Ci - Cj diagrams
• Reaction above might be handled by a T vs.
  CK/CH diagram

29
Reactions and Chemographics

• We can use chemographics to infer reactions

• Any two phases in a binary system can react to
  from a phase between them

• Fo Qtz En Mg2SiO4 SiO2 Mg2Si2O6
• En Per Fo Mg2Si2O6 2 MgO 2 Mg2SiO4
• Per Qtz Fo or En
• If we know the chemographics we can determine
  that a reaction is possible (and can dispense
  with balancing it)

30
Reactions and Chemographics

• What reaction does this ternary system allow?

Fig. 26-12. From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
31
Reactions and Chemographics

• A B C X

above x-in isograd
below x-in isograd
32
Reactions and Chemographics

• What reaction does this system allow?

Fig. 26-13. From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
33
Reactions and Chemographics

• What reaction is possible between A-B-C-D?

A compatibility diagram for some metamorphic zone
Fig. 26-14a. From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
34
Below the isograd
Fig. 26-14. From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
A B C D
At the isograd
Above the isograd
This is called a tie-line flip, and results in
new groupings in the next metamorphic zone
35
Petrogenetic Grids

• P-T diagrams for multicomponent systems that show
  a set of reactions, generally for a specific rock
  type

Petrogenetic grid for mafic rocks
Fig. 26-19. Simplified petrogenetic grid for
metamorphosed mafic rocks showing the location of
several determined univariant reactions in the
CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system
(C(N)MASH). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
36
Text figures that I dont have time to cover in
my 1-semester class
Fig. 26-7. T-XCO2 phase diagram fro 2 reactions
in the CaO-MgO-SiO2-H2O-CO2 system at 0.5 GPa,
assuming ideal mixing of non-ideal gases,
calculated using the program TWQ by Berman (1988,
1990, 1991). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
37
Text figures that I dont have time to cover in
my 1-semester class
Figure 26-15. The Al2SiO5 T-P phase diagram from
Figure 21-9 (without H2O). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
38
Text figures that I dont have time to cover in
my 1-semester class
Figure 26-16. Schematic one-component T-P phase
diagram showing the topology of a four-phase
multisystem in which all invariant points are
stable. Because only three phases (C2) coexist
at an invariant point, a complete system should
have four invariant points, each with one phase
absent. Phases absent at invariant points are in
square brackets, phases absent for univariant
reactions are in parentheses. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
39
Text figures that I dont have time to cover in
my 1-semester class
Figure 26-17. A portion of the P-T phase diagram
for SiO2 (Figure 6-6) showing two stable
invariant points Trd and Liq and two
metastable ones. b-Qtz occurs at negative
pressure, and Crs is truly metastable in that
it is the intersection of metastable extensions.
From Spear (1993) Metamorphic Phase Equilibria
and Pressure-Temperature-Time Paths. Mineral.
Soc. Amer. Monograph 1. MSA.
40
Text figures that I dont have time to cover in
my 1-semester class
Figure 26-18. a. Hypothetical reaction D E F
in a two-component phase diagram. Note that the
D-absent and E-absent curves must both lie on the
side of the initial univariant curve opposite to
the field in which D E is stable. Likewise the
F-absent curve must lie on the side opposite to
the field in which F is stable. b. A second
hypothetical univariant curve (D-absent) is
added. c. The complete topology of the invariant
point can then be derived from the two initial
reactions in (b). The chemographics may then be
added to each divariant field. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
41
Figure 26-20. a. Sketch from a photomicrograph
showing small crystals of kyanite (purple) and
quartz (blue) in a larger muscovite grain
(green). Small crystals of fibrolitic sillimanite
also occur in the muscovite. Glen Cova, Scotland.
b. Sillimanite needles in quartz (blue) embaying
muscovite (green). Pink crystals are biotite.
Donegal, Ireland. After Carmichael (1969).
Contrib. Mineral. Petrol., 20, 244-267.
42
Text figures that I dont have time to cover in
my 1-semester class
Figure 26-21. A possible mechanism by which the
Ky ? Sil reaction can be accomplished while
producing the textures illustrated in Figure
26-20a and b. The exchange of ions shown between
the two local zones is required if the reactions
are to occur. After Carmichael (1969). Contrib.
Mineral. Petrol., 20, 244-267.
43
Text figures that I dont have time to cover in
my 1-semester class
Figure 26-21. An alternative mechanism by which
the reaction Ky ? Sil reaction can be
accomplished while producing sillimanite needles
associated with biotite with plagioclase
occupying embayments in the biotite. The exchange
of ions shown between the two local zones is
required if the reactions are to occur. After
Carmichael (1969). Contrib. Mineral. Petrol., 20,
244-267.