METAMORPHIC PETROLOGY

1. INFLUENCEOFFLUIDSON
METAMORPHICPHASEEQUILIBRIUM

2. CONTENT

3. WHAT IS A FLUID ?
• A fluid is a substance that continually deforms under an applied shear stress or
external force. Fluids are a phase of matter and include liquids, gases and plasmas.
They are substances with zero modulus or in simpler terms substances which
cannot resist any shear force applied to them.
• Fluids plays an influential role in many igneous and metamorphic processes, where
they may partake in various types of reactions or act as a transporting medium for
the movement of soluble material. The role of fluids can be seen in seafloor and
hydrothermal metamorphism and many other processes.
• Fluid can change the transport properties of a rock by dissolution or precipitation
of material, or by creating fractures due to fluid pressure (“hydrofracturing”)

4. The litho static pressure is more than the fluid pressure as the density of
mineral is more than the fluid, as the depth increase say at around 10 km
the mineral starts to dissolve so the pressure difference between the fluid
and the mineral is no longer different but they become equal. This
equilibrium is maintain as the metamorphism proceeds, but as it reaches
peak metamorphic conditions, the liquid that remained un release is again
dissolved in the previously dehydrated minerals.

5. METAMORPHICFLUIDS
 During metamorphism, metamorphic rocks contains an inter-
granular fluid phase.
 All the inter-granular high temperature /high pressure fluids that
are in equilibrium with the metamorphic mineral assemblage
during peak metamorphic conditions escapes as pressure.
 The only direct evidence for metamorphic fluids that remains is
found in the small high density fluids inclusions trapped in many
metamorphic rocks.
 The existence of fluids trapped in a rock, however, is not an
indicator of the quantity of fluids that existed during
metamorphism.
 Besides the temperature and pressure, the composition of the
fluid phase is the most important factor that governs
metamorphism.

6. Pressure-Temperature Phase Diagram

7. COMPOSITIONOFFLUID
• The fluid phase is either H2O or CO2 or both.
• Another component O2 which is present in small amount is important when minerals
are sensitive to redox reactions that are involved in metamorphism.
• The term fluid is used to used to avoid the exact physical nature of the phase.
• Water is the chief substance that is responsible for metamorphism, but it may be
reinforced locally by CO2 or other substances.
• At low pressure the fluid is either liquid(H2O) or gas(CO2/O2) and mixture of both ,
but at P and T above the critical point there is no difference and the non solid phase is
called supercritical fluid.

8. ROLEOFH2OANDCO2
 H2O or CO2 pressure is useful in order to stabilize hydrous and /or carbonate
minerals in metamorphic rocks at the temperature of metamorphism.
 Without such fluids these minerals would quickly dissolve or disappear.
 Fluids librated by the dehydration and decarbonation reactions contribute to
the metamorphic inter-granular fluids until equilibrium is attained.
 The volatile bearing minerals in many rocks and the reactions that involves
them occurred at metamorphic grades requires the existence of a fluid in
equilibrium with the solid phases.

9.  Inter-granular metamorphic fluids are usually dominated by H2O but Co2 may
also present in some rocks.
 In dehydration rocks, the equilibrium temperature is controlled by the molar
concentration of water (XH2O)in the fluid phase.
 The equilibrium is shifted towards lower temperature ( i,e towards left) with
decreasing XH2O in the fluid phase, while with an increase of XH2O curve will
shift towards higher temperature (i,e towards right)
ROLEOFH2OINDEHYDRATIONREACTIONS
KAl2(AlSi3)O10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O
Muscovite Quartz K-Feldspar Sillimanite

10. With increase in molar concentration of H2O the original curve PQ shifted to the
position of RQ and with decrease in molar concentration of H2O the same is
shifted to the position of SQ

11. ROLEOFCO2INDECARBONATIONREACTIONS
• In case of decarbonation reaction the equilibrium temperature is controlled by
molar concentration of CO2.
• In decarbonation reaction the equilibrium is shifted towards higher temperature
with increase in molar concentration of CO2 (Xco2) in the fluid phase and with
decrease of molar concentration of CO2 (Xco2) the equilibrium is shifted towards
lower temperature.
CaCO3 + SiO2 = CaSiO3 + CO2
Calcite Quartz Wollastonite

12. In the figure,
it is cleared that the equilibrium
curve shifts towards the right with
increase in molar concentration of
CO2 (Xco2) and with decrease of
molar concentration of CO2 (Xco2)
the curve moves towards left.
Temperature vs Composition of CO2 diagram

13. • The equilibrium in case of reactions involving fluids has been theoretically
analyzed by Greenwood (1962,1967).
• The general shape of the isobaric equilibrium curves for reactions involving
fluids are indicated in the figure given below.
One gas species reactant and the other
a product : the equilibrium curve has a
point of inflexion (5)
Both gas species products : the
equilibrium curve has a maximum. (3)
Only one gas species (CO2 or H2O)
involved in the reaction : the
equilibrium temperature increases
with its molar fraction.(1 and 2)

14. ROLEOFO2
• The partial pressure of oxygen in fluid phase essentially controls oxidation and
reduction in metamorphic reactions
Hypersthene + water + oxygen = talc + magnetite
• This reaction demonstrate the way of oxidizing the iron.
• This reaction shows that Mg and Fe are almost inseparable can be completely
separated by partly oxidizing the iron.
• The most useful indicator for partial pressure of oxygen in the crust are iron
oxides i,e haematile , magnetite and wustite.
• These minerals are related to each other by the following reactions--
(Mg,Fe)SiO3 H2O O2 Mg3Si4O10(OH)2 Fe3O4

15. Reactions–
 6Fe2O3 = 4Fe3O4 + O2
 2Fe3O4 = 6FeO + O2
 2FeO = 2Fe + O2
 ½ Fe3O4 = 3/2Fe + O2
Haematite magnetite
Magnetite wustite
Wustite
Haematite

16. • It is clear from the figure that Haematite is stable at high partial pressure of
oxygen .
• As partial pressure of oxygen decreases magnetite, wustite and lastly native
Fe becomes progressively stable.
• Wustite and native iron are practically unknown in metamorphic rocks.
• Magnatite is most common iron oxide in metamorphic rocks.
• From the graph we can conclude that the partial pressure of oxygen s usually
of the order 10^-10 to 10^-40 bar.

17. CONCLUSION
Metamorphic fluids plays a very vital role in various metamorphic
processes. The fluids also acts as a transporting medium. The pressure,
temperature and the chemical composition of the metamorphic fluid are
the most important factors which controls the course of metamorphism.
Within all the metamorphic fluids the water is the dominant one and
very important also. Fluids are present in metamorphic rocks, but there is
little direct evidence concerning the nature in the rock that we now
collect on the earth’s surface.

18. PrinciplesofIgneousAndMetamorphic PetrologybyJohnD.Winter
IgneousAndMetamorphicPetrologybyMyronG.Best
MetamorphicPetrologybyB.BhaskarRao
http://wikipedia.org/metaphic_phase_equilirium
BIBLIOGRAPHY

19. THANKYOUFORYOURPATIENCE