Chemical equilibrium in metamorphic rocks, Retrograde metamorphism

Here i share the PPT of Chemical equilibrium in metamorphic rocks, Retrograde metamorphism .

1. Name : Darji Darshan. R Geology paper no: GEL 409 Roll No: 03 Supervision: Dr.Rahul SIR M. G. Science institute ( Geology department) Navrangpura, Ahmedabad, Gujarat 380009 [email protected] Chemical equilibrium in metamorphic rocks ( equilibrium reaction ) Retrograde metamorphism

2. Content Introduction Metamorphic Reaction Types of Metamorphic Reaction Grade of Metamorphic Reaction Retrograde Metamorphism References

3. Metamorphic Reaction A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism where in on assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions resulting in the final stable state of the observed metamorphic rock.

4. INTRODUCTION The equilibrium model for metamorphism is founded on the metamorphic facies principle, the repeated association of the same mineral assemblages in rocks of different bulk composition that have been metamorphosed together.

5. Conti…. Examples would include the production of talc under varied metamorphic conditions: serpentine + carbon dioxide—> talc + magnesite + water chlorite + quartz—> kyanite + talc + water.

6. Types Of Metamorphic Reaction Mainly have a two types Of metamorphic reaction Univariant reaction diavariant reaction

7. Univariant Reactions A univariant reaction is one that plots as a line or curve on a pressure-temperature diagram. If all phases in the reaction are present in the rock, then we know that the rock must have been metamorphosed at some pressure and temperature along the reaction boundary Consider for example the simple Al2SiO5 system with excess SiO2 and H2O .

8. Conti… In low grade metamorphic in this system, the reaction: Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O Pyrophyllite Ky or Andal Qtz fluid

9. defines a reaction boundary on a P-T diagram. This boundary can be determined experimentally or can be calculated using thermodynamic properties of the phases involved. If we find a rock that contains pyrophyllite, quartz, and an Al2SiO5 mineral, then we know that metamorphism took place somewhere along the trajectory of the reaction boundary.

10. Divariant Reactions In the cases discussed above, the univariant reactions that were considered involved reaching a point in pressure temperature space where a reaction occurred resulting in a sudden change in mineral assemblage. These reactions can be considered discontinuous reactions because they occur along specific pressure temperature curves. Because many minerals are solid solutions, it is also possible to have discontinuous reactions that result in a gradual change in composition of the minerals, but not necessarily the formation of new minerals.

11. Conti…. These reactions are also considered divariant reactions because they occur over a wide range of pressure and temperature conditions. Consider the hypothetical case of rocks that contain minerals like chlorite and garnet, which are both Mg-Fe solid solutions. The reaction that occurs with increasing temperature (at constant pressure) is: Chlorite +Qtz=>Garnet + Mg-richer Chlorite + H2O

12. This reaction begins at a temperature of T1 where an initial Mg-poor garnet is produced. As temperature increases, say to T2, both the garnet and the chlorite become more Mg-rich. The reaction continues over a range of temperature until eventually the temperature reaches T3 at which point the much more Mg-rich chlorite disappears leaving garnet with Mg/(Mg/Fe) ratio the same as that in the initial chlorite.

13. Conti…. We say that this reaction is a continuous reaction because there is no change in mineral assemblage between T1 and T3, but there is a reaction occurring and its effect is to change the compositions of the solid solution minerals. Note the similarity of this idea to the melting behavior of Fe-Mg solid solution

14. Grade of Metamorphism Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. https://egyankosh.ac.in/

15. Continue… • Low-grade metamorphism • High-grade metamorphism • Prograde (= progressive) metamorphism • Retrograde (= retrogressive) metamorphism

16. Low Grade Metamorphism Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O, and non-hydrous minerals become more common

17. Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Retrograde metamorphism (diaphthoresis, retrogressive metamorphism) is the mineralogical adjustment of relatively high-grade metamorphic rocks to temperatures lower than those of their initial metamorphism

18. Retrograde metamorphism In general, the changes in mineral assemblage and mineral composition that occur during burial and heating are referred to as prograde metamorphism, whereas those that occur during uplift and cooling of a rock represent retrograde metamorphism. If thermodynamic equilibrium were always maintained, one might expect all the reactions that occur during prograde metamorphism to be reversed during subsequent uplift of the rocks and reexposure at Earth’s surface; in this case, metamorphic rocks would never be seen in outcrop. However, two factors mitigate against complete retrogression of metamorphic rocks during their return to Earth’s surface.

19. Continue… First is the efficient removal of the water and carbon dioxide released during prograde devolatilization reactions by upward migration of the fluid along grain boundaries and through fractures. Because almost all the water released during heating by reactions—such as when chlorite (Fe9Al6Si5O20(OH)16) reacts with quartz (4SiO2) to yield garnet (3Fe3Al2Si3O12) and water (8H2O) —is removed from the site of reaction, the reaction cannot be reversed during cooling unless water is subsequently added to the rock. Thus, garnet can be preserved at Earth’s surface even though it is thermodynamically unstable at such low temperatures and pressures.

20. Continue….. garnets are often rimmed by small amounts of chlorite and quartz, indicating that limited quantities of water were available for the reverse of the reaction given above to proceed during cooling. Retrograde features such as these reaction rims can be mapped to yield information on pathways of fluid migration through the rocks during uplift and cooling. In other rocks, such as high-temperature gneisses, mineral compositions often reflect temperatures too low to be in equilibrium with the preserved mineral assemblage. In these samples, it is clear that certain exchange reactions operated in a retrograde sense even when the net-transfer reactions were frozen in during prograde metamorphism.

21. https://opengeology.org/

22. References https://www2.tulane.edu/ https://www.britannica.com/ WWW.WIKIPEDIA.COM https://egyankosh.ac.in