Metamorphism - Lecture 6 Metamorphic Reactions

I. Metamorphic reactions and composition diagrams

In addition to showing graphically what the equilibrium mineralogy should be given a bulk composition, a major reason to use composition diagrams is to show what reactions have occurred between grades.

Example:

Two different pelitic units with slightly different bulk composition in Barrovian metamorphic sequence.

  Chlorite zone Biotite zone
UNIT A kspar>musc>chl ksp>musc>biot
UNIT B chl>musc>ksp chl>musc>biot

Both rock types have the same minerals in the chlorite zone, but the abundances are different

Question: What is the biotite forming reaction in the area?

 

 

II. Types of metamorphic reactions

A. Solid - solid reactions

DS/DV remains constant for all solid-solid reactions, thus P/T slope remains the same. So reaction is very dependent upon pressure

 

Types of solid-solid reactions

1. Polymorphic transformations

2. Coupled reactions (= net transfer reaction)

3. Exsolution or solvus reactions

 

B. Dehydration reactions

With increasing metamorphism, minerals get more dehydrated. Shape of curves can be gotten from Clapeyron equation (dP/dT = DS/DV), but you'll be spared that this quarter. Dehydration reactions are always curved in P-T space.

 

 

Progressive dehydration or "drying out" of rock:

Mud loose clay sized particles

(pore water expelled during burial & compaction)

10-30 vol.% porosity filled with H2O

 

Shale

clay minerals (~14% H2O in mineral)

ex) kaolinite Al2Si2O5(OH)4

5-10 wt.% H2O found in mineral
Slate/Schist mica minerals

ex) muscovite KAl2Si3O10(OH)2

4 wt.% H2O
High grade gneiss

(granulite)

biotite + amphibole

ex) amph ~2 wt.% H2O, but <10% of rock

<0.5 wt.% H2O

 

 

III. Reading metamorphic reactions off composition diagrams

A. Appearance/Disappearance of a phase

Example: Dehydration of talc

Talc -> enstatite + quartz + fluid

Mg3Si4O10(OH)2 -> 3MgSiO3 + SiO2 + H2O

B. Tieline flip

Example: dehydration of muscovite

Muscovite + quartz -> kspar + sillimanite + H2O