Metamorphism of Carbonate Rocks

See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html

A. Types

-Carbonate >50%: limestone (calcite); dolostone (dolomite) --> both become marble
-Carbonate <50% : calc-silicates; metamorphosed to skarn
-Meta-ls = Cal over wide range of conditions. We''ll consider impure (siliceous) dolostone.

B. Minerals

-System CaO-MgO-SiO2 + Co2 + H2O
-Minerals: Cal (CaCO3); Dol (CaMgCO3);Wo (CaSiO3) ; Di (CaMgSiO3) ; Q
-Carbonates in left half; carbonaceous ultramafics in right half

II. Contact metamorphism

A. Petrogenetic grid

-T increases as distance to igneous contact decreases.
-Low-T assemblage = Dol + Cal + Q
-First reaction to occur depends on XCO2, Let X < 0.63
-Talc isograd: reaction is Dol + Q + water = Tlc + Cal + CO2; this is a tie-line flipping reaction - See Figure 29-4b.
-Next?? :As we discussed before, when dehydration/decarbonation reactions are involved, we must distinguish between an externally-controlled fluid and one that is internally controlled.

B. Open system (externally-controlled fluid)

-Typical of shallow metamorphism of permeable rocks: fluid produced by reactions doesn''t change the overall composition of the fluid, so rock follows a vertical path; Tlc-forming reaction is discontinuous and proceeds at T = 390 oC until 1 reactant is used up.
-Tremolite isograd: next reaction on Figure 29-3 would produce Tr, but it doesn''t occur, since no more Q: next reaction is Tlc + Cal = Dol + Tr + CO2 + water (this is a tie-line flipping reaction)
-Forsterite isograd: next 2 reactions will not affect siliceous dolostones (no Di, since Q is gone/ no Fo since not enough MgO); reaction is Tr + Dol = Fo + Cal + CO2 + water; this is a tie-line flipping reaction
-[Wollastonite isograd - siliceous limestones only]: reaction is Cal + Q = Wo + CO2. Dolostones unaffected.
-Periclase isograd: reaction is Dol = Per + Cal + CO2; this removes Dol as a phase.
-For other XCO2, isograds would vary:

-XCO2 > 0.97 (point B): Di-Fo
-0.7 < XCO2 < 0.97: Tr-Di-Fo
-0.63 < XCO2 < 0.7: Tlc-Tr-Di-Fo

A. Closed system (internally buffered fluid)

-Typical of somewhat deeper, less permeable rocks
-Let XCO2 < 0.63: Talc isograd (same reaction as before:


-Dol + Q + water = Tlc + Cal + CO2 ( But this time, fluid produced by reaction stays in system, mixes with original fluid to change its composition/ Continuous reaction. T keeps rising and reaction proceeds along reaction curve toward point A/If Q is used up before A, T is free to rise vertically).
.
-Tremolite isograd (Fluid composition reaches A and Tr forms/Fluid is buffered at XCO2 = 0.7 until 1 reactant is used up. System will then proceed along curve that lacks that phase/Assume Q is used up: Tlc + Cal = Dol + Tr + CO2 + water (Reaction produces fluid with X = 0.5 < A. This mixes with fluid at A, making it more water-rich/T rises during continuous reaction along reaction curve away from A until: one reactant is used up, or overall fluid composition reaches 0.5)
-Forsterite isograd (Same sort of behavior: fluid is buffered along reaction curve toward C).
-Di isograd (At C, Di forms/System is fixed until something is used up, then proceeds along reaction curve to right or left).


-Let 0.7 < XCO2 < 0.97: Tr - continuous reaction toward B - Di - continuous reaction toward C - Fo

III. Regional metamorphism

A. Petrogenetic grid at higher P

-medium-P metamorphic gradient
-Reactions shift to higher T
-Invariant points shift to more water-rich composition

B. Open system

-Let XCO2 < 0.1: Isograds: Tlc - Tr - Di - Fo � Wo; oR Tlc - Tr - Fo - Wo for nearly pure water
-Let 0.1 < XCO2 < 0.85: Isograds: Tr - Di - Fo � Wo;
-Let XCO2 > 0.85: isograds: Di - Fo - Wo.

C. Closed system

-Let XCO2 < 0.1: isograds: Tlc - Tr - Di - Fo
-Let 0.1 < XCO2 < 0.85: isograds: Tr - Di - Fo
-Let XCO2 > 0.85: isograds: Di - Fo - Wo