Transcript
Page 1: Geol 2312  Igneous and Metamorphic Petrology

GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 26

Metamorphic Reactions (cont.)

Chemographics and Petrogenetic Grids

April 8, 2009

Page 2: Geol 2312  Igneous and Metamorphic Petrology

TYPES OF METAMORPHIC REACTIONS

• Phase Transformations

• Exsolution Reactions

•Solid-Solid Net Transfer Reactions

• Devolatilization Reactions

• Continuous Reactions (THIS LECTURE)

• Ion Exchange Reactions

• Oxidation/Reduction (redox) Reactions

•Reactions involving Dissolved Species

Page 3: Geol 2312  Igneous and Metamorphic Petrology

CONTINUOUS REACTIONSINVOLVING SOLID SOLUTION PHASES

Winter (2001) Figure 26-8.

The bulk chemistry of each unit is homogeneous, but differs somewhat from the other units in the area

The garnet-in field isograd varies from unit to unit, occurring at different grades.

WHY?

An idealized field area of steeply dipping meta-pelitic sediments that strike directly up metamorphic grade

Page 4: Geol 2312  Igneous and Metamorphic Petrology

CONTINUOUS REACTIONSTwo possible reasons:

1. Each unit has contrasting compositions such 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)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 or 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

Let’s evaluate the second situation

Page 5: Geol 2312  Igneous and Metamorphic Petrology

CONTINUOUS REACTIONS

Winter (2001) Fig. 6-10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213

““Melt-in” Melt-in” isograd?isograd?

Temperature is strongly

X-dependent

Recall the igneous situation with solid solution

Page 6: Geol 2312  Igneous and Metamorphic Petrology

CONTINUOUS VS. DISCONTINUOUS REACTIONS

Consider the reaction: Chl + Ms + Qtz Grt + Bt + H2O

F = C –+ 2 = 5 – 4 + 2 = 1 (univariant)

But effectively F=0 since T and P are controlled by moving along a geothermalor field gradient

Therefore, the reaction occurs at a particular T for a givengradient and would be consideredDISCONTINUOUS

Page 7: Geol 2312  Igneous and Metamorphic Petrology

CONTINUOUS VS. DISCONTINUOUS REACTIONS

This is a continuous reaction and we expect to find chlorite, muscovite, quartz, biotite, and garnet all together in the same rock over an interval of metamorphic grade above the garnet-in isogradThe composition of solid solution phases will vary across the interval, and the proportions of the minerals will change until one of the reactants disappears with increasing grade

However, when considering the solid solution of Mg and Fe in garnet, biotite, and chlorite, the general reaction is (Fe,Mg)Chl + Ms + Qtz (Fe,Mg)Grt + (Fe,Mg)Bt + H2O

Page 8: Geol 2312  Igneous and Metamorphic Petrology

CONTINUOUS REACTIONS

Perhaps a more realistic way to portray this continuous reaction (minus the K component) is this.

Thus, the offsets in the idealized field area may be due to differences in the Mg/Fe ratios of the different rock layers.

Page 9: Geol 2312  Igneous and Metamorphic Petrology

TYPES OF METAMORPHIC REACTIONSDEVOLATILIZATION

Another example of a continuous reaction over a range in T due the potential for the fluid composition

to changeCLOSED (buffered)Volatiles are trapped in the rock with degassing

XCO2 must follow the

reaction equibrium curve to the max T.

a – degassing of CO2 & H2O drives system to b and increased Tb – Trem, Cal or Qtz must be consumed before T can increase c – degassing of CO2 causes system to move along reaction curve toward XCO2

= 1

Fluid composition is controlled (buffered) by the progress of the

reaction

Page 10: Geol 2312  Igneous and Metamorphic Petrology

REACTIONS AND CHEMOGRAPHICS

We can use chemographics to infer reactions. Consider the 2-component system MgO-SiO2

MgO SiO2

Per Fo En Qtz

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)

However, thermodynamics determines whether such a reaction is probable

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REACTIONS AND CHEMOGRAPHICS

For a ternary system, any phase that is coplanar with three other phases can be related by a chemical reaction

2A + B + C = X

Again, whether this reaction is probable under natural conditions of P & T depends on Thermodynamics.

If the diagram represents a projection from another phase or component, then that phase is implied in the reaction.For component D, the reaction might be:

2A + B + C + #D = XHowever, the amount of D in the reaction would have to be figured out by balancing the reaction.

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REACTIONS AND CHEMOGRAPHICS

At P/T Conditions A

At P/T Conditions B

2A + B + C = X

X-inIsograd

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REACTIONS AND CHEMOGRAPHICS

Another Possibility If phase X can be defined by two different reactions:

2A+B+C = X

D+E = X

Then, the reaction:

2A+B+C = D+E

is also true

Page 14: Geol 2312  Igneous and Metamorphic Petrology

REACTIONS AND CHEMOGRAPHICS

A + B = C + D

A Tie-line Flip - results in new groupings in the next metamorphic zoneBecause A+B and C+ D share a common tie-line, they can be related by the reaction:

Increasing Grade

Below the Isograd At the Isograd Above the Isograd

Page 15: Geol 2312  Igneous and Metamorphic Petrology

PETROGENETIC GRIDSGRAPHICALLY PORTRAYING SEVERAL

REACTIONS IN PRESSURE – TEMPERATURE SPACE

Univariant Curves can be labelled by Absent

Phase

MetastableExtensions

Simple One Component SystemSimple One Component System

Page 16: Geol 2312  Igneous and Metamorphic Petrology

PETROGENETIC GRIDSMULTISYSTEMS > C +2

One-Component SystemFour Phases (ABDE)

Four invariant points(labelled by the absent phase)

Seven univariant lines(labelled by the absent phases)

Four divariant fields(labelled by stable phase)

Page 17: Geol 2312  Igneous and Metamorphic Petrology

PETROGENETIC GRIDSMULTISYSTEMS

Theoretically Possible vs. Geologically Probable

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Many Petrogenetic Grids will overlay grids of multiple

component systems that pertain to a specific protolith type

Winter (2001) 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”).

PETROGENETIC GRIDSMULTISYSTEMS

Petrogenetic Grid for Mafic Rocks

Actinolite

Pyrophyllite

Lawsonite

P-T Range of Rock with Lawsonite + Actinolite + Pyrophyllite


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