Lecture 15 More metamorphic Rocks

REVIEW from last time:

We’ve defined metamorphic grade by sequences of index minerals in either pelitic (muddy) rocks or basalts. For example:

Low grade (slate)	Medium grade (schist)	High grade (gneiss)
Chlorite è Biotite è	Garnet è staurolite è kyanite [cordierite è andalusite]	Sillimanite

This is called a Barrovian sequence (with the lower pressure Buchan sequence shown in brackets].

Amphibole type	Stability Range	Metamorphic Rock Name
green amphibole	low T, low P	greenschist
black amphibole	moderate T,P	amphibolite
blue amphibole	low T, high P	blueschist

We use the latter to define the primary metamorphic facies

Metamorphic Reactions

1. Solid-solid reactions

A simple example of both the kinds of chemical reaction and their role in defining conditions of metamorphism is provided by the one component system Al₂SiO₅. This system consists of three minerals – kyanite, andalusite, and sillimanite – each of which has the same chemical composition but different structure. That is, just like graphite and diamond, these minerals are polymorphs. The phase diagram for this system shows that each mineral is stable over a particular pressure and temperature range. For example, andalusite forms only at pressures less than those of the “triple point”, that is, the single point in P-T space where all three phases can coexist. Similarly, sillimanite can form only at temperatures greater than that of the triple point. Additionally, this diagram shows that progressive heating at low pressure (P < 4000 bars) produces first kyanite, then andalusite then sillimanite. The transition from kyanite to andalusite is known as the andalusite isograd, while the transition from andalusite to sillimanite is called the sillimanite isograd.

Use of this diagram requires that we have an equilibrium assemblage. One test of that is known as Gibbs Phase Rule:

F = C – P + 2

where F is the “degrees of freedom”, C is the number of components (here, Al₂SiO₅), and P is the number of phases; the “2” accounts for P, T. The phase rule says that, in the Al₂SiO₅ system, rocks containing one phase are divariant (2 degrees of freedom), rocks with 2 phases are univariant (1 degree of freedom), and rocks containing all three phases are invariant (they can equilibrate at only one P-T condition).

2. Dehydration reactions

Sedimentary rocks, especially those rich in clay minerals, may contain up to 6 wt% H₂O. As these rocks undergo metamorphism, resultant minerals contain increasingly less water. Many of the reactions that produce these minerals are dehydration reactions, that is, reactions that break down hydrous minerals to produce minerals with lower (or no) water, thus releasing water in the process. All of the reactions that produce index minerals during the progressive metamorphism of shales (e.g., biotite, garnet, staurolite, kyanite, sillimanite) are dehydration reactions.

EX: KAl₂Si₃AlO₁₀(OH)₂ + SiO₂ = KAlSi₃O₈ + Al₂SiO₅ + H₂O

muscovite + quartz = K-feldspar + aluminosilicate + water

You can also have decarbonation reactions, such as CaCO₃ + SiO₂ = CaSiO₃ + CO₂

NOTE: The Transition from Metamorphism to Magmatism

An important dehydration reaction is one that occurs only at very high grades of metamorphism (at ~ 700˚C). It involves the breakdown of white mica (muscovite) in the presence of quartz to produce K-feldspar, sillimanite, and H₂O. Remember (from the igneous section of the class) that “normal” continental geothermal gradients are not sufficiently high to produce melt. However, with H₂O present, melting may occur at much lower temperatures. If we look at a diagram for dry and wet melting of muscovite + water, we see that the “wet” melting curve permits melting at ~ 700˚C and 5000 bars pressure. This melting mechanism is probably responsible for formation of granite deep in the continental crust, probably the origin of many large batholiths (such as the Idaho Batholith).

3. Continuous reactions

Another important type of metamorphic reaction is one that simply exchanges cations among solid solution minerals that coexist in a metamorphic rock. For example, Mg²⁺ and Fe²⁺ can be exchanged between garnet and biotite:

KMg₃AlSi₃O₁₀(OH)₂ + Fe₃Al₂Si₃O₁₂ çè KFe₃AlSi₃O₁₀(OH)₂ + Mg₃Al₂Si₃O₁₂

Mg-biotite Fe-garnet Fe-biotite Mg-garnet

This reaction is virtually independent of pressure, but strongly dependent on temperature. Thus, the composition (Fe/Mg ratio) of coexisting garnet and biotite in a metamorphic rock is primarily determined by temperature – for this reason, this reaction can be used as a geothermometer, if properly calibrated in the lab.

Similarly, reactions that are strongly dependent on pressure can be used as geobarometers. By careful use of geothermometers and geobarometers, metamorphic petrologists are often able to constrain T to within about 30˚C and P to within 500 bars.