Ig Met Pet Lecture 19: Metamorphic Toolbox Two

Why is metamorphism predictable and reproducable?

Because the same bulk composition affected by the same conditions will produce the same mineral assemblage

Different bulk compositions result in predictable…

characteristic equilibrium mineral assemblages

What are the categories of protolith (bulk composition)

Mafic, ultramafic, pelitic, calcareous, and quartzo-feldspathic

Why do we have the categories of protolith (bulk composition) that we do?

They represent the range of compositions found in crustal rocks

Special emphasis is placed on mafic and pelitic protoliths. Why is this?

Mafic and pelitic rocks are the most sensitive to changes in metamorphic condition, providing the most variance in mineral assemblage. This is valuable for interpretation

What is characteristic of the zeolite facies?

Zeolite facies rocks are generally undeformed and retain relict igneous or sedimentary textures

What is characteristic of the greenschist facies?

The Barrovian chlorite, biotite, and garnet zones are all in it

What is characteristic of the amphibolite facies?

The Barrovian staurolite, kyanite, and sillmanite zones are all in the amphibolite facies. At
lower pressures than the Barrovian sequence, andalusite, sillmanite, and cordierite are common.

What is common of the blueschist facies?

Glaucophane or lawsonite

What is common of the eclogite facies?

UHP coesite is generally converted to qtz in the rock matrix as the rocks are brought to the surface, but can be preserved as inclusions
in high-strength minerals like garnet and omphacite.

What is typical of hornfels?

A typically undeformed, fine-grained rock produced from contact metamorphism with plutons

When does sanidinite form?

Under the highest temperature conditions of contact metamorphism, approaching the melting temp of rocks

Why are higher grade minerals generated from lower grade minerals as you go up in metamorphic grade?

Lower grade minerals become unstable, and are replaced by more stable configurations

What needs to happen for higher grade minerals to replace lower grade ones?

A chemical reaction, where some minerals are consumed, and others are produced

Everything about mineral assemblages hinges off the assumption that…?

The metamorphic mineral assemblage was at or close to equilibrium at the time of metamorphism

How do we know that the metamorphic mineral assemblage was at or close to equilibrium at the time of metamorphism?

There’s no way to know for sure, but there are qualitative and quantitative checks

What are some qualitative checks on whether the metamorphic mineral assemblage was at or close to equilibrium at the time of metamorphism?

How many minerals are there, do they appear happy together texturally, and should they occur within the same facies?

What is an important quantitative check for whether the metamorphic mineral assemblage was at or close to equilibrium at the time of metamorphism?

Gibb’s phase rule equilibrium

Are rocks likely to equilibriate on a reaction line or at an invariant point?

No, this is very rare in natural systems

If the number of phases is > or = the number of components…

Things are great, or may be made complicated by a solid solution

If the number of phases is < the number of components, what are the two likely possibilities?

equilibrium has not been attained, or the number of components was chosen incorrectly

Is it common for metamorphic rocks not to reach equilibrium, and what indicates this occurred?

Yes, relict textures and incomplete reactions are visible

What are the types of reactions which can occur?

Polymorphic, exsolution, solid-solid

What is a solid-solid reaction?

The reaction of a solid to a different solid, without consuming or producing a volatile phase

If a solid solution is limited, what happens to solid-solid net-transfer?

it is discontinuous

What is characteristic of discontinuous solid-solid reactions?

tendency towards completion at a single temperature, and abrupt change at reaction isograd

What are two types of devolatization reactions?

Dehydration and decarbonization

What typically happens to pelitic protoliths in terms of volatization reactions?

Dehydration

What typically happens to mafic protoliths in terms of volatization reactions?

Hydration upon initial burial then dehydration

Why are devolatization reactions on phase diagrams curved?

At differing pressures, the density of fluids change.

What are the two types of reaction progressions?

Continuous and discontinuous

How do continuous reactions affect stability of a mienral?

compositional variation leads to stability over wide range of conditions

Discontinuous reaction progress

stuck on the reaction line or point, occurring at a specific temperature until the entire reaction is complete or one reactant has been consumed

How do solid solution compositions vary with changing metamorphic grade?

The proportions of the minerals vary until one of the reactants disappears with increasing grade

Ion exchange reaction

The modal amount of the phases involved remains constant, but their compositions changes due to a chemical exchange

Which reaction is best for geochemistry and why?

Ion exchange, because the partitioning of elements between phases is highly temperature dependent

Redox reactions

involves a change in the oxidation state of an element, resulting in oxygen buffers, and controlling the Fe-Mg solid solutions in metamorphic rocks