Metamoprhic reactions 22`

What is the fundamental principle of metamorphic rocks?

The mineral assemblages reflect the physical P-T conditions all the time when the rock formed

How to get a rock in equilibrium

long enough exposure to the same specific P-T conditions, and the rock will assemble into the most stable conditions possible

What is the most stable mineral composition?

The state with the lowest gibbs free energy

What is a petrogenetic grid used for?

Bulk composition of a constant

What is the value of a P-T grid

The actual range of physical conditions under which specific assemblages are stable relative to others

Compositional phase diagrams

Constructed for a specific value of P + T

* only phases connected by Tie lines can exist
* The system will usually crystallize as a 3 phase system according to the triangular field

What happens if a mineral is not directly connected by tie line?

It cannot stably co-exist

What happens when two minerals react together?

The tie line that connects them is removed

what happens when 2 minerals grow together for the first time?

a new tie line connects them together

What is serpentinite?

Hydrated ultramafic rock, common in mountain belts.

* They represent the metamorphosed ocean floor or upper mantle rocks that have been thrust upwards during the orogenic process

Antigorite= Forsterite + talc+ h2O reaction

As we get closer to the pluton, the T of metamorphism is increasing. Antigorite has dehydrated and reached its high-T stability limit to form Fo+ TC+h2O

crossing tie line reaction

A crossing tie line reaction occurs when a rock crosses from one tie line to another as the conditions change during metamorphism.