Evolution of Igneous Rocks

Simple Eutectic
• Two components that don’t mix in the solid state
• One or the other begins to form as melt cools
• When temperature minimum reached, other component starts to form
• Both components crystallize
• Temperature remains constant until melt completely solidifies

As
Component A is Removed, the Melt
Shifts Toward
B

At Any Given
Point We Can
Determine
How Much
Melt and
Solid Are
Present

Once the
Temperature
Minimum
(Eutectic) is
Reached, B
Forms as
Well

The Overall
Eutectic
Diagram

A Familiar
Eutectic

Evolution of a
Eutectic
Melt

Evolution of a
Eutectic
Melt

Intermediate Compounds

Solid Solution
• Two components mix freely in solid state
• A melts at higher temperature than B
• As melt cools, the first crystals to form are richer in A than the melt
• As A is taken out, the mineral and the remaining melt become richer in B
• Last melt is much richer in B than the original melt
• Final solid has same composition as original melt.

First Solid is
Richer in Fo than the Melt

As Fo is
Removed,
Both the
Remaining
Melt and
Resulting
Solid Get
Richer in Fa

A Simple
Rule Allows
Us to Tell
How Much of
Each
Component
We Have

The Final
Melt is Much
Richer in Fa
Than the
Original

Simple
Solid
Solution

How Our
System
Evolves on the Phase
Diagram

A Solid
Solution,
Animated

How To Read
Any Phase
Diagram
1. Read the Field
Labels
2. Note What
Changes at
Boundaries
3. Track All Phases
4. Use Proportions to Determine
Quantities

Bowen's Reaction Series
• The geologist N.L. Bowen found that minerals tend to form in specific sequences in igneous rocks
• These sequences could be assembled into a composite sequence.

Bowen's Reaction Series
• Why “Reaction?”
– Solid Solutions may or may not remain in equilibrium with liquid
– Some solids (enstatite) break down on melting and others may dissolve in their own magma
• Why “Series?”
– Solid solutions evolve as melt solidifies
– Eutectic relationships determine solidification sequence.

Bowen's Reaction Series
No igneous rock ever displays the whole sequence, just a slice across the sequence.

Bowen's Series and Igneous Rocks

Incongruent Melting
• Some minerals break down as they melt
• Example: Enstatite (MgSiO
3
) breaks down to Forsterite (Mg
2
SiO
4
) + Liquid
• When cooling, the reverse happens:
Fortsrite and Liquid react to make
Enstatite
• Sometimes Forsterite disappears completely

Incongruent
Melting

Incongruent
Melting

Incongruent
Melting

Incongruent
Melting

Incongruent
Melting

Incongruent
Melting

Incongruent
Melting

Incongruent Melting

A Ternary System

A Ternary System

A Ternary System

A Ternary System

How Simple Ternary Systems Evolve
• First phase crystallizes. Melt moves radially away from that corner of plot
• Second phase starts to form. Melt moves away from both corners of plot toward eutectic
• Once eutectic is reached, all three phases crystallize

A Ternary System

A Ternary System

Bowen's Series and Igneous
Rocks
Volcanic Rocks
(Rare) Basalt Andesite
Plutonic Rocks
Dunite Gabbro Diorite
1200 C Melting Point
Mg, Fe
Rapid
Rich In...
Weathering
Usually Dark Color
Rhyolite
Granite
700 C
Si, Na, K
Slow
Often Light

Bowen's Series and Volcanoes
Volcanic Rocks
(Rare) Basalt Andesite
Dunite Gabbro Diorite
Fluid
Plutonic Rocks
Lava Is...
Rhyolite
Granite
Viscous
Mild Eruptions
Type of Volcano
Violent
Shield Volcano Stratovolcano Plug Dome