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 Phases4. 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 (MgSiO3) breaks down to Forsterite (Mg2SiO4) + 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

33

A Ternary System

33

A Ternary System

33

A Ternary System

33

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

33

A Ternary System

Bowen's Series and Igneous Rocks

Volcanic Rocks

(Rare)  Basalt   Andesite                 Rhyolite

Plutonic Rocks

Dunite  Gabbro   Diorite                   Granite

1200 C            Melting Point              700 C

Mg, Fe              Rich In...             Si, Na, K

Rapid              Weathering                   Slow

Usually Dark       Color              Often Light

Bowen's Series and Volcanoes

Volcanic Rocks(Rare)  Basalt   Andesite                 Rhyolite

Plutonic RocksDunite  Gabbro   Diorite                   GraniteFluid               Lava Is...                 ViscousMild                Eruptions                 Violent

Type of VolcanoShield Volcano     Stratovolcano    Plug Dome