Transcript Evolution of Igneous Rocks

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 (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
3
A Ternary System
3
A Ternary System
3
A Ternary System
3
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
3
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
Plutonic Rocks
Dunite Gabbro Diorite
Fluid
Lava Is...
Mild
Eruptions
Type of Volcano
Shield Volcano Stratovolcano
Rhyolite
Granite
Viscous
Violent
Plug Dome