Evolution of Igneous Rocks
Dec 17, 2015
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
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.
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.
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
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
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