Quiz:
Geo 406 Lecture 6 –Ternary Phase Diagrams
Reading: Ch 6
next: handouts
Announcements
Homework on Binary PDs – discuss
Due on Wed.
Goals for today
Be able to read phase diagrams
TWO COMPONENT SYSTEM – BINARY (BRIEF)
Examine PD with solid soln – olivine
Plag is the same
Show A-B Eutectic diagram on Keynote
Temperature vs. composition
Fields of pure liquid (1 phase), liquid + solid (2 phase), pure solid (2 phase)
Crystallization temperature is highest for pure endmembers
Eutectic - lowest T at which melt can exist in system
cf eutectic composition, eutectic point, eutectic temperature
Liquidus - line separating region of pure liquid and solid from pure solid
(temperature at which material is completely molten
Solidus - line separating region of liquid & solid from pure solid (temperature at
which magma completely solidifies
Identify phase fields on Ice-Sucrose PD
Now we’ll examine what happens during crystallization, based on these
diagrams
I. Equilibrium Crystallization
SLOW, so all atoms remain part of the system (can “talk to” each other)
Go through how liquid changes composition as minerals crystallize
“liquid line of descent”
why should we care?
a set of volcanic rocks might track this over time as a magma chamber crystallized
Draw An-Diopside PD and discuss crystallization from melt (note Di is higher-T
phase)
Liquid (a) crystallizes pure anorthite when temperature reaches liquidus temp.
Liquid becomes more diopside rich as anorthite is removed. Bulk composition
(liquid + anorthite) stays the same.
Liquid travels down liquidus (becoming more diopside rich) as more anorthite is
crystallized. Just above solidus, have only crystals of anorthite.
At eutectic, both diopside and anorthite crystallize in eutectic proportions. Bulk
composition stays the same (crystals + liquid). The temperature must remain the
same.
When all melt is gone, have crystals of diopside and anorthite in proportions the
same as the bulk composition.
Same for compositions to left of eutectic. But this time excess diopside
Relative "time" of this process:
T vs. time
II. Equilibrium melting - reverse of crystallization (use same figure)
Heat added to crystals of anorthite and diopside (use composition b above as
example)
Melting will begin at Teutectic. Both anorthite and diopside melt in eutectic
proportions (which is richer in anorthite than the bulk composition b). Liquid
produced has eutectic composition
Mixture stays at constant T until ALL anorthite has melted
Once last bit of anorthite has melted, still have diopside, T can rise again. More of
diopside melts, liquid gets more diopside rich.
Diopside disappears at temperature where the liquid composition lies directly above
the bulk composition. Solid has entirely melted into liquid of bulk composition of
starting material.
Important points to remember:
All mixtures of anorthite and diopside will melt at same T
Bulk compositions lying on diopside side will lose anorthite first, bulk compositions
lying on anorthite side will lose diopside first.
Fractional crystallization – covered more next week
In equilibrium crystallization - assume that crystals and liquid remain in equilibrium
(in contact) throughout crystallization.
In fractional crystallization (more realistic) - crystals will settle out by gravity, or
somehow get separated from liquid. Crystals form cumulates.
In fractional crystallization, crystals of diopside settle out, liquid gets depleted in
CaMgSi2O6 component.
Get to eutectic and system composition has changed. End up with pile of crystals
(lower layer) of diopside overlain by pile of crystals diopside + anorthite (upper
layer)
For solid solution: melt can become more Na-rich than in equilibrium xllzn
Think about plagioclase zoning to remove early-formed plag
Ternary PDs
Ternary lever rule
Show progress as more phases are removed.
Simple 2 component with 2 endmember phases
Two component with intermediate compound
A. Congruent melting/equilibrium crystallization
B. Incongruent melting
Solid Solution
II. A. Congruent melting - solid phase with composition intermediate between
endmembers
Compounds melt to form liquid of their own composition. Like two simple diagrams put
end to end.
Example: see keynote: Nepheline, Albite, Quartz
Albite is thermal maximum. Melting point of pure albite is depressed by adding either
nepheline or quartz.
Pure albite melts to a liquid of albite, but even the tiniest amounts of nepheline or quartz
will cause melting to occur at eutectic temperature to produce eutectic liquid.
Nepheline, albite, quartz - illustrates concept of silica saturation. Can't go from silica
oversaturated to silica undersaturated or vice versa because of thermal maximum.
Incongruent melting / peritectic (qz-en-fo PD)
Intermediate compound melts to liquid not of its own composition
SOLID orthopyroxene
SOLID olivine + liquid
L is peritectic liquid with composition Lp
Stay at that T until all orthopyroxene converted to olivine. Lots of liquid generated.
Examples:
1. Composition c. Below solidus, equal parts Forsterite and Enstatite. At T slightly
above MPenstatite, get 60% olivine and 40% liquid. So olivine increased
50% enstatite
40% L + 10% olivine
2. Composition r. Below solidus, 10% enstatite, 90% quartz.
- r melts at TE to liquid LE, stays at TE until all quartz gone.
- Enstatite continues to melt, Liquid gets more enstatite rich.
- But then hits MPenstatite (~60% liquid, 40% crystals enstatite).
- All enstatite converts to olivine + L.
So above MP enstatite, 7% olivine, 93% liquid