1. Eutectic and Peritectic Systems
Winter, Chapter 6

2. Eutectic Systems Example: Diopside - Anorthite No solid solution
Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40,

3. Cooling from composition a:
bulk composition = An70
At a: phi = 1
F = C - phi + 1 = = 2
Good choice is T and X(An)Liq

4. First crystal forms at 1455oC (point b) with a compositon of pure An
What happens at b? Pure anorthite forms (point c)
phi = 2
F = = 1
composition of all phases determined by T
Xliq is really the only compositional variable in this particular system

5. Cooling continues as Xliq varies along the liquidus
Continuous reaction: liqA ® anorthite + liqB
Lever rule shows less liquid and more anorthite as cooling progresses

6. At 1274oC f = 3 (three phases co-exist)
Therefore, F = = 0 This is an invariant point
(P) T and the composition of all phases is fixed
Must remain at 1274oC as a discontinuous reaction proceeds until a phase is lost
What happens at 1274oC?
get new phase
Pure diopside g joins liquid d and anorthite h
d is called the eutectic point: MUST reach it in all cases

7. A discontinuous reaction occurs at d
Temperature remains constant at 1274
Use geometry to determine liquid crystal ratios
d between g and h, so reaction must be:
diopside + anorthite = liquid
must run right -> left as cool (toward low entropy)
therefore first phase lost is liquid
Below 1274oC have diopside + anorthite
phi = 2 and F = = 1
Only logical variable in this case is T (since the composition of both solids is fixed)

8. Left of the eutectic there is a similar relationship
Cool An oC (e) and diopside forms first at f
Continue cooling: continuous reaction with F = 1 as liqA -> liqB + diopside
At 1274oC anorthite (h) joins diopside (g) + liquid (d)
Stay at 1274oC until liquid consumed by discontinuous reaction: liquid -> diopside + anorthite

9. The melt crystallizes over a T range up to ~280oC
A sequence of minerals forms over this interval
- And the number of minerals increases as T drops
The minerals that crystallize depend upon T
- The sequence changes with the bulk composition

10. Augite Forms Before Plagioclase
Gabbro of the Stillwater Complex, Montana
This forms on the left side of the eutectic

11. Plagioclase Forms Before Augite
Ophitic texture
Diabase dike
This forms on the right side of the eutectic

12. The last melt to crystallize in any binary eutectic
The last melt to crystallize in any binary eutectic mixture is the eutectic composition
Equilibrium melting is the opposite of equilibrium crystallization
Thus the first melt of any mixture of Di and An must be the eutectic composition as well

13. Fractional Crystallization
Since solids are not reactants in eutectic-type continuous reactions, the liquid path is not changed
Only the final rock will differ: = eutectic X, and not bulk X
Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40,

14. Partial Melting: if remove liquid perfectly as soon as it forms:
melt Di + An to 1274oC
discontinuous reaction: Di + An -> eutectic liquid d
consume either Di or An first depending on bulk X
then melting solid = pure Di or An and jump to 1- C system
Thus heat 118 or 279oC before next melt

15. Binary Peritectic Systems
Three phases involved: enstatite = forsterite + SiO2
Figure Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.
Cool bulk composition a (42 %)
At a
phi = 1 (liquid)
F = = 2
Cool to 1625oC
Cristobalite forms at b
phi = 2
F = = 1
Xliq = f(T)

16. Binary Peritectic Systems
Figure Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.
As T lowered Xliq follows path to c: the eutectic
At 1543oC, enstatite forms: d
Now f = 3 and F = = 0 invariant
Discontinuous reaction:
liq = En + Crst colinear
Stay at this T until liq is consumed
Then have En + Crst
phi = 2
F = = 1 univariant
At 1470oC get polymorphic transition Crst -> Trid
Another invariant discontinuous rxn

17. At 1557oC get opx (En) forming phi = 3 F = 2 - 2 + 1 = 0 invariant
Figure Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci.
Next cool f = 13 wt. %
at 1800oC get olivine (Fo) forming
phi = 2
F = = 1 univariant
Xliq = f(T)
At 1557oC get opx (En) forming
phi = 3
F = = 0 invariant

18. i m k d c i = “peritectic” point
At 1557oC there is colinear equilibrium of Fo-En-liq
geometry indicates a reaction: Fo + liq = En
consumes olivine (and liquid) ® resorbed textures c d i k m Fo En
1557
When is the reaction finished?
Expanded view of the Fo-SiO2 diagram:
When is the reaction finished??
When a phase is used up
Which phase will it be?
Since the bulk composition lies between En and Fo, liq must be used up first
Then have En + Fo: f = 2 F = 1
Bulk X

19. Olivine with orthopyroxene rims, Mt McLoughlin, OR

20. Same photo with olivine at extinction to make Opx more visible

21. 1543 c d i k m Fo En
1557
bulk X x y Cr
Now begin with bulk composition shown, hi T: F = 2
olivine y forms at 1590oC liq = x phi = 2 F = 1
1557oC En joins Fo and liq phi = 3 F = 0
again discontinuous reaction: Fo + liq = En
this time olivine consumed before liquid
olivine appears then disappears
Xliq follows liquidus -> c
below m:
En + liq
phi = 2 F = 1
At 1543oC get En + Crist + liq
Eutectic invariant
liq -> En + Cr

22. Incongruent Melting of Enstatite
Melt of En does not produce a melt of same composition
Rather En goes to Fo + Liq i at the peritectic
Partial Melting of Fo + En (harzburgite) mantle
En + Fo also ® firsl liq = i
Remove i and cool
Result = ?
1543 c d i Fo En
1557
bulk X Cr

23. Consequence For Basalt Magma
The Fo-En-Q system causes compositions
to migrate from alkali
basalt toward tholeiite
as the degree of crystallication
or melting progresses