Ab-Or system at 5 kilobars with excess H2O, i.e, PH2O = 5 kb

Note:1. This is actually a 3 component system. Why?

Discuss the cooling of composition a from above the V-saturated liquidus (T>800ºC).

2. What are the phases that coexist at 701ºC? What is the variance of the system at this temperature?

3. Why is the eutectic temperature much lower than in the “dry” system?

5. How does the equilibria in this system help to explain 2-feldspar granites.

900

800

700

600

500Ab 20 40 60 80 Or

L + V

Ab+L+V Sa+L+V

Sass+VAbss+V

Abss+Sass+V

701

a

Wt % Or

H2O is also a component along with Ab and Or. This figure is a projection of the V-saturated equilibria from H2O.

Phases: Abss, Sass, L, Vf = 3-4+1 =0 (Isobaric invariant)

Effect of H2O on silica-rich melts is pronounced

After Morse (1970) Journal of Petrology

4. How much water is dissolved in eutectic melt at 5 kilobars

(~ 10 wt. %)

c

a

(001)

(100)

Plane // (010)

cleavage

perthite

Sketch showing the orientation of perthitic lamellae of albite in a K-feldspar host

TEM image of albite lamellae (showing polysynthetic twinning) in a K-feldspar host

Optical photomicrograph of wispy albite lamellae in a microcline host

Albite-H2O: Binary system with H2O to 1.0 Gpa (10 kb)

This P-T projection shows two lines (a) dry melting of albite and (b) melting of albite under water-saturated conditions (studied by numerous experimentalists). Note: pronounced lowering of melting point with addition of water to the system. Why is there a dramatic change in slope of V-satd melting curve?

What is not obvious from this figure is that the amount of H2O dissolved in albite melt at vapor-saturation increases dramatically along the the vapor-saturated curve (shown below and on next figure).

Phase rule:

Along V-saturated melting curve, c=2, p=?, f = ?

P (kb) Wt% H2O

1 4.4 40 2 6.4 50 4 8.7 586 11.1 648 13.4 69

10 15.8 73

Mol % H2O

Figure from Winter (2001) An Introduction to Igneous and Metamorphic Petrology

Albite – H2O system (cont.)As discussed in the previous figure, it takes a lot of water to saturate a melt of albitecomposition (or any silicate melt for that matter). Chances are that there is not enough water in the melting region to saturate the melt under all conditions. What then? We can use the Ab-H2O system to illustrate this situation.

Red curves: melting for fixed mol fr. H2O in meltBlue curves: water content of a H2O-satd meltLet’s examine the melting path of compn a at P= 5 kb with 50 mol% (6.4 wt %) H2OAt b: system begins to melt. System is isobaric invariant so it must remain at b until one of the phases is gone. Which one? Note that albite can dissolve 10 wt % H2O at 5 kb but we only have 6.4%.

All H2O available is dissolved in melt and we are left with melt and some albite crystals

From b to c, T increases and the albitecontinues to melt (no V phase in the system). When is melting complete?

Melting is complete at c. Can show in (TX)Pfigure on next slide

ab c

Figure from: Burnham and Davis (1974) AJS, 274

ab c

Albite - H2O system (cont.)

T(ºC)

600

1400

800

1000

1200

Ab H2O20 40 60 80

L

L + V

V

Ab+L

Ab + V

770

PH2O = 5 kbIsobaric TX section

Wt % H2O

a

b

c

Figure from: Burnham and Davis (1974) AJS, 274

Albite-H2O system (Cont.) Isothermal decompression from 10 Kb to the surface starting at a (950ºC). Assume acontains 50 mol% (6.4 wt %) H2O.What is the initial state of the system?

Since albite melt at 950ºC & 10kb can hold 52% H2O at satn, the system will consist of 96% melt containing 52% H2O + 4% albitecrystals giving a bulk compn of 50% H2O.

What happens at b as melt decompresses?All crystals melt, leaving melt with 50% H2O

What happens between b and c?Melt becomes superheated (no V phase)

What happens at c?Melt starts to outgas and vesiculate

What happens between c and d?

What happens at d?Melt continues to outgas and vesiculate

At d, melt contains ~30 mol% H2O and crystallization begins. Melt stays at d simultaneously outgassing and crystallizing until totally crystallized.

Do real magmas behave like this? Yes!!

a

b

c

d

Figure from: Burnham and Davis (1974) AJS, 274

Albite-H2O system (cont.)

What happens if the fluid phase is a mixture of species? In nature the fluid phase is likely to be a mixture of H2O, CO2, SO2along with other gaseous species.

This figure shows the effect of “diluting” the fluid phase with addition of CO2. The activity of H2O is reduced below 1 and consequently its effect in lowering the melting temperature is reduced.

Figure from Winter (2001) An Introduction to Igneous and Metamorphic Petrology.

Ternary (c = 3) systemsThis diagram is an attempt to show a 3D diagram in a 2D perspective. Anorthite

Forsterite

Diopside

Tb

c

TE

a

The red lines show the binary eutectic between anorthite and forsterite, the blue lines show the eutectic between forsterite and diopside and the green lines show the eutectic between diopside and anorthite.

Black lines extending from each of the binary eutectics extend into the 3D object and meet at the ternary eutectic labeled TE. The black lines are called cotectics. TE is the lowest temperature in the system and it can be compared to a hole where three valleys meet.

An isothermal plane (not shown) parallel to the base passing through TE is the ternary solidus plane.

This system which has no solid solution is the simplest ternary systemTo depict this system in 2D, the liquidus surface is projected parallel to the T axis on to the base. This diagram can then be contoured with isotherms much like elevation contours on a topo map

After: Winter (2001) An Introduction to Igneous and Metamorphic Petrology.

Ternary (c = 3) systems: Fo-An-Di systemThe liquidus surface projected onto the base of the triangle with the position of the ternary cotectic lines shown in orange. Fine lines are isothermal contours on the liquidussurface. The ternary euctectic is at M (1270ºC); this is lowest temperature at which liquid can exist in this system.

The orange lines are also called field boundaries since they delineate distinct fields of crystallization

The composition of M can be read off the ternary figure as Di50-An43-Fo7.

Phase rule at M:C = 3, p= ?, f = ?

P = 1 atm

After: Morse (1994) Basalts and phase diagrams. Krieger

a

b

Eqm crystallization of composition a:Fo crystallizes at 1500ºC and liquid follows path directly away from Fo corner to b(mass balance). At b, An joins Fo and both crystallize in ratio given by the tangent to the cotectic (30Fo + 70An) and the liquid follows the cotectic to M. At M, diopsidejoins the crystallization sequence and the three crystalline phases form in the relative amounts corresponding to composition M.

Fractional crystallization follows the same path, the only difference being that the crystals are removed from the system as they form.

Equilibrium melting is the reverse of eqm crystallization. The first liquid to form from any ratio of An:Fo:Di is at M.

Fo-Di-An system at P = 1 atm

After: Morse (1994) Basalts and phase diagrams. Krieger

aaDiDi

xx

FoFomm

Illustration of the ternary lever rule

T =1300ºC: Bulk composition = aliquid x crystallizing Di and Fo

Ratio of xals/liq = ax/am

Ratio of Di/Fo = Fo-m/Di-m

After: Morse (1994) Basalts and phase diagrams. KriegerAfter: Winter (2001) An Introduction to Igneous and Metamorphic Petrology.

Ternary system (Fo-An-SiO2) with intermediate compound (peritectic)

Phase rule:At points c and d: f = ?

Reaction along line labeled “peritectic”:Fo + L ↔ En

All other field boundaries are cotectics, e.g., from 1543 to d, reaction is:L ↔ En + Silica

After: Morse (1994) Basalts and phase diagrams. Krieger