1 - C Systems. The system H 2 O. Heat an ice at 1 atm from-5 to 120 o C. Heat vs. Temperature

Transcription

1 1 - C Systems The system H 2 O Heat an ice at 1 atm from-5 to 120 o C Heat vs. Temperature Fig After Bridgman (1911) Proc. Amer. Acad. Arts and Sci., 5, ; (1936) J. Chem. Phys., 3, ; (1937) J. Chem. Phys., 5,

2 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures)

3 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals generally increases as T decreases

4 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 3. Minerals that form do so sequentially, with considerable overlap

5 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 3. Minerals that form do so sequentially, with considerable overlap 4. Minerals that involve solid solution change composition as cooling progresses

6 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 3. The minerals that form do so sequentially, with consideral overlap 4. Minerals that involve solid solution change composition as cooling progresses 5. The melt composition also changes during crystallization

7 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 3. The minerals that form do so sequentially, with consideral overlap 4. Minerals that involve solid solution change composition as cooling progresses 5. The melt composition also changes during crystallization 6. The minerals that crystallize (as well as the sequence) depend on T and X of the melt

8 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 3. The minerals that form do so sequentially, with consideral overlap 4. Minerals that involve solid solution change composition as cooling progresses 5. The melt composition also changes during crystallization 6. The minerals that crystallize (as well as the sequence) depend on T and X of the melt 7. Pressure can affect the types of minerals that form and the sequence

9 Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 3. The minerals that form do so sequentially, with consideral overlap 4. Minerals that involve solid solution change composition as cooling progresses 5. The melt composition also changes during crystallization 6. The minerals that crystallize (as well as the sequence) depend on T and X of the melt 7. Pressure can affect the types of minerals that form and the sequence 8. The nature and pressure of the volatiles can also affect the minerals and their sequence

10 2 - C Systems A. Systems with Complete Solid Solution 1. Plagioclase (Ab-An, NaAlSi 3 O 8 - CaAl 2 Si 2 O 8 ) Coupled substitution Fig Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1913) Amer. J. Sci., 35,

11 Bulk composition a = An 60 = 60 g An + 40 g Ab X An = 60/(60+40) = 0.60

12 At 1450 o C, liquid d and plagioclase f coexist at equilibrium A continuous reaction of the type: liquid B + solid C = liquid D + solid F

13 The lever principle: Amount of liquid Amount of solid = ef de where d = the liquid composition, f = the solid composition and e = the bulk composition liquidus d de e ef f solidus

14 Note the difference between the two types of fields The blue fields are one phase fields Any point in these fields represents a true phase composition Liquid The blank field is a two phase field Any point in this field represents a bulk composition composed of two phases at the edge of the blue fields and connected by a horizontal tie-line Liquid Plagioclase plus Plagioclase

15 When X plag h, then X plag = X bulk and, according to the lever principle, the amount of liquid 0 Thus g is the composition of the last liquid to crystallize at 1340 o C for bulk X = 0.60

16 Final plagioclase to form is i when = 0.60 Now f = 1 so F = = 2 plag X An

17 * The actual temperatures and the range depend on the bulk composition Note the following: 1. The melt crystallized over a T range of 135 o C * 2. The composition of the liquid changed from b to g 3. The composition of the solid changed from c to h 4. The ratio of solid to liquid increases with cooling

18 Fractional crystallization: Remove crystals as they form so they can t undergo a continuous reaction with the melt At any T X bulk = X liq due to the removal of the crystals

19 Plagioclase Differentiation Mechanisms zoned plag Crystal Settling Perthitic pyroxene Gabbro - Plane Polarized Light Plagioclase zoning

20 2. The Olivine System Fo - Fa (Mg 2 SiO 4 - Fe 2 SiO 4 ) also a solid-solution series Fig Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24,

21 2-C Eutectic Systems Example: Diopside Anorthite No solid solution [basalt composition] Fig Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40,

22 Cool composition a: bulk composition = An 70

23 Cool to 1455 o C (point b)

24 Continue cooling as X liq varies along the liquidus Continuous reaction: liq A anorthite + liq B

25 at 1274 o C f = 3 so F = = 0 invariant (P) T and the composition of all phases is fixed Must remain at 1274 o C as a discontinuous reaction proceeds until a phase is lost

26 Discontinuous Reaction: all at a single T Dipside+liquid=diopside +Anorthite

27 Left of the eutectic get a similar situation

28 #s are listed points in text Note the following: 1. The melt crystallizes over a T range up to ~280 o C 2. A sequence of minerals forms over this interval - And the number of minerals increases as T drops 6. The minerals that crystallize depend upon T - The sequence changes with the bulk composition

29 Augite forms before plagioclase Gabbro of the Stillwater Complex, Montana This forms on the left side of the eutectic

30 Plagioclase forms before augite Ophitic texture Diabase dike This forms on the right side of the eutectic

31 Also note: 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

32 Fractional crystallization: No effect on the liquid path The composition of the final rock is affected Fig Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40,

33 C. Binary Peritectic Systems Three phases enstatite = forsterite + SiO 2 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.

34 C. 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.

35 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.

36 i = peritectic point 1557 o C have colinear Fo-En-liq geometry indicates a reaction: Fo + liq = En consumes olivine (and liquid) resorbed textures When is the reaction finished? 1557 k i m d c Fo Bulk X En

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39 y k x i m 1557 d c 1543 Cr Fo En bulk X

40 Cool X = n At 1960 o C hit solvus exsolution 2 liquids o and p f = 2 F = 1 both liquids follow solvus At 1695 o C get Crst also Reaction? Immiscible Liquids Mafic-rich liquid 1695 Silica-rich liquid Crst