1. Diversification of Magmas

2. Today Updates: Have fun next week Topics: Evolving melt compositions:
Partial melting
Fractional crystallization
Crystal settling
Cumulates

3. Magmatic Differentiation? A B

4. Magmatic Differentiation

5. Partial Melting

6. Effects of only partially melting+segregating
Anorthite
Forsterite

7. Partial melt and grain boundaries
The ability to form an interconnected film is dependent upon the dihedral angle () a property of the melt
Figure 11-1 After Hunter (1987) In I. Parsons (ed.), Origins of Igneous Layering. Reidel, Dordrecht, pp

8. Minimum amount of melt

9. Stages in ascent Eruption (Fragmentation) Vesiculation Renewed ascent
Storage
mixing
assimilation
crystallization
Buoyant ascent
Partial melting

10. Why storage? crust crust
Why do some magmas stall and pond in chambers during ascent?
crust
denser
stronger
crust

11. Processes during storage in magma chambers

12. Fractional Crystallization

13. Gravity settling

14. Gravity settling and cumulates

15. Gravity settling and cumulates

16. Frax crystallization example: majors
Cumulate
Melt
Figure 11-2 After Murata and Richter, 1966 (as modified by Best, 1982)

17. Buoyancy, sinking: Stoke’s Law
2gr 2 ( r - r )
V = the settling velocity (cm/sec)
g = the acceleration due to gravity (980 cm/sec2)
r = the radius of a spherical particle (cm)
rs = the density of the solid spherical particle (g/cm3)
rl = the density of the liquid (g/cm3)
h = the viscosity of the liquid (1 c/cm sec = 1 poise) V = s l 9 h

18. Sinking olivine in basalt
Olivine (rs = 3.3 g/cm3, r = 0.1 cm)
Basaltic liquid (rl = 2.65 g/cm3, h = 1000 poise)
V = 2·980·0.12 ( )/9·1000 = cm/sec
that’s ~1m per day

19. Sinking xls in rhyolite
Rhyolitic melt
h = 107 poise and rl = 2.3 g/cm3
hornblende crystal (rs = 3.2 g/cm3, r = 0.1 cm)
V = 2 x 10-7 cm/sec, or 6 cm/year
feldspars (rl = 2.7 g/cm3)
V = 2 cm/year
= 200 m in the 104 years that a stock might cool
If 0.5 cm in radius (1 cm diameter) settle at meters/year, or 6.5 km in 104 year cooling of stock

20. Caution

21. Consequences
Figure 11-3 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall