1. 3- Mixing, assimilation, etc.
Evolution of magmas
3- Mixing, assimilation, etc.

2. Why FC cannot explain everything?
Need large amounts of cumulate (where??)
Difficult in a viscous magma
No good evidence for omnipresent magma chamber

3. Minor(?) magma differenciation processes
Liquid unmixing
Magma mixing
Assimilation

4. Liquid Immiscibility Liquid immiscibility in the Fo-SiO2 system Liquid
1557
Mg2SiO4 20 40 60 80
SiO2
1400
1600
1800
2000
1713
T oC
MgSiO2
1543
1470
Enstatite
Forsterite
+ Enstatite
Liquid +
1695
Enstatite + Tridymite
Enstatite + Cristobalite
Cristobalite + Liquid
Two Liquids
Liquid
Several early geologists appealed to liquid immiscibility as a mechanism for magmatic differentiation
Might be responsible for the separation of a granitic liquid from an evolving system (presumably from an initial basaltic parent)
Liquid immiscibility in the Fo-SiO2 system
Very limited compositional range
How behave as add more components?

5. Figure Two immiscibility gaps in the system fayalite-leucite-silica (after Roedder, 1979). Yoder (ed.), The Evolution of the Igneous Rocks. Princeton University Press. pp Projected into the simplified system are the compositions of natural immiscible silicate pair droplets from interstitial Fe-rich tholeiitic glasses (Philpotts, 1982). Contrib. Mineral. Petrol., 80,
Liquid immiscibility is now widely accepted as a phenomenon in natural magmas
But the extent of the process, and its importance in generating large bodies or a significant proportion of evolved magmatic rocks is still dubious

6. Some Examples
Late silica-rich immiscible droplets in Fe-rich tholeiitic basalts
Sulfide-silicate immiscibility (massive sulfide deposits)
Carbonatite-nephelinite systems
Iron-silicates (primitive Earth)
Granites & rhyolites? Bimodal basalt-rhyolite? Unlikely, since don’t correspond to any known experimental gaps

7. Mingling and mixing
Closepet granite (2.5 Ga), South India

8. Closepet granite (2.5 Ga), South India

9. Closepet granite (2.5 Ga), South India

10. Closepet granite (2.5 Ga), South India
Low strain
High strain
Closepet granite (2.5 Ga), South India

11. Hoedjiespunt granite (540 Ma), Saldanha, RSA

12. Hoedjiespunt granite (540 Ma), Saldanha, RSA

13. Hoedjiespunt granite (540 Ma), Saldanha, RSA

14. Closepet granite (2.5 Ga), South India

15. Vredenburg granite (540 Ma), Britannia Bay, RSA

16. Vredenburg granite (540 Ma), Britannia Bay, RSA

17. Vredenburg granite (540 Ma), Britannia Bay, RSA

18. Vredenburg granite (540 Ma), Britannia Bay, RSA

19. ~ 1 mm
Closepet granite (2.5 Ga), South India

20. Closepet granite (2.5 Ga), South India

21. Closepet granite (2.5 Ga), South India

22. Closepet granite (2.5 Ga), South India

23. Assimilation

24. Figure Gradational border zones between homogeneous igneous rock (light) and country rock (dark). After Compton (1962), Manual of Field Geology. © R. Compton.

25. Possible reactions Mg2SiO4 + SiO2 = 2MgSiO3
Olivine + Quartz =Orthopyroxene
CaMg2Si2O6 + Al2SiO5 = CaAl2Si2O8 + MgSiO3
Diopside + Sillimanite = Plagioclase + Orthopyroxene

31. Problems with assimilation
Source of heat?
> AFC processes
Direct evidences not commonly found; lot of case interpreted as assimilation could be magma mixing
Generaly invoked to explain geochemical (isotopic) characteristics with no field evidence…